2 2 0 0 1 1 0 0 H H a a n n g g z z h h o o u u W W o o r r k k s s h h o o p p o o n n Q Q u u a a n n t t u u m m M M a a t t t t e e r r B B O O O O K K L L E E T T May 18-22, 2010 Run-Run Shaw Science Building, Level 1, Zhejiang University Organized by International Collaborative Center on Quantum Matter and Department of Physics, Zhejiang University Department of Physics and Astronomy, Rice University Sponsored by Zhejiang University Rice University Max-Planck Institute for Chemical Physics of Solids Program of Changjiang Scholarship and Innovation Research Team National Science Foundation of China
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Introduction to the 2010 Hangzhou Workshop
Purpose and Scope The 2010 Hangzhou Workshop on Quantum Matter will be held in Hangzhou, China, on the Yuquan Campus of Zhejiang University, in May 18-22, 2010. It will be a satellite conference to the 9th international conference on spectroscopies of novel superconductors (Shanghai, May23-28, 2010, http://www.sns2010.org). This workshop is featured with invited talks and poster session. After the conference, there will be a one-day tutorial program for graduated students and junior scientists on May 22, 2010, with four lectures covering the subjects of heavy fermion superconductivity, quantum phase transition, ARPES and its application, and topological insulator. The workshop is part of the series sponsored by the International Collaborative Center on Quantum Matter. The purpose of this center is to explore the long–term institutional collaborations between Universities in China and abroad in the area of quantum matter. This frontier subject concerns modern condensed matter and atomic systems, in which quantum correlation and quantum coherence strongly influence their physics properties. The first such workshop was advocated by the Presidents of Zhejiang University and Rice University, and was held in Hangzhou in 2006. In 2009, the International Collaborative Center on Quantum Matter was inaugurated. On this occasion, the second workshop was held. Jointly sponsored by Zhejiang University, Rice University, MPI for the Chemical Physics of Solids, London Centre for Nanotechnology, USTC, and Hangzhou Normal University, the workshop brought together over 50 speakers from China, USA, Germany, UK and elsewhere to discuss the newly discovered quantum phases and novel quantum phenomena in strongly correlated electron systems, quantum nanostructures and cold atoms.
Topics The 2010 Workshop will cover the following topics: 1) Unconventional superconductivity 2) Quantum phase transition 3) Topological states
Advisory Committee Gabriel Aeppli (University College London) Qimiao Si (Rice University) Frank Steglich (MPI-CPfS) Fu-Chun Zhang (Hong Kong University/Zhejiang University)
Introduction to the 2010 Hangzhou Workshop Page 4/84
Local organizing Committee Zhuan Xu (Zhejiang University, Co-Chair) Huiqiu Yuan (Zhejiang University, Co-Chair) Guanghan Cao (Zhejiang University) Qijin Chen (Zhejiang University) Jianhui Dai (Zhejiang University) Minghu Fang (Zhejiang University) Yi Zhou (Zhejiang University)
Venue
The 2010 Workshop will be held at the Yuquan Campus of Zhejiang University
Website
http://zimp.zju.edu.cn/~iccqm/workshop2010/ .
Partner group of MPI-CPFS at Zhejiang University Page 5/84
Partner group of Max-Planck-Institute for Chemical Physics of Solids at Zhejiang University In order to strengthen the collaboration between Germany and China, the Max-Planck Society recently approved to establish a Partner Group of the Max-Planck Institute for Chemical Physics of Solids at Zhejiang University, led by Dr. Huiqiu Yuan, a Changjiang professor of Physics at Zhejiang University. The main research topic of the MPG partner group is on non-centrosymmetric superconductors. Joint research efforts will be devoted to the emergent quantum states in correlated materials, with emphasis on revealing the unique pairing states, the vortex dynamics and the possible exotic magnetic states, arising from an antisymmetric spin-orbital coupling due to the lack of inversion symmetry. On the occasion of the official inauguration of the “Zhejiang University – Max-Planck Institute for Chemical Physics of Solids Partner Group”, there will be a ceremony in the morning of May 19, 2010 on the Yuquan campus of Zhejiang University, followed by an academic conference – “The 2010 Hangzhou workshop on quantum matter”. The "Max Planck Partner Groups" are an instrument created by the Max Planck Society for the purpose of strengthening the ties between Max Planck Institutes and foreign research institutes and of intensifying cooperation between individual scientists through jointly conducted projects. Partner groups are headed by visiting scientists with proven research records and profiles who, after completing their research residency at a Max Planck Institute, return to their home base to lead an appropriately equipped research group. Within the scope of Partner Group funding, scientists are given the opportunity to continue their research at their home institute under favourable working conditions and in close contact with their former host institute. For the Max Planck Society, the Partner Groups are a means of facilitating access to high-performance research institutes overseas. The Partner group is supported by a bilateral Scientific Advisory Board, initially for a three-year period. After successful evaluation by the Board, the Partner Group can be extended by a further two years. As a rule the Max Planck Society will allocate 20,000 euros for the partner group each year.
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Scientific Program
May 18, 2010 Hotel
14:00-18:00 Registration JinXi Hotel (Invited Speakers and Guests) LingFeng Hotel (Other Participants)
18:00-20:00 Reception JinXi Hotel (All the Participants) May 19, 2010
Session 1: Heavy Fermions, non-centrosymmetric superconductor Chair: Lu Yu
09:10-09:35 Frank Steglich Unconventional quantum criticality in YbRh2Si2 09:35-10:00 Sungkit Yip Novel Properties of non-centrosymmetric superconductors
10:00-10:25 Qimiao Si Quantum Criticality and Global Phase Diagram of Heavy Fermion Metals
10:25-10:50 Kazumasa Miyake
Role of Critical Valence Fluctuations in Ce-Based Heavy Fermions
10:50-11:15 Tea Break Session 2: Topological states
Chair: Xi Dai11:15-11:40 S.C. Zhang Topological insulators and topological superconductors 11:40-12:05 Rui-Rui Du Recent Progress on the n = 5/2 Topological State
12:05-12:30 Xi Chen Molecular beam epitaxy growth and novel properties of topological insulator films
Chair: M. Le Tacon15:30-15:55 K. Ishida NMR Studies on Iron-Pnictide Superconductor BaFe2(As1-xPx)2
15:55-16:20 Nan-Lin Wang In-plane and out-of-plane charge dynamics of BaFe2As2 probed by optical spectroscopy
16:20-16:45 Weiqiang Yu Comparative NMR Study of Magnetism and Spin fluctuations on NaFeAs and LiFeAs Single Crystals
16:45-17:10 P. C. Dai Neutron Scattering studies of iron arsenide superconductors May 20, 2010
Campus Session 4: Iron-based superconductors
Chair: Nan-Lin Wang
08:30-08:55 M. Le Tacon Inelastic x-ray scattering investigations of iron pnictides superconductors
08:55-09:20 Zhuan Xu Effect of non-magnetic impurity Zn on superconductivity in LaFeAsO
09:20-09:45 Liling Sun Pressure dependence of phase diagrams of Fe-based superconductors
09:45-10:10 Minghu Fang Magnetism and Superconductivity in Fe(Te,Se,S) system 10:10-10:35 Tea Break
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Session 5: Emergent phenomena, Novel quantum states Chair: S. C. Zhang
10:35-11:00 Davor Pavuna NanoEngineering and Direct ARPES on In-situ Grown Ultra-thin Quantum Matter
11:00-11:25 Xi Dai Quantized Anomalous Hall Effect: From Science fiction to real materials
10:25-10:50 T K Ng Fermionic theory for S>1/2 spin-liquid states 11:50-12:15 Sean Giblin Magnetic Charge Transport in Spin Ice 12:15-14:00 Lunch 14:00-17:30 Excursion - West Lake 18:00-20:00 Banquet - Wei Zhuang
May 21, 2010 Campus
Session 6: Heavy fermions, quantum criticality Chair: Frank Steglich
08:30-08:55 F. M. Grosche Quantum phase transitions in transition metal com-pounds
08:55-09:20 Meigan Aronson
Geometric Frustration and Quantum Criticality in Heavy Fermions with the Shastry-Sutherland Lattice
09:20-09:45 Tuson Park Quantum criticality in Cd-doped Ce115 compounds
09:45-10:10 Peijie Sun Electron correlations and enhanced thermoelectricity in narrow gap FeSb2
Chair: Qimiao Si 10:35-11:00 Changqing JIN “111” type iron pnictide superconductors
11:00-11:25 Shiyan Li Superconducting gap structure of iron-based superconductors probed by heat transport measurement
11:25-11:50 Fa Wang Functional Renormalization Group Study of Iron-based Superconductors.
11:50-12:15 Jiangping Hu Disorder effect on Iron-Based superconductors 12:15-14:00 Lunch Session 8: High-Tc cuprates
Chair: Pengcheng Dai
14:00-14:25 Jeff Sonier Emergence of Static Electronic Moments in the Heavily Overdoped Regime
14:25-14:50 Ming Shi ARPES Studies of Electronic Excitations in Cuprate High Temperature superconductors
14:50-15:15 Hae Young Kee Nematicity and Symmetry Constraints in Correlated systems
15:15-15:40 Jian-Xin Li Evolution of electronic structure in doped Mott insulators:Fermi arc and Fermi pocket
15:40-16:00 Tea break Session 9: Emergent phenomena, Novel quantum states
Chair: Rui-rui Du 16:00-16:25 Kamran Behnia Nernst effect in bismuth and graphite across the quantum limit 16:25-16:50 Yue Yu Correlation-hole induced paired quantum Hall states 16:50-17:15 Supeng Kou Symmetry protected topological orders 17:15-17:40 Jung Hoon Han Skymion Spin Texture in Magnetic Metals 17:40-18:00 Lu Yu Summary and concluding remarks 18:00-19:00 Dinner 19:30-21:00 Show - Impression.West Lake
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Tutorial Program
May 22, 2010 Campus (Academic Building 12, Room 201) Chair: Zhuan Xu 08:30-10:00 S. C. Zhang Topological insulators 10:00-10:15 Break 10:15-11:45 P. C. Dai Neutron scattering and its applications 12:00-14:00 Lunch 14:00-15:30 Q. M. Si Heavy fermions and quantum criticality 15:30-15:45 Break 15:45-17:15 Y. H. Zhang On spins of electrons
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Posters
No. Group Name Institution Poster Title
1 A Rong Yu Rice University Mott Transition in Multi-Orbital Hubbard Models for Iron Pnictides
2 A Jack Gillett Cambridge XLD and XMCD measurements of Sr(Fe1-xCox)2As2
3 A Li Zhang China Jiliang College
Correlation between the combinative energy and Tc in SmFeAsO1-xFx and SmFe1-xMxAsO(M=Co,Ni)
4 A Huiqiu Yuan Zhejiang University Nearly isotropic superconductivity in iron pnictides
5 A Qian Tao Zhejiang University Nernst effect of BaFe2-xNixAs2 system
6 A YuKe Li Zhejiang University
Effect of Zn-impurity on Tc and its implication to pairing symmetry in under- and over-doped LaFeAsO1−xFx
7 A Jun Tong Zhejiang University The magnetic properties of EuCuAs
8 A Lin Jiao Zhejiang University Electronic duality in Ba(Fe1-xCox)2As2
9 A Hangdong Wang Zhejiang University From antiferromagnetic order to spin-glass state in Fe1.05-xTe system
10 A Chiheng Dong Zhejiang University
Effect of annealing on superconductivity and transport properties in Fe(Te,S) system
11 A Shuai Jiang Zhejiang University Superconductivity and local-moment magnetism in Eu(Fe0.89Co0.11)2As2
12 B Jedediah Pixley Rice University Scaling and relaxational dynamics near Kondo-destroying quantum critical points
13 B Jianda Wu Rice University
Entropy Accumulation, Divergent Gruneisen Ratio, and Crossover Energy Scales near Quantum Critical Points
14 B Yanhua Dai Rice University
Geometric Resonance in Triangular or Honeycomb Lattices Patterned on Very High-Mobility Quantum Wells^1
15 B Eteri Svanidze Rice University
Crystal field anisotropy and geometric frustration effects in triangular rare-earth based intermetallic compounds
16 B ZhiFeng Ma Zhejiang University
La2Co2Se2O3: A Quasi-two-dimensional Mott insulator with Unusual Co Spin State and Possible Orbital
17 C Ivan Knez Rice University
TOWARDS QUANTUM SPIN HALL EFFECT IN InAs/GaSb QUANTUM WELLS
18 C JunHui He YangZhou University
Enhanced room-temperature magnetoresistance and cluster spin glass behavior in Mo doping La0.67Sr0.33Mn1-xMoxO3 system
19 C Zhe Qu CAS High temperature resistive relaxation in lightly Ti doped CuCrO2
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20 C MengBo Luo Zhejiang University Creep of vortex glasses in type-II superconductors
21 C Hui Xing Zhejiang University
Emergent order in the spin-frustrated system DyxTb2−xTi2O7 studied by ac susceptibility measurements
22 C
Aleksej Mialitsin
Rutgers the State University
Shared electron-phonon coupling: Coexistence of the charge density wave and superconductivity in the layered transition metal dichalchogenide 2H-NbSe2
A Iron-based superconductors B Strongly correlated electrons C Others
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Invited talks
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Unconventional quantum criticality in YbRh2Si2
F. Steglich
MPI for Chemical Physics of Solids, Noethnitzer Str. 40. 01187 Dresden, Germany
In the heavy-fermion metal YbRh2Si2 a quantum critical point (QCP) has been established
by driving a continuous antiferromagnetic (AF) phase transition from TN ≈ 70 mK at B = 0
to TN = 0 via application of a tiny magnetic field BN (⊥c) ≈ 60 mT. New results on the Hall
coefficient and thermoelectric power support the conclusion drawn from earlier studies that
this AF QCP coincides with a Kondo-breakdown QCP. Positive and negative chemical
pressure was applied to YbRh2Si2 to explore the evolution of its B-T phase diagram under
changes of the unit-cell volume: Clear signatures of the Kondo-breakdown QCP were
observed within the magnetically ordered phase under volume compression (i.e., Co
substitution for Rh). Here, the AF QCP appears to be of the conventional (3D SDW) type.
Under slight volume expansion (doping with 2.5 at % Ir) the AF instability and the
Kondo-breakdown QCP were found to still coincide at BN (⊥c) ≈ 45 mT. For 6 at% Ir
doping, however, AF order appears to be largely suppressed (BN ≈ 15 mT), while the
Kondo-breakdown QCP remains virtually unchanged. For this composition, a new low-T
phase (probably of spin-liquid type) shows up in a finite range of magnetic fields.
In collaboration with: M. Brando, S. Friedemann, P. Gegenwart, C. Geibel, S. Hartmann, S.
Kirchner, C. Krellner, Q. Si, T. Westerkamp and S. Wirth.
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Novel Properties of non-centrosymmetric superconductors
Sungkit Yip
Institute Institute of Physics, Academia Sinica, Nankang 115, Taipei, Taiwan
Antiferromagnetic (AF) fluctuations are believed to promote anisotropic superconductivity in strongly correlated metals such as Ce-based heavy fermion compounds. In particular, the superconductivity (SC) appearing around AF quantum critical point (QCP) under pressure can be naturally understood by this mechanism. On the other hand, some Ce-based heavy fermion compounds, such as CeCu2(Si,Ge)2, exhibit an sharp enhancement of superconducting transition temperature (Tc) at pressure far above that of AF-QCP. Namely, SC in Ce-based heavy fermions under pressure are classified into two classes, “pea” and “potato” as shown in Figure 1 [1]. SC of “pea” is considered to be induced by AF spin fluctuations and Tc is considerably low compared to the Neel temperature (TN). On the other hand, Tc of “potato” is comparable to TN and SC phase extends in wider pressure region than the “pea”. In CeCu2(Si0.9Ge0.1)2, in particular, “pea” and “potato” are separated to form two domes of Tc [2]. The origin of SC of “potato” can be traced back to a sharp valence change or transition of Ce ion under pressure[3].
Non-Fermi liquid behavior is observed at the pressure around Pv, roughly corresponding to the maximum of Tc of “potato”: A T-linear resistivity, an enhancement of the residual resistivity, and a non-monotonous variation of the Sommerfeld constant as a function of the pressure. These anomalous properties, together with a sharp valence crossover and an enhancement of Tc of d-wave SC, can be understood coherently on the extended Anderson model with Coulomb repulsion between f- and conduction electrons [3]. Recent theoretical development on this issue is that the QCP of valence transition is controlled effectively by a magnetic field [4], and the transition in CeRhIn5 at around P=2.4GPa can be understood as a phenomenon induced by the valence crossover of Ce ion [5]. These findings imply that critical valence fluctuations are more ubiquitous than thought earlier, and it seems possible to understand a series of anomalous properties associated with valence transition phenomena observed in Ce- and Yb-based heavy fermion compounds.
Figure 1: Superconducting duo of Ce-based heavy fermion compounds under pressure. [1] A. T. Holmes et al., Phys. Rev B, 69 (2004) 024508; J. Phys. Soc. Jpn. 76 (2007) 051002. [2] H. Q. Yuan et al., Science 302 (2003) 2104. [3] K. Miyake, J. Phys.: Condens. Matter 19 (2007) 125201 and references therein. [4] S. Watanabe et al., Phys. Rev. Lett. 100 (2008) 236401; J. Phys. Soc. Jpn. 78 (2009) 104706. [5] S. Watanabe and K. Miyake, J. Phys. Soc. Jpn. 79 (2010) 033707.
Abstracts of Invited talks Page 17/84
Topological insulators and topological superconductors
S.C. Zhang
Department of Physics, Stanford University Recently, a new class of topological states has been theoretically predicted and experimentally realized. The topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction[1] of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation[2]. The edge states of the QSH state supports fractionally charged excitations[3]. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators[4]. Topological insulators Bi2Te3, Bi2Se3 have been discovered theoretically and experimentally to have surface states consisting of a single Dirac cone[5,6,7]. I shall present a realistic experimental proposals to observe the magnetic monopoles on the surface of topological insulators[8]. Topological superconductors and superfluid have been theoretically proposed recently [9], in both two and three dimensions. They have a full pairing gap in the bulk, and their mean field Hamiltonian look identical to that of the topological insulators. However, the gapless surface states consists of a single Majorana cone, containing only half the degree of freedom compared to the single Dirac cone on the surface of a topological insulators. I shall discuss their physics properties and the search for these novel states in real materials. [1] A. Bernevig, T. Hughes and S. C. Zhang, Science, 314, 1757, (2006) [2] M. Koenig et al, Science 318, 766, (2007) [3] J. Maciejko, Chaoxing Liu, Yuval Oreg, Xiao-Liang Qi, Congjun Wu, and Shou-Cheng Zhang, , Phys. Rev. Lett. {\bf 102}, 256803 (2009). [4] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, Phys. Rev B. 78, 195424 (2008) [5] Haijun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang, and Shou-Cheng Zhang, Nature Physics 5, 438 (2009). [6] Y. Xia, L. Wray, D. Qian, D. Hsieh, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. Hor, R. Cava, et al., Nat. Phys. 5, 398 (2009). [7] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, et al., Science 325, 178 (2009). [8] Xiao-Liang Qi, Run-Dong Li, Jiadong Zang and Shou-Cheng Zhang, Science 323, 1184 (2009). [9] Xiao-Liang Qi, Taylor L. Hughes, Srinivas Raghu and Shou-Cheng Zhang, Phys. Rev. Lett. 102, 187001 (2009)
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Recent Progress on the Filling Factor 5/2 Topological State
Rui-Rui Du
Rice University
The factional quantum Hall effect (FQHE) at the Landau level filling factor 5/2 presents a l
ongstanding challenge for understanding. As proposed by theories, the 5/2 is described by t
he Moore-Read Pfaffian (Pf) wavefunction, or its particle-hole symmetry conjugate, the An
ti-Pfaffian (APf). Either Pf or APf supports e/4 quasiparticles that obey non-Abelian statisti
cs, and the non-trivial breading of these entities form the bases for fault-tolerant topological
quantum computation. This talk will briefly describe/discuss recent experimental work on
5/2, 1) Alternating e/4 and e/2 period interference oscillations consistent with filling factor
5/2 non-Abelian quasiparticles, by Willett (Bell Labs) et al; and 2) Spin polarization at 5/2
as revealed by transport in tilted magnetic fields, by Eisenstein group (CalTech) and by Du
group (Rice). These findings are consistent with the predicted properties of non-Abelian sta
tes.
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Molecular beam epitaxy growth and novel properties of topological insulator films
Xi Chen
Department of Physics, Tsinghua University
In this talk, I will summarize our recent activities in the state-of-art molecular beam epitaxy
(MBE) growth and characterization of nontrivial surface states of topological insulator
films of Bi2Te3 and Bi2Se3 on Si(111) and 6H-SiC(0001) substrates. We study the growth
dynamics and epitaxial relationship under different flux ratios and substrate temperatures,
and identified the optimized conditions for stoichiometric and layer-by-layer MBE
deposition of both Bi2Te3 and Bi2Se3 films by real time reflection high energy electron
diffraction (RHEED). We demonstrate the atomically flat morphology and intrinsic
topological property of the resulted films by angle resolved photoemission spectroscopy
(ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS). By direct imaging
standing waves associated with nonmagnetic impurities and steps on Bi2Te3 and Bi2Se3
surfaces, we show that the topological states have a surface nature and are protected by the
time reversal symmetry. I will also discuss our recent observation of Landau quantization
of the topological surface states.
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NMR Studies on Iron-Pnictide Superconductor BaFe2(As1-xPx)2
K. ISHIDA1,2, Y. NAKAI1,2, S. KITAGAWA1,2, T. IYE1,2, S. KASAHARA3, T. SHIBAUCHI1,
Y. MATSUDA1, AND T. TERASHIMA3
1 Department of Physics,Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2TRIP, JST, Sanban-cho, Chiyoda, Tokyo 102-0075, Japan 3Research Center for Low Temperature and Materials Science, Kyoto University, Kyoto
We present recent our NMR results on iron-pnictide superconductor BaFe2(As1-xPx)2,
in which superconductivity is induced by substituting isovalent P for As. We have investigated the evolution of electronic and magnetic properties with P doping from systematic P-NMR measurements. The nuclear spin-lattice relaxation rate divided by temperature 1/T1T in BaFe2(As0.67P0.33)2 with a maximum Tc ~ 30 K in BaFe2(As1-xPx)2 continues to increase down to Tc, indicating the development of the antiferromagnetic (AF) fluctuations, and sharply decreases below Tc due to opening of the SC gap. The AF fluctuations detected with 1/T1T measurements are suppressed and Tc also decreases with increasing P content. From the analyses of the NMR results in the normal state, we found that the maximum-Tc BaFe2(As0.67P0.33)2 is located in the vicinity of the quantum critical point of the AF ordering, and that the AF fluctuations are intimately related to the superconductivity in BaFe2(As1-xPx)2. We discuss the similarity between BaFe2(As1-xPx)2 and heavy-fermion superconductors.
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In-plane and out-of-plane charge dynamics of BaFe2As2 probed by optical spectroscopy
Nan Lin Wang
Institute of Physics, Chinese Academy of Sciences, Beijing, China
I present optical spectroscopy investigations on the ab-plane and c-axis charge dynamics of parent compound BaFe2As2. Our study revealed a clear difference in optical conductivity for E//ab-plane and E//c-axis in the SDW state. The very pronounced energy gap structure seen at a higher energy scale for E//ab-plane is almost invisible for E//c-axis, whereas the smaller energy gap could be seen in both polarizations. We propose a novel picture for the band structure evolution and suggest different driving mechanisms for the two energy gaps. The strong ($\pi,\pi$) nesting between disconnected 2D cylinder-like Fermi surfaces is the main driving force for the SDW instability, leading to the opening of larger energy gap in the 2D Fermi surfaces. The cutting of the magnetic Brillouin zone on the 3D Fermi surface leads to a smaller gap at the crossing region. Work done with W. Z. Hu, G. Li, Z. G. Chen, J. Dong, T. Dong, R. H. Yang, B. F. Hu, B. Cheng, P. Zheng, G. F. Chen, and J. L. Luo, Z. Fang, and X. Dai.
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Comparative NMR Study of Magnetism and Spin fluctuations on NaFeAs and LiFeAs Single Crystals
Weiqiang Yu
Department of Physics, Renmin University of China,
Beijing 100872, China The reported small spin-density-wave (SDW) moment of NaFeAs and the absence of long-range SDW order in LiFeAs are still not well understood so far. In this talk, I will present our comparative NMR study on high-quality NaFeAs and LiFeAs single crystals, and discuss our results on the magnetism, the spin fluctuations, the chemical non-stoichiometry, and the superconductivity in both compounds. * The work is in collaboration with L. Ma, G. F. Chen and Z. Y. Lu.
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Neutron Scattering studies of iron arsenide superconductors
Pengcheng Dai,
The University of Tennessee/Oak Ridge National Laboratory/Institute of Physics, CAS
In this Talk, I describe the most recent progress in the field of iron-based superconductors. Using neutron scattering as a probe, we study the electron doping evolution of the spin excitations in the Ni-doped FeAs-based superconductors BaFe2As2. We show that the resonance is directly connected with superconducting electronic gap, and the three-dimensional nature of the mode persist from underdoped to overdoped regime. These results suggest that spin excitations are the most promising candidate for electron pairing and superconductivity in iron-based superconductors.
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Inelastic x-ray scattering investigations of iron pnictides superconductors
M. Le Tacon,1 T. R. Forrest,2 Ch. Rüegg,2 A. Bosak,3 A. C. Walters,2,3 N. D. Zhigadlo4, J. Karpinski,4 J. P. Hill,5
M.Krisch,3 and D. F. McMorrow2
1Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany 2London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London
WC1E 6BT, United Kingdom
3 European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France 5Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland
6Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, New York 11973, USA
I will present measurements of the phonon density of states (PDOS) and dispersion in iron oxypnictide superconductors by inelastic x-ray scattering. In the PDOS, we observe softening of phonon branches under fluorine doping in NdFeAsO1-xFx and SmFeAsO1-xFx, and Raman-scattering experiments lead us to conclude that this softening is not related to zone-center phonons. It consequently implies an important softening of the relevant phonon branches at finite-momentum transfer Q [1]. Further experiments on the parent SmFeAsO and superconducting SmFeAsO0.60F0.35 single crystals have been carried out in order to map the phonon dispersion. Particular attention was paid to thedispersions along the 100 and 110 directions of three optical modes close to 23 meV, polarized out of theFeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping [2]. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron pnictides. Fig. 1: Doping dependence of the phonon density of in SmFeAsO(1-x)Fx.
Fig. 2: Momentum dependence of the doping induced renormalization of the 26 meV, B1g
(upper panel) and 21 mV, A1g (lower panel) modes in the (qa 0 qc) plane. The dashed ellipse indicates the q-resolution. The insets show schematics of the respective q=0 eigenvectors, with the atoms color coded as indicated.
[1] M. Le Tacon, M. Krisch, A. Bosak, et al., Physical Review B 78, R140505 (2008). [2] M. Le Tacon, T. R. Forrest, C. Rüegg, et al., Physical Review B 80, R220504 (2009).
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Effect of non-magnetic impurity Zn on superconductivity in LaFeAsO
Zhuan Xu
Department of Physics, Zhejiang University, Hangzhou 310027, China
We report a systematic investigation of non-magnetic Zn-impurity effect on both the magnetism of the 1111 phase parent compound LaFeAsO and on superconductivity in the underdoped, optimally doped, and overdoped regimes of F-doped LaFeAsO superconductors. For parent compounds, a significant suppression of antiferromagnetism of Fe 3d electrons is observed. For the F-doped superconducting LaFeAsO1-xFx, we found a strong doping dependence of the non-magnetic impurity effect, which suggests a switch of the pairing symmetry of the superconductivity from a Zn-insensitive s-wave state in the underdoped regime to a Zn-impurity sensitive anti-phase s-wave or d-wave state in the overdoped regime as the doped carrier density (F content in LaFeAsO1-xFx) increases. The measurements of Hall effect and thermopower confirm that the charge carrier density does not change with Zn content. These findings provide a way to reconcile seemingly conflicting previous experimental studies of Zn impurity effect in the pnictides and they should have important implication to theoretical models.
Abstracts of Invited talks Page 26/84
Pressure dependence of phase diagrams of Fe-based superconductors
Liling Sun and Zhongxian Zhao
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics,
Chinese Academy of Sciences, Beijing 100190, P. R. China
Understanding the physics in newly discovered iron-base superconductors is one of central issues of current research in solid state physics. The influence of pressure, chemical substitution and doping etc. on the superconducting transition provides an attractive source for the investigation of the superconductivity mechanism. This talk will present pressure dependence of diagrams of REFeAsO1-xFx (RE=La, Ce, Nd, Sm), EuFe2As2, AFeAs (A=Li, Na) and FeSe(Te). Particularly we will focus on CeFeAsO1-xFx superconductors, for which the pressure-induced full suppression of the superconductivity is demonstrated and its physical origin is discussed based on the experimental results of in-situ electrical resistance, x-ray diffraction and x-ray absorption spectrum measurements in a diamond anvil cell.
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Magnetism and Superconductivity in Fe(Te,Se,S) System
Minghu Fang
Department of Physics, Zhejiang University, Hangzhou 310027, China
The Fe-chalcogenide superconductors attract us as the simplest structure of Fe-based superconductors. Although their Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits a different antiferromagnetic order compared with the other pnictide parent compounds. In this report, firstly, we will discuss the effect of Cu substitution for Fe on magnetism in Fe1.05-xCuxTe system: from antiferromagnetic ordering to spin-glass state; secondly, we will report the effect of annealing on superconductivity in both Fe1+y(Te1-xSx) and Fe1+y(Te0.6Se0.4) system; finally, we will discuss the weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals.
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NanoEngineering and Direct ARPES on In-situ Grown
Ultra-thin Quantum Matter Davor Pavuna, Daniel Ariosa, Nathaniel Wooding, Guy Dubuis, Ofer Yuli, Claudia
Cancellieri
Physics of Complex Matter – http://cream.epfl.ch - ICMP-FSB, station 3 Swiss Federal Institute of Technology at Lausanne (EPFL)
CH-1015 Lausanne - EPFL, Switzerland
In order to nano-engineer new electronic functionalities in complex high-Tc heterostructures it is important to master the nanophase separation and control the doping, electronic dispersion, local dielectric function and interfaces. Since 2001 we have demonstrated in-situ angle resolved photoelectron spectroscopy (ARPES) on in-situ grown, non-cleaved, ultra-thin (<30nm) cuprate films grown by laser ablation (PLD). We have probed low energy electronic structure and measured properties of high-Tc films under different degree of epitaxial (compressive vs tensile) strain [1-5]. In overdoped in-plane compressed LSCO-214 films we doubled Tc, from 20K to 40K, yet Fermi surface (FS) remained essentially 2-dimensional (2D) [1]. In contrast, tensile strained films exhibit 3D dispersion; Tc is drastically reduced [2]. It seems that the in-plane compressive strain pushes the apical oxygen away from the CuO2 plane [1-3], enhances the 2D character of the dispersion and enhances Tc, while the tensile strain seems to act in the opposite direction and the resulting dispersion is 3D [2-5]. Evidently, such studies are directly relevant for the mechanism of high-Tc, as we have also obtained the FS topology for both cases. As the actual lattice of cuprates consists of rigid CuO2 planes that alternate with softer 'reservoir' (that strains distort differently), our results tend to rule out 2D rigid lattice mean field models [3]. Following our initial work in Wisconsin Synchrotron, at our EPFL PLD-Scienta laboratory we have successfully mapped the 3D FS topology from the observed wavevector quantization in cuprate films thinner than 18 units cells [5]. Moreover, at our Center (http://cream.epfl.ch ), we have successfully grown doped Bi-2201 thin films and performed XRD, transport and in-situ ARPES studies [6]. Last but not least, we also study the interface superconductivity and preéiminary transport, kinetic inductance and µSR studies indicate that the results of Bozovic et al. (BNL), as well as other groups, offer a very promising direction for further Tc enhancements [6].
1. M. Abrecht et al. PRL 91, 057002 (2003); EPFL Thesis no. 2792 (2003) 2. D. Cloetta et al., Phys. Rev. B74, 014519 (2006); EPFL Thesis no. 3333 (2005) 3. D. Pavuna et al., Journal of Physics 108, 012040 (2008) and references therein 4. C. Cancellieri et al., Phys. Rev. B76, 174520 (2007); EPFL Thesis no. 4120 (2008) 5. D. Ariosa et al. Appl. Phys. Lett. 92, 092506 (2008) and references therein 6. D. Oezer, M. Sc. Thesis, EPFL (2008); T. Schnyder, M. Sc. Thesis, EPFL (2010); N.
Wooding et al., unpublished (2010) ; G. Dubuis et al. unpublished (2010)
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Quantized Anomalous Hall Effect: From Science fiction to real materials
Xi Dai
IOP, CAS
The anomalous Hall effect is a fundamental transport process in solids arising from the spin-orbit coupling. In a quantum anomalous Hall insulator, spontaneous magnetic moments and spin-orbit coupling combine to give rise to a topologically non-trivial electronic structure, leading to the quantized Hall effect without an external magnetic field. Based on first principles calculations, we predict that the tetradymite semiconductors Bi2Te3, Bi2Se3, and Sb2Te3 form magnetically ordered insulators when doped with transition metal elements (Cr or Fe), in contrast to conventional dilute magnetic semiconductor where free carriers are necessary to mediate the magnetic coupling. Magnetic order in two-dimensional thin films gives rise to a topological electronic structure characterized by a finite Chern number, with the Hall conductance quantized in units of e2/h..
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Fermionic theory for S>1/2 spin-liquid states
Tai-Kai Ng
HKUST I shall show in this talk how the fermion representation for $S=1/2$ spins can be generalized to spins with arbitrary magnitudes. The symmetry properties of the representation is analyzed where a fundamental difference between integer and half-odd-integer spins is found. I shall show how a particle-hole symmetry mean-field theory in the fermionic representation can produce results in agreement with the Haldane conjecture in 1D and similar difference exists between spin liquid states in higher dimensions.
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Measurement of the charge and current of magnetic monopoles in spin ice
The transport of electrically charged quasiparticles (based on electrons, holes or ions) plays a pivotal role in modern technology as well as determining the essential function of bio-logical organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges are generally considered to be, at most, convenient macroscopic parameters rather than sharply defined quasiparticles. However, the recent proposition of emergent magnetic monopoles in certain materials may change this point of view. Here we address the question of whether these magnetic charges and their associated currents (`magnetricity') can be directly measured in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes, we show that this is possible, and devise an appropriate method. Then, using muon spin rotation as a convenient local probe, we apply the method to a real material: the spin ice Dy2Ti2O7. Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb interaction, and have measurable currents. We further characterise deviations from Ohm's Law, and determine the elementary unit of magnetic charge to be 5μ BA-1. We show further results from magnetic susceptibility confirming the Wein effect and that the surface of the crystal behaves like a capacitor, storing charge, with the resultant relaxation described by the dissociation and recombination of charge carriers. The measurement of magnetic charge and observation of magnetic current emphasises the reality of these quantities and establishes an instance of a perfect symmetry between electricity and magnetism.
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Quantum phase transitions in transition metal compounds
F. M. Grosche
Cavendish Laboratory, University of Cambridge, Cambridge, UK The search for novel electronic states in condensed matter involves the dual notions of tunability of the effective quasiparticle interaction and of crossing phase boundaries at low temperatures: quantum phase transitions. The classification of low temperature states purely in terms of broken symmetries has to be refined in the face of an increasing variety of topologically protected phenomena, such as flux lines, skyrmions, monopoles and topological insulators, but the notion of tunability remains robust and useful. We examine the low temperature states of two transition metal compounds as functions of pressure, composition and applied magnetic field: (i) NbFe2 is poised on the threshold of ferromagnetism and can be pushed into a spin-aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe2 has been reported as a rare example of low-temperature spin-density-wave order in a d-metal system. Near the quantum critical point (qcp), we find distinct non-Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca3Ru2O7 undergoes first a magnetic transition (TN = 56 K) and then a structural transition (TS = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both TN and TS. For p> 3.5 GPa, the carrier density increases by two orders of magnitude, corresponding to about 0.6 electrons/Ru, and a third low temperature state is observed, which is eventually suppressed at p : 10 GPa. Competing scenarios for the low temperature states of both systems and their relevance to related materials are discussed.
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Geometric Frustration and Quantum Criticality in Heavy Fermions with the Shastry-Sutherland Lattice
Meigan Aronson
Brookhaven
Geometrical frustration leads to the destabilization of magnetic order, and in some cases to the formation of strongly fluctuating `spin liquid’ states with unconventional dynamics and only short ranged correlations. Since frustration arises from competition among short-ranged interactions, its impact is most evident in insulating systems, and is thought to be a weak effect in metallic systems where the phase behavior is likely to be controlled by long-ranged Kondo and RKKY physics. We review here the essential properties of the R2T2X (R=rare earth, T=transition metal, X=Pb,Sn,Sb,Bi) layered compounds, where the R atoms lie on triangular units in the geometrically frustrated Shastry-Sutherland lattice (SSL)[1]. Depending on the relative strengths of the first and second neighbor exchange interactions, these compounds are separated by a quantum critical point (QCP) into antiferromagnets and spin liquids. The metallic R2T2X compounds present the first opportunity to span the entire SSL phase diagram within a single structure and moment type. Far from the QCP, conventional Kondo and mixed valence behavior is found for R=Ce,Yb, while typical SSL phenomena such as magnetization plateau and strong fluctuations are found in the magnetically ordered compounds. Yb2Pt2Pb and Ce2Pt2Pb are of special interest, as they lie very near the quantum critical point. Yb2Pt2Pb orders antiferromagnetically at 2 K, with unusually strong fluctuations in the paramagnetic state. The ordered state is Fermi liquid-like with a Sommerfeld coefficient γ=0.05 J/mol-K2[3]. In contrast, Ce2Pt2Pb appears to be very near the QCP, and here the ground state is heavy fermion-like, with γ-0.6 J/mol-K2.
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Quantum criticality in Cd-doped Ce115 compounds
Tuson Park (1, 2), Z. Fisk (3), and J. D. Thompson (2)
(1) Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea (2) Los Alamos National Laboratory, Los Alamos, NM 87545, USA (3) Department of Physics, University of California, Irvine, California 92697, USA
The heavy fermion class CeMIn5 (M=Co, Rh, Ir) has served as one of the exemplary systems for exploring quantum criticality because high-quality crystals are available and modest applied pressure can tune the continuous antiferromagnetic transition to absolute zero temperature, a quantum critical point. Recently, it has been reported that a slight amount of Cd (or Hg) doping (percent level) introduces antiferromagnetism to the heavy fermion superconductor CeCoIn5, opening a venue to study the interplay between magnetism and superconductivity even at ambient pressure where many experimental tools are available [1, 2]. In this talk, we discuss resistivity and heat capacity measurements of the Cd-doped CeCoIn5 under pressure. Disorder effects that are necessarily introduced by Cd-doping will be discussed in the context of quantum criticality. [1] L. D. Pham, Tuson Park, S. Maquilon, J. D. Thompson, and Z. Fisk, “Reversible tuning of the heavy-fermion ground state in CeCoIn5,” Phys. Rev. Lett. 97, 056404 (2006). [2] E. D. Bauer et al., “Occurrence of magnetism inCeMIn5-xHgx (M=Rh, Ir),” Physica B 403, 1135 (2008).
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Electron correlations and enhanced thermoelectricity in narrow gap FeSb2
P. Sun1, M. Søndergaard2, S. Johnsen2, B. B. Iversen2, M. Baenitz1, F. Steglich1
1Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
2Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark FeSb2 was recently identified as a new model system of narrow-gap, strongly correlated semiconductor[1-2], sharing high similarity with FeSi. At around 100 K, its magnetic susceptibility shows a crossover from low-temperature diamagnetism to enhanced paramagnetism at high temperatures, indicative of a spin gap of ~ 80 meV. Electrical transport properties, on the other hand, exhibit two energy gaps of 5-10 meV and 30-40 meV. Largely enhanced thermopower (|S| > 10 mV/K) is observed in this material below 30 K [2]. All these observations suggest FeSb2 as a new playground for studying electron-electron correlations in a d-based narrow band gapped system. Comparative investigations on electrical, thermal transport and thermodynamic properties of FeSb2 and non-correlated reference systems like RuSb2 and FeAs2 corroborate the argument that electron correlations play a crucial role in FeSb2. Substitution of Sb by Te is studied and we find a shift of FeSb2 from a correlated semiconductor to a correlated metal for only less than 0.5% Te. The large enhancement of thermopower, however, is found to be relatively intact crossing the metal-insulator transition. [1] C. Petrovic et al., Phys. Rev. B 72 (2005) 045103. [2] A. Bentien et al., Europhys. Lett. 80 (2007)17008. [3] P. Sun et al., Phys. Rev. B 79 (2009) 153308; Appl. Phys. Express 2 (2009) 091102; Dalton Trans. 39 (2010) 1012.
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“111” type iron pnictide superconductors
Changqing JIN
Institute of Physics, Chinese Academy of Science
The discovery of iron pnictide superconductors opens a new era for unconventional superconductivity. We will present our recent work on “111” system AFe(As, P) (where A presents an alkaline metal element). We will discuss LiFeAs, LiFeP & NaFeAs superconductors: addressing superconductivity of LiFeAs including effects of pressure, the pressure enhanced superconductivity up to 31 K for NaFeAs, the superconducting properties of LiFeP.
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Superconducting gap structure of iron-based superconductors probed by heat transport measurement
Shiyan Li
Department of Physics, Surface Physics Laboratory (National Key Laboratory), Laboratory of Advanced Materials, Fudan University, Shanghai 200433, People’s Republic
of China The in-plane thermal conductivity κ of iron-based superconductors FeSex (Tc = 8.8 K), BaFe1.9Ni0.1As2 (Tc = 20.3 K), BaFe1.73Co0.27As2 (Tc = 9.2 K), and KFe2As2 (Tc = 3 K) were measured down to 50 mK. In zero field, the absence of a residual linear term κ0/T in FeSex, BaFe1.9Ni0.1As2, and BaFe1.73Co0.27As2 samples is strong evidence for nodeless superconducting gap. For FeSex, the field dependence of κ0/T is very similar to that in NbSe2, a typical multi-gap s-wave superconductor. From optimally doped BaFe1.9Ni0.1As2 to overdoped BaFe1.73Co0.27As2, the behavior of κ0/T(H) changes dramatically upon increasing doping. For the extremely hole-doped KFe2As2, we found a field-induced antiferromagnetic quantum critical point at Hc2 and nodal superconductivity, which is attributed to the intraband pairing via antiferromagentic spin fluctuations. Finally, we give a full scenario of the pairing and superconducting gap structure in FeAs-based superconductors.
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Functional Renormalization Group Study of Iron-based Superconductors
Fa Wang
MIT
We apply the fermion functional renormalization group(FRG) method to determine the pairing symmetry and pairing mechanism of several iron-based high-Tc superconductors. From realistic five-orbital models fit to the band structure calculations and local repulsive interactions we find electronic driven pairing instabilities. For all materials considered(LaFeAsO, LaFePO, FeSe, and FeTe), the leading pairing instability is unconventional s-wave(s+-). But the details vary significantly between different materials. In particular the pairing in LaFePO is found to have accidental nodes on electron Fermi surfaces. By analyzing the RG flow we propose that the pairing mechanism is the inter-Fermi-surface pair scattering generated by the antiferromagnetic correlation. References: Fa Wang, Hui Zhai, Dung-Hai Lee, arXiv:1002.3358, Fa Wang, Hui Zhai, Dung-Hai Lee, EPL 85, 37005 (2009) Fa Wang, Hui Zhai, Ying Ran, Ashvin Vishwanath, Dung-Hai Lee, PRL 102, 047005 (2009)
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Disorder effect on Iron-Based superconductors
Jiangping Hu
Purdue University I will discuss the effect of quenched disorder on magnetic and superconducting transitions in Iron-based superconductors. I will show that a novel anti-collinear AFM state can exist if Fe is replaced by nonmagnetic atoms, such as Zn. The structural transition can be modified strongly by the disorder as well if it is driven by the magnetic transition , as predicted theoretically before. In contrast to the magnetic transition, the sign changed s-wave superconducting state in t-J1-J2 model is rather robust against the disorder caused by nonmagnetic impurities. Our predictions provide a direct test for the relationship between the structural and magnetic transitions. Moreover, they suggest that strong electron-electron correlation is the fundamental reason why the sign changed s-wave is insensitive to nonmagnetic impurity, an experimental results challenging theories based on weak coupling approach.
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Emergence of Static Electronic Moments in the Heavily Overdoped Regime
Jeff Sonier
Simon Fraser University
Recent high transverse-field (TF) μSR measurements on La2-xSrxCuO4 in the pseudogap region show a inhomogeneous line broadening associated with residual antiferromagnetic correlations. On the high doping side of the pseudogap region a new kind of heterogeneous spin magnetism emerges that results in a monotonic increase of the TF-μSR line width and a Curie term in the dc susceptibility with doping. Such behaviour is suggestive of local staggered moments induced by intrinsic disorder and/or defects. However, at doping levels beyond the superconducting dome, the TF-μSR line width and Curie term decrease with increased doping and frozen moments are detected at x = 0.33. I will discuss these results in the context of microscopic disorder and the possible occurrence of ferromagnetism in the heavily overdoped regime. I will also briefly discuss related TF-μSR measurements that point to a different conclusion than NMR on the inhomogeneity in bulk YBa2Cu3Oy.
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ARPES Studies of Electronic Excitations in Cuprate High Temperature superconductors
Ming Shi and Joel Mesot
Paul Scherrer Institut
In this talk we will present angle-resolved photoemission (ARPES) results on La2-xSrxCuO4 and Bi2Sr2CaCu2O8+δ high temperature superconductors over extensive doping range starting from non-superconducting insulating samples to highly doped superconducting ones. We shall show how the underlying Fermi surface, the superconducting and pseudogaps evolute from a highly underdoped samples to overdoped ones. We observe that there are sharp quasiparticles all around the underlying Fermi surface in the superconducting state for all doping values. The superconducting gap anisotropy follows a simple d-wave form for all doping values. We will also show the dichotomy of the dispersion observed in the gapless and the gaped regions of the Brillouin zone in the pseudogap phase of the underdoped cuprate. The implication of the dichotomy of the dispersion in the pseudogap phase will be discussed.
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Nematicity and Symmetry Constraints in Correlated systems
Hae Young Kee
University of Toronto In correlated electron systems, electrons can organize themselves in states that are analogous to classical liquid crystal phases. The search for such phases in solid state systems, in particular for the quantum version of an anisotropic liquid crystal state, dubbed electronic nematic phase, has been of great interest. I will review theoretical studies on the nematic phase, and related phenomena observed in Cuprates and Ruthenates. For example, anisotropic metal bounded by two consecutive meta-magnetic transitions was reported in bilayer Ruthenates. I will show that the nematic phase accompanying the meta-magnetic transitions can be formed by an effective momentum-dependent interaction, and discuss its microscopic origin. Then I will present intimate relations between the nematicity and other orders discussed in high temperature cuprates based on a symmetry group. Consequences of such relations will be also discussed.
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Evolution of electronic structure in doped Mott insulators: Fermi arc and Fermi pocket
Jian-Xin Li
Department of Physics and National Laboratory of Solid State Microstructure, Nanjing
University, Nanjing 210093, China Motivated by the recent experiments on the coexistence of Fermi arcs and Fermi pockets in high-Tc cuprates [Nature 462,335(2009)] and the possible pesudogap behavior in layered organic superconductors \kappa-(BEDT-TTF)2X, we investigate the evolution of electronic structure with the Hubbard interaction U in the two-dimensional Hubbard model by means of the cluster perturbation theory. For the half-filled triangular lattice which is applied to layered organic superconductors, an anisotropic pseudogap is found when U>3.5t. With the increase of U, the pseudogap develops into an anisotropic Mott gap which leads to an unambiguous Fermi arc for U~4.5-5.5t. With the further increase of U, the Mott gap opens in the entire Fermi surface. For the square lattice, we find a coexistence of the Fermi arc and Fermi pocket at low dopings, which is consistent with the recent ARPES result on high-Tc cuprates. These features are ascribed to the short-range correlations. * In collaboration with Jing Kang, Shun-Li Yu and Tao Xiang
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Nernst effect in bismuth and graphite across the quantum limit
Kamran Behnia
ESPCI, Paris The quantum limit is attained when the magnetic field puts all electrons in their lowest Landau level . With available magnetic fields, it can be crossed in bismuth and graphite for particular orientations of the magnetic field. The fate of a three-dimensional electron gas pushed to this ultraquantum regime has been barely explored. According to recent studies on bismuth and graphite in the vicinity of the quantum limit, whenever a Landau level intersects the Fermi level, the Nernst response sharply peaks and the oscillating signal exceeds by far the monotonous background. Both these features are absent in two-dimensional systems including graphene. Beyond the quantum limit, Nernst effect in bismuth detects field scales unexpected in the one-particle picture. Our recent angular-dependent Nernst measurements find that the band picture, quite successful in explaining the complex electronic spectrum of bismuth up to 9 T, is inadequate as the quantum limit is crossed. Additional topological phase transitions in addition to those expected in the one-particle picture are detected. This is contrasted to the case of graphite where a thermodynamic phase transition occurs beyond the quantum limit.
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Correlation-hole induced paired quantum Hall states
Yue Yu
Institute of Theoretical Physics, CAS A theory is developed for the paired even-denominator fractional quantum Hall states in the lowest Landau level. We show that electrons bind to quantized vortices to form composite fermions, interacting through an exact instantaneous interaction that favors chiral p-wave pairing. There are two canonically dual pairing gap functions related by the bosonic Laughlin wavefunction (Jastrow factor) due to the correlation holes. We find that the ground state is the Moore-Read pfaffian in the long wavelength limit for weak Coulomb interactions, a new pfaffian with an oscillatory pairing function for intermediate interactions, and a Read-Rezayi composite Fermi liquid beyond a critical interaction strength. Our findings are consistent with recent experimental observations of the 1/2 and 1/4 fractional quantum Hall effects in asymmetric wide quantum wells.
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Symmetry protected topological orders
SuPeng Kou
Beijing Normal University
Topological order is a new type of quantum state beyond Laudau paradigm. Recently, many different systems with topologically ordered ground states on spin model (we call them symmetry protected topological orders) were found. How to classify symmetry protected topological orders and how to describe the quantum phase transitions between them (topological quantum phase transitions)? What's topological field theory of them? We try to answer these questions.
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Skymion Spin Texture in Magnetic Metals
Jung Hoon Han
Department of Physics, SungKyunKwan University Observation of a Skyrmionic spin texture in condensed matter systems began with quantum Hall ferromagnets. When the filling factor deviates slightly away from one (a single Landau level occupied with fully polarized spins), the excess amount of reversed spins spontaneously organize themselves into an intricate texture known as the Skyrmion. More recently, careful small-angle neutron diffraction studies on magnetic metals lacking inversion symmetry, such as MnSi and Fe1-xCoxSi, revealed a sixfold symmetric Bragg spots. The spots appear in the so-called A-phase in the phase diagram of these materials, where spin texture is now believed to be that of a triangular crystalline array of Skyrmions. Motivated by such recent advances in experiments, we present a theory of Skyrme crystal formation in chiral magnets, which are ferromagnets with an additional Dzyaloshinskii-Moriya interaction and a spiral magnetic ground state. A new CP1 formulation of the Ginzburg-Landau energy for chiral magnets is shown to give a richer description of the possible complex magnetic phases than a conventional description based on O(3) non-linear sigma model order parameter. A phase diagram obtained from Monte Carlo simulation is presented that agrees nicely with recent experiments on magnetic thin films.
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Posters
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Mott Transition in Multi-Orbital Hubbard Models for Iron Pnictides
Rong Yu,1 and Qimiao Si1
1Department of Physics & Astronomy, Rice University, Houston, Texas 77005, USA
A central question in the iron pnictides is how strong the electron correlations are. Towards this end, it would be important to establish how Mott transition may in principle happen in microscopic models appropriate for the iron pnictides, i.e., models with even number of electrons partially occupying multiple orbitals. Here we study multi-orbital Hubbard models using slave-rotor and slave-spin mean-field theories. We show that a metal-insulator transition generally exists, and that the insulating phase is a Mott insulator. We determine the critical values of the Coulomb interactions for several choices of band parametrization, and also discuss the role of Hund's rule coupling in the Mott transition. Funding for this work provided by the NSF Grant No. DMR-0706625, the Robert A. Welch Foundation Grant No. C-1411, and the W. M. Keck Foundation.
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XLD and XMCD measurements of Sr(Fe1-xCox)2As2
Ross Springell1, Jack Gillett2, Suchitra Sebastian2, Julio Cezar Crijinski3, Fabrice Wilhelm3
1Department of Physics and Astronomy, UCL, London WC1E 6BT, UK 2Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK
3European Synchrotron Radiation Facility, BP220, F-38043, Grenoble Cedex, France
The recently discovered iron pnictide superconductors have the highest critical temperatures of any superconductors yet discovered aside from the cuprates, with highest temperatures of near 55K [1]. The similarities between the phase diagrams in the two materials have prompted widespread debate about the role of competing magnetic order in high-temperature superconductivity. The AFe2As2 (with A = Ba, Sr or Ca) has been of particular interest since superconductivity can emerge from electron-doping, hole-doping, or pressure [2-4]; and because of the relative ease of growing large crystalline samples. In this submission we will discuss recent measurements taken at the ESRF synchrotron in Grenoble.
We have measured the x-ray absorption spectra at the Fe(Co) L2,3 edges (probing transitions from the 3p3/2 and 3p5/2 states to the empty 3d levels) and the Fe(Co,As) K edges (probing s to the p transitions), across a wide range of dopings in single-crystal samples cleaved in high vacuum. Our study is twofold; one is an investigation of the x-ray linear dichroism (XLD) and the other is to investigate the magnetic properties, using XMCD.
XLD measurements in the cuprates [5] have supported the charge-transfer nature of doping, but behavior in the iron pnictide systems is more ambiguous. By comparing our experimental results to calculations, using the FDMNES code of Y.Joly, we can detect discrepancies from the ideal structure in terms of inter-site mixing of each individual
element. Figure 1 shows the XLD from a SrFe2As2 parent compound for a series of sample angles. We have measured XMCD spectra at the Fe L edges at
low temperature in a magnetic field of 6T, applied perpendicularly to the Fe-As layers and along the incident x-ray beam (see Fig. 2). By probing the difference in population of spin up and spin down bands in this way, we can apply the SUM rules to calculate spin and orbital moments, respectively. Although these will not be accurate representations of the ordered moment values, the ratio of the spin to orbital moment is a reliable quantity. By comparison with neutron data for the total ordered magnetic moment it is then possible to determine the separate contribution from the spin and orbital moments. *This work is supported by the ESRF, Grenoble; Trinity College, Cambridge; and the EPSRC, UK. [1] Z. A. Ren et al., Chin. Phys. Lett. 25, 2215 (2008) [2] A. Leithe-Jasper et al., Phys. Rev. Lett. 101, 207004 (2008) [3] K. Sasmal et al., Phys. Rev. Lett. 101, 107007 (2008) [4] P. L. Alireza et al., J. Phys.: Condens. Matter 21, 012208 (2009) [5] C. T. Chen et al., Phys. Rev. Lett. 68, 2543 (1992)
Figure1 – XLD at Fe L2,3 edges for SrFe2As2 sample at 6K, as a function of sample angle. (0º is normal incidence).
Figure 2 – The x-ray near edge absorption spectra (XANES) and XMCD for the field applied parallel (red) and antiparallel (blue) to the beam direction. Measurements were made in a field of 6T at 6K.
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Correlation between the combinative energy and Tc in SmFeAsO1-xFx and SmFe1-xMxAsO(M=Co,Ni)
L. Zhang1, *, Y. K. Li2, Q. Tao2, S. L. Shi1, X.Y. Wang1, and Y. Zhou1
Department of Physics, China Jiliang University, Hangzhou 310018, People’s Republic of
China, Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of
China
Based on a block model, we developed a program to calculate the cohesive energy of the whole cell and the combinative energy between the Fe-As block and Sm-O block in F-, Co-, and Ni-doped SmOFeAs systems. A relationship between the combinative energy and Tc is established, which is similar to our previous reports in cuprate HTSCs to some extent. While the nearly unchanged combinative energy in SmFe1-xMxAsO(M=Co,Ni) no matter whether the electron is supposed at the Fe, As site or at the centre of the Fe-As layer, suggesting an itinerant character in SmOFeMAs(M=Co,Ni), which is severely different from the local carrier picture in cuprate HTSCs. Our results may shed light on the similar and different mechanism in understanding superconductivity in the two important HTSCs. Keywords:combinative energy, structural block, superconductivity, SmOFeAs
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Nearly isotropic superconductivity in iron pnictides
H. Q. Yuan1, L. Jiao1, J. L. Zhang1, J. Singleton2, F. F. Balakirev2, H. D. Wang1, M. H. Fang1, G. F. Chen3, J. L. Luo3, N. L. Wang3, L. Fang3, H. H. Wen3
1Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, China 2NHMFL, Los Alamos National Laboratory, MS E536, Los Alamos, NM 87545, USA
3 Institute of Physics, Chinese Academy of Science, Beijing 10080, China.
Discovery of superconductivity in the layered iron pnictides has attracted considerable interests. Comparison of iron pnictides with cuprate oxides might provide important insights into the nature of high temperature superconductivity. By means of measuring the transport properties in a pulsed magnetic field, we have determined the upper critical field Hc2 for variant series of the iron pnictides, including LaFeAs(O,F) [1], (Ba,K)Fe2As2 [2], Ba(Fe,Co)2As2 [3] and Fe(Te,Se) [4]. All these superconductors show a rather common feature: a very high upper critical field and weak anisotropy of superconductivity. In particular, the 122 series and the 11 series demonstrate nearly isotropic upper critical field at low temperature. The weak anisotropy of superconductivity in iron pnictides might come from a three-dimension-like Fermi surface, being rather different from other layered superconductors, e.g., the cuprates and the organic superconductors. Furthermore, the observations of three-dimensional superconductivity in layered compounds indicate that reduced dimensionality is not the prerequisite conditions for high temperature superconductivity. [1] G. F. Chen et al., Phys. Rev. Lett. 101, 057007 (2008). [2] H. Q. Yuan et. al., Nature 457, 565 (2009). [3] L. Jiao et al., unpublished. [4] M. H. Fang et al., Phys. Rev. B81, 020509(R) (2010)
Department of Physics, Zhejiang University, Hangzhou 310027, China
We systematically studied the Nernst effect of BaFe2-xNixAs2 single crystals with different Ni doping lever. The Nernst effect of “122” type parent compound BaFe2As2 exhibits a huge enhancement when it enters SDW state. The normal state Nernst effect decreases gradually with increasing Ni doping, and it finally behaves like conventional metal in the overdoped region. The Nernst effect of the optimal doped sample (x=0.1) indicated that there exists a quite broad vortex liquid region. This vortex liquid region is obviously smaller than that of “1111” type superconductors such as LaFeAsO1-xFx, implying the three-dimension nature of “122” type superconductors. The anomalous Nernst signal associated with preformed pairs above Tc is not observed, in contrast to high-Tc cuprates. However, the normal state Nernst signal is very large. We proposed that the large Nernst signal may result from the unusual inter-band scattering between electron and hole bands.
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Effect of nonmagnetic impurity in LaO1-xFxFe1-yZnyAs and LaFe1-x-yCoxZnyAsO systems
Yuke Li, Jun Tong, Qian Tao, Guanghan Cao, Zhu’an Xu
Department of physics, Zhejiang University, Hangzhou 310027, China
We study the effect of nonmagnetic Zn impurity in both LaO1-xFxFe1-yZnyAs and LaFe1-x-yCoxZnyAsO superconductors by measuring transport and magnetic properties. In the F-doped LaO1-xFxFeAs superconductors, the effect of Zn impurity is strongly dependent on the charge carrier doping level (i.e. F content). With addition of Zn, the superconducting transition temperature (Tc) increases in the under-doped regime, remains unchanged in the optimally doped regime, and is severely suppressed in the over-doped regime, suggesting a switch of the symmetry of the superconducting order parameters from a s++ wave to s+/- or d-wave states as the charge carrier doping increases. In the Co-doped LaFe1-x-yCoxZnyAsO system, the effect of Zn impurity shows a contrast behavior. Even in the under-doped LaFe0.95-yCo0.05ZnyAsO system, Tc is suppressed gradually with increasing Zn content. In the optimal-doping regime (x=0.075), Tc decreases sharply with increasing y. Furthermore, the resistivity measurements show that the addition of Zn impurity causes semiconductor-like resistivity in normal state. If the charge carrier density is not affected by the Zn impurity, the results strongly suggest that the symmetry of superconducting paring could not only change with charge carrier doping level, but also be different in different chemical doping systems.
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Magnetic properties of EuCuAs
Jun Tong, Yuke Li, Yongkang Luo, Qian Tao, Guanghan Cao, Zhu’an Xu Department of Physics, Zhejiang University, Hangzhou 310027, China
The Eu-based compound EuCuAs with Ni2In-type structure has been synthesized by solid state reaction method and its magnetic and transport properties has been studied by the measurements of magnetization, resistivity and magnetoresistivity. The X-ray powder diffraction pattern of EuCuAs can be indexed based on Ni2In-type hexagonal structure. Resistivity exhibits a good metallic behavior and there is a sharp peak around 16 K. The DC magnetic susceptibility also shows a peak at 16 K, implying that the compound undergoes an antiferromagnetic (AFM) order below 16 K. The magnetic moment is 7.7 μB/fr obtained by fitting the magnetic susceptibility to the Curie-Weiss law for T > 16 K, closing to the value of the effective moment of free Eu2+ ions (7.94 μB), confirming that AFM order below 16 K comes from Eu ions. A metamagnetic transition was observed by measuring M-H curves at 2 K. Finally, a magnetic phase diagram is established according to the magnetic measurements.
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Electronic duality in Ba(Fe1-xCox)2As2
L. Jiao1, J. Singleton2, F. F. Balakirev2, C. Setty3, J. P Hu3, L.J.Li1, G. H. Cao1, Z. A. Xu1, H. H. Wen4, H. Q. Yuan1
1Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, China
2NHMFL, Los Alamos National Laboratory, MS E536, Los Alamos, NM 87545, USA
3Department of Physics, Purdue University, 525 Northwestern Ave.,West Lafayette, IN47907, US
4 Institute of Physics, Chinese Academy of Science, Beijing 10080, China. The origin of the magnetic transition observed in the iron pnictides remains rather contradictory. Distinct approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of the 3d electrons in the parent compounds. In this presentation, we report the nontrivial transport properties of the single crystalline Ba(Fe1-xCox)2As2 (x=0, 0.05) in a pulsed magnetic field up to 60T. It is found that the magnetoresistance is negligible and the Hall resistivity follows a regular linear field-dependence at temperatures above the magnetic transition. In contrast, a large magnetoresistance is observed below the magnetic transition and its Hall resistivity deviates from the linear field dependence, showing parabola-like behavior in the parent compound (x=0). The low temperature magnetoresistance and Hall resistivity are analyzed in terms of a two-band model, which gives a strong field dependence of the charge carrier and its mobility. Remarkably, we found that the magnetic transitions in both x=0 and x=0.05 are extremely robust again magnetic field up to 60T, giving strong evidence that the magnetic ordering is formed by local moments. We argue that the 3d-electrons in Ba(Fe1-xCox)2As2 bear a dual nature; partial of them become localized at TM and contribute to the magnetic order, and the others are more itinerant, giving rise to complex transport properties and also superconductivity.
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From antiferromagnetic order to spin-glass state in Fe1.05-xTe system
Yang Lin (杨琳), Li Zujuan (李祖娟), Yuan Huiqiu (袁辉球), Fang Minghu (方明虎)
Department of Physics, Zhejiang University, Hangzhou 310027, China
The Cu doping effect on physical properties of Fe1.05-xCuxTe system was studied in detail by electronic transport and magnetization measurements both in polycrystalline samples and single crystal samples. For x = 0.0, a metal-insulator transition accompanied with an anti-ferromagnetic transition was observed around 70K as reported previously. The study of the transport properties indicated that the temperature of the metal-insulator transition decreases monotonously with the increasing of Cu content first. Then for x > 0.05, it exhibits a semiconducotr behavior in the whole measuring temperature range. On the other hand, the anti-ferromagnetic ordering was detected only at the metallic region and was suppressed by the Cu substitution. However, before the TN drops to zero, a spin-glass state is induced at low temperatures. Besides, such spin-glass state preserves in a large Cu content range. In the end, the evolution of the transport and the magnetic properties are summarized and a phase diagram of Fe1.05-xCuxTe is proposed.
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Enhance of superconductivity by annealing in Fe1+y(Te1-xSx) system
Dong Chiheng1, Wang Hangdong1, Yang Jinhu1, Qian Bin2, Chen Jian1, Li Zujuan1,
Yuan Huiqiu1, Fang Minghu1,*
1Department of Physics, Zhejiang University, Hangzhou 310027, China
2Department of Physics, Changshu Institute of Technology, Changshu 215500, China We have synthesized polycrystalline samples of Fe1.11(Te1-xSx) and single crystals of Fe1+y(Te0.88S0.12), and characterized their properties. Our results show that the solid solution of S in the Fe1.11Te tetragonal lattice is limited, ~ 10%. We observed superconductivity at ~ 8 K in both polycrystalline samples and single crystals. Magnetization measurements reveal that the volume fraction is small for this superconducting phase in both polycrystalline samples as-synthesized and single crystals as-grown. It is found that annealing in air enhances the superconducting fraction, the maximum fraction is almost 100% in the single crystals annealed in the air at 3000C. We discuss the effect of annealing on superconductivity and transport properties at the normal state in Fe1+y(Te1-xSx) system in term of decrease of excess Fe.
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Superconductivity and local-moment magnetism in Eu(Fe0.89Co0.11)2As2
Shuai Jiang
Department of Physics, Zhejiang Univeristy
We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped Eu(Fe0.89Co0.11)2As2 single crystal. We observed a resistivity drop at Tc～21 K, which shifts toward lower temperatures under external fields, suggesting a superconducting transition. The upper critical fields near Tc show large anisotropy, in contrast with those of other '122' FeAs-based superconductors. Low-field magnetic susceptibility data also show evidence of superconductivity below 21 K. Instead of expected zero-resistance below Tc, however, a resistivity reentrance appears at 17 K under zero field, coincident with the magnetic ordering of Eu2+ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the Eu2+ spins is proposed. External magnetic fields easily changes the helimagnetism into a ferromagnetism with fully polarized Eu2+ spins, accompanying by disappearance of the resistivity reentrance. Therefore, superconductivity coexists with ferromagnetic state of Eu2+ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both H||ab and H||c.
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Scaling and relaxational dynamics near Kondo-destroying quantum critical points
Jedediah Pixley
(Rice University)
Stefan Kirchner (Max Planck Institute for the Physics of Complex Systems)
Matthew Glossop (Rice University)
Qimiao Si (Rice University)
Experimental findings in antiferromagnetic heavy fermion metals have pointed to a new class of quantum critical points. Most theoretical proposals, including local quantum criticality, have focused on the novel excitations associated with a critical Kondo destruction. In order to elucidate the finite-temperature dynamical scaling properties, we study Kondo-destroying quantum critical points in two quantum impurity models. These are the pseudogap Anderson model [1] and the Bose-Fermi Kondo model [2]. In particular, we determine the scaling behavior of the local spin susceptibility and the single particle Green’s function in the vicinity of the quantum critical point, using a combination of continuous time quantum Monte Carlo and large-N techniques. We establish that both correlators have a linear-in-temperature relaxation rate. Our results shed further light on the dynamical scaling behavior of the quantum-critical heavy fermion metals. In particular, the scaling behavior of the single particle Green’s function is consistent with recent experimental results in the quantum critical regime of the heavy fermion compound YbRh2Si2 [3]. [1] M. T. Glossop, S. Kirchner, J. H. Pixley, and Q.Si, http://arxiv.org/abs/0912.4521 (2009). [2] J. H. Pixley, S. Kirchner, and Q. Si, to be published. [3] S. Friedemann et al., to be published (2009). Funding for this work is provided by NSF and the Welch Foundation.
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Entropy Accumulation, Divergent Gruneisen Ratio, and Crossover Energy Scales near Quantum Critical Points
Jianda Wu (Rice University)
Lijun Zhu (LANL) Qimiao Si
(Rice University) Quantum critical points (QCP) arise at the points of second order phase transition at zero temperature, which are of extensive current interest, in part because they strongly influence the physical properties at finite temperatures.The Gruneisen ratio has been shown to diverge at QCPs [1], and this divergence has been experimentally observed in heavy fermion metals [2]. An important consequence of this divergence is that entropy will be maximized in the quantum critical regime, and this has recently been directly observed in an elegant experiment on the field-induced QCP in Sr3Ru2O7 [3]. Here, we further address the relationship between the accumulation of entropy, the divergence as a function of both temperature (T) and control parameter (r) in the Gruneisen ratio, and the crossover energy scales in the T-r phase diagram. We consider these in some detail in the transverse-field Ising chain for which experimental realizations are just starting to emerge[4], and the transitions into itinerant magnets, which may describe Sr3Ru2O7. We report the result of microscopic calculations of the entropy as a function of the control parameter r in both models. We show that, for the transverse-field Ising chain, there is an unusual contrast between the T- and r- dependence of the Gruneisen ratio. [1] L. Zhu et al, PRL 91, 066404 (2003); [2] P.Gegenwart et al, Nature Phys. 4, 186 (2008); [3] A.W. Rost et al, Science 325, 1360 (2009) [4] R. Coldea et al, Science, 327, 177 (2010).
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Geometric Resonance in Triangular or Honeycomb Lattices Patterned on Very High-Mobility Quantum Wells1
YANHUA DAI, R. R. DU
Rice University L. N. PFEIFFER, K. W. WEST
Princeton University In our magnetotransport studies we introduce spatial modulation in the 2D electron system by patterning the Hall bar samples with antidot lattices. Our samples are high-mobility GaAs/AlGaAs quantum wells with electron densities (3- 6) x 1011/cm2 and mobilities > 9 x 106 cm2/Vs. The antidot lattices (triangular or honeycomb) which have a lattice constant between 3 and 10 μm and a dot diameter between 0. 4 and 2 μm were patterned with e-beam lithography. Our low temperature (300 mK) magnetotransport measurements reveal exceptionally sharp geometric resonances (GR) in Rxx up to 8th order in these samples, with the even-peaks commonly stronger than the odd-peaks. The data cannot be explained satisfactorily by the model of localization of pinned orbits. In particular, our data indicate that in the very high-mobility samples, the primary GR peaks are related to the Zener tunneling between electron Landau orbits as their diameter approaches the lattice constant. Ref. Z. Q. Yuan et al, Phys. Rev. B 74, 075313(2006). 1. Work at Rice was supported by NSF-DMR0706634.
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Crystal field anisotropy and geometric frustration effects in
triangular rare-earth based intermetallic compounds
Eteri Svanidze, Victor Leyva+, Mark Hendriks*, Emilia Morosan
Department of Physics and Astronomy, Rice University Houston TX 77005, USA +Department of Mechanical Engineering and Materials Science, Rice University
*REU summer student, Department of Chemistry, Harvey Mudd College Hexagonal compounds with the Mn5Si3 – type structure present an opportunity for studying the role of reduced dimensionality and geometric frustration on their magnetic and electronic properties. We have recently observed unique magnetic ordering in a particular series of such compounds, R5Pb3 (where R = rare earth). We are presenting temperature- and field-dependent magnetization data on polycrystalline R5Pb3 compounds (R = Y, Gd - Er) illustrating these intriguing properties: the R = Gd – Er members of this series have very high magnetic ordering temperatures Tord (e.g. about 300 K for Gd5Pb3). Moreover these compounds feature Tord/θW larger than unity (θW is the Weiss temperature), which is unexpected and, to our knowledge, not observed in any other magnetically ordered systems. In addition, the field-dependent magnetization shows several metamagnetic transitions in Ho5Pb3 and Er5Pb3, despite the apparent ferromagnetic ordering in one or both of these compounds.
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La2Co2Se2O3: A Quasi-two-dimensional Mott insulator with Unusual Co Spin State and Possible Orbital
ZhiFeng Ma
Zhejiang University
A new oxyselenide La2Co2Se2O3 containing Co2O square-planar layers has been successfully synthesized using solid-state reaction in vacuum. The compound crystallizes in space group I4/mmm with lattice parameters a = 4.0697(8) Å and c = 18.419(4) Å. Magnetic susceptibility measurement indicates an antiferromagnetic transition at ~220 K. The magnetic entropy associated with the transition is close to Rln2, suggesting an unusual low-spin state for the Co2+ ions. The as-prepared sample shows insulating behavior with room-temperature resistivity of ~107 W×cm, which decreases by four orders of magnitude under a pressure of 7 GPa. Band structure calculations using LSDA+U approach reproduce the insulating ground state with low spin for Co, and suggest strong orbital polarization for the valence electrons near the Fermi level. It is also revealed that the spin and orbital degrees of freedom in the antiferromagnetic checkerboard spin lattice are mutually coupled.
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TOWARDS QUANTUM SPIN HALL EFFECT IN InAs/GaSb QUANTUM WELLS
1Ivan Knez, 1R.R. Du, 2G. Sullivan
1Department of Physics and Astronomy, Rice University
2Teledyne Scientific Company LLC Recently, it has been proposed that inverted InAs/GaSb composite quantum wells (CQWs) should exhibit the Quantum Spin Hall Effect (QSHE), characterized by the energy gap in the bulk and gapless edge modes which are protected from backscattering by time reversal symmetry. We have successfully fabricated a double-gated device on high-quality MBE-grown InAs/GaAs CQWs in the inverted regime, in which we were able to vary the band structure via an electrical field, and tune the Fermi level into mini-gap regime. We observed clear evidence for an energy gap in the inverted regime, with values of the gap consistent with those theoretically predicted; however, the mini-gap exhibits residual conductivity of non-trivial origin, which complicates transport investigation of proposed edge channels. We will discuss our progress towards observing QSHE in this unique material system.
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Enhanced room temperature magnetoresistance and cluster spin
glass behavior in Mo doping La0.67Sr0.33MnO3 L. Chen a, J.H. He a, Y. Mei a, Y.Z. Cao a, B.Q. Liu a, Z.W. Zhu b, Z.A. Xu b
a College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, P. R. China
b Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China
The structural, magnetic, and transport properties of Mo doping La0.67Sr0.33Mn1–xMoxO3 (x=0–0.04) manganite system have been investigated by x-ray diffraction, SEM, magnetization, and magnetoresistance measurements。The Mo doping in Mn site is found to lower the Curie temperature Tc slightly. However, it induces cluster spin glass state in ferromagnetic background of La0.67Sr0.33MnO3，which results to the reduction of magnetization and the increase of resistivity for Mo doping samples. The room temperature magnetoresistance for Mo doping La0.67Sr0.33Mn1–xMoxO3 system is significantly enhanced and is ~ 50% (x=0.04) higher than that of La0.67Sr0.33MnO3 (x=0.00). The induced cluster spin glass state in Mo-doping samples is expected to play an important role in the enhancement of MR at room temperature. For undoped La0.67Sr0.33MnO3, the antiferromagnetic (AFM) interactions between Mn3+–O2––Mn3+ or Mn4+–O2––
Mn4+ are overwhelmed by the FM double exchange interaction between Mn3+–O2––
Mn4+，and the long range FM order is established accordingly. As for Mo doping La0.67Sr0.33Mn1-xMoxO3, the small content of Mo doping induces a large variation in the Mn valence and breaks the Mn–O–Mn network. The previous reports indicate that the Mo-doping induce ferromagnetism in lightly Mo doping CaMn0.96Mo0.04O3 and LaMn0.96Mo0.04O3, where the DE interactions between Mn3+–O2––Mn4+ and Mn2+–O2––Mn3+ exists respectively. Therefore, it can be deduced that there exist the FM interactions between Mn3+–O2––Mn4+ or Mn2+–O2––Mn3+ in Mo doping La0.67Sr0.33Mn1-xMoxO3, meanwhile the AFM interaction between Mn3+–O2––Mn3+ or Mn4+–O2––Mn4+ is enhanced due to local existence of the superfluous Mn3+ and Mn4+ ions. The competition between the FM and AFM exchange interaction is reinforced in the background of the dominant FM exchange interaction and the the cluster spin glass state with short range FM order is induced and is evolved with Mo doping. Under external magnetic field, the ferromagnetic clusters with short range order are forced to align uniformly and the spatial magnetic disorder is reduced. As a result, the resistivity is depressed and the room temperature magnetoresistance is enhanced. Therefore, the enhancement of magnetoresistance at room temperature is found to be closely related to the formation of cluster spin glass state. This result suggests that the enhancement of room temperature magnetoresistance may be achieved by introducing appropriate short range ordered ferromagnetic clusters.
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High temperature resistive relaxation in lightly Ti doped
CuCrO2
Zhe Qu
CAS
Triangular lattice antiferromagnets (TLAs) have attracted considerable interests because geometrical spin frustration inherent to these systems results in exceptionally rich physical properties. CuCrO2, which has a delafossite structure, is a typical example. It orders antiferromagnetically below 26 K and shows ferroelectricity driven by this magnetic order. Here we report that lightly Ti dopants strongly tunes the transport properties, while barely affected the magnetic behaviors. With increasing Ti content, the resistivity increases rapidly. More interestingly, it shows significant resistive relaxation at high temperature e.g. 400 K, whose magnitude increases with increasing Ti content. The possible origin is discussed.
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Creep of vortex glasses in type-II superconductors
Mengbo Luo
Department of Physics, Zhejiang University, Hangzhou, 310027, China Dynamics of three-dimensional flux lines with point-like defects are studied by large-scale accurate molecular dynamical simulation based on Langevin dynamics. A continuous depinning transition is observed at zero temperature, where scaling function is described as
)]/1([),( 0/1/1 FFTSTFTv c−= βδ−δ with Fc0 the zero temperature depinning force. We
obtain two universal classes with different exponents β and δ for vortex glass (VG) and Bragg glass (BrG), respectively. Especially, the product βδ ≈ 3/2 in the BrG indicates a non-Arrhenius creep motion, in contrast to βδ = 1 in the VG. The creep motions of flux lines in both VG and BrG are also studied. The mean velocity of flux lines can be well described by v ∝ exp[-U(Fc0/F)μ/T] at low driving forces F < Fc0/2. The power-law dependence of the creep activation barrier on the driving force is in agreement with collective creep theory. Two universal classes of creep motion are found with the exponent μ = 0.5 ± 0.02 for the BrG and μ = 0.28 ± 0.02 for the VG. The former is in good agreement with the prediction by the scaling theory and the functional-renormalization-group theory on creep, while the latter is a new estimate whose estimate was theoretically very difficult since the usually adopted elastic restoration force is not available. The value of μ for BrG is larger than VG since the motion in the BrG is in much larger spatial scale due to the relatively strong inter-vortex force which establishes the crystalline order. The different dynamics of flux lines in the VG and in the BrG are investigated accordingly.
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Emergent order in the spin-frustrated system DyxTb2−xTi2O7 studied by ac susceptibility measurements
Hui Xing,1,2 Mi He,1 Chunmu Feng,3 Hanjie Guo,1 Hao Zeng,2 and Zhu-An Xu1,*
1Department of Physics, Zhejiang University, Hangzhou 310027, China 2Department of Physics, University at Buffalo–the State University of New York, Buffalo,
New York 14260, USA 3Test and Analysis Center, Zhejiang University, Hangzhou 310027, China
We report the ac susceptibility study of DyxTb2−xTi2O7 with x in [0, 2]. In addition to the single-ion effect at Ts (single-ion effect peak temperature) corresponding to the Dy3+ spins as that in spin ice Dy2Ti2O7 and a possible spin freezing peak at Tf (Tf < 3 K), a new peak associated with Tb3+ is observed in χac (T) at nonzero magnetic field with a characteristic temperature T* (Tf < T < Ts). T* increases linearly with x in a wide composition range (0 < x < 1.5 at 5 kOe). Both application of a magnetic field and increasing doping with Dy3+ enhance T*. The T* peak is found to be thermally driven with an unusually large energy barrier as indicated from its frequency dependence. These effects are closely related to the crystal-field levels and the underlying mechanism remains to be understood.
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Shared electron-phonon coupling: Coexistence of the charge density wave and superconductivity in the layered transition
metal dichalchogenide 2H-NbSe2
A. Mialitsin, A. Nevidomskyy, G. Blumberg
Department of Physics and Astronomy, Rutgers the State University of New Jersey
We examine the temperature dependence of the soft charge density wave (CDW) amplitude modes in 2H-NbSe2 by means of polarized Raman scattering through out both the superconducting (SC) and the CDW phases and their interaction with the SC phase modes. The larger of the two SC gaps is directly measured in c-axis geometry (for the first time in Raman). The SC phase signature modes are resolved in great detail (ab-plane geometry) together with the fundamental gap. Our fundamental gap value corrects the recent ARPES estimates. We compare the energy scales of the SC-CDW excitations to those seen by scanning-tunneling microscopy and discuss the electronic vs. Peirels like scenarios of their origin.
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List of Participants
Invited Speakers, Organizers and Committee Members
JinXi Hotel → Zhejiang University 07:40 May-19 Zhejiang University → Both Hotels 20:00 JinXi Hotel → Zhejiang University 08:00 May-20
WeiZhuang → Both Hotels 20:10 JinXi Hotel → Zhejiang University 08:00 May-21
West Lake → Both Hotels 21:10 JinXi Hotel → Zhejiang University 08:00 Lily Hotel → Zhejiang University 08:10
Shuttle Bus Info
May-22 Zhejiang University → Both Hotels TBA
Trips (flight, hotel, cars etc.) information (English): http://english.ctrip.com/ Bus information (Chinese): http://hot.dahangzhou.com/top/hzgj/index.htm Train information (Chinese): http://hot.dahangzhou.com/top/hzhc/index.htm Local Tours An excursion to West Lake will be organized for all registered Conference Participants free-of-charge on May 20 afternoon. Also, a show, “Impression.West Lake”, will be arranged for all registered Conference Participants free-of-charge on May 21 evening. For more Hangzhou travel information, see the website (English): http://www.chinatravel.net/china-destinations/Hangzhou/cityintroduction-5.html Tips 1) Since there are several campuses of Zhejiang University in Hangzhou, you have to tell the taxi driver it is Yuquan Campus (玉泉校区) that you will go. 2) During 4:00pm to 6:00pm, it’s very hard to find a taxi, because of drivers’ handover. 3) Currency exchange: The currency unit in China is Renminbi (RMB/CNY). It is pegged to the US dollars at a rate of CNY6.83 to US$1, with little fluctuation. Most banks provide currency exchange services. For less than 100 USD, you can exchange at JinXi Hotel. 4) Emergency Numbers: Emergency Line (24 hours): 110 Ambulance Service (24 hours): 120 Maps You can find a Hangzhou map (English version) in your document bag.
