What is the origin of anomalous dielectric
response in the spin liquid organic system
kappa-(BEDT-TTF)2Cu2(CN)3:
an in-depth study of
anisotropic charge dynamics
Silvia TomićInstitut za fiziku, Zagreb, Croatia
Research
in collaboration with
J.A.Schlueter, Argonne NLK.Kanoda, K.Miyagawa, University Tokyo
IPhys Zagreb
Marko PinterićMatija ČuloOgnjen MilatBojana Hamzić
University Stuttgart
Tomislav IvekMartin Dressel
University Zagreb
Mario BasletićEmil TafraAmir Hamzić
IRB Zagreb
Nadja Došlić
• Ferroelectricity and magnetism: κ-(BEDT-TTF)2Cu[N(CN)2]Cl ?
Ferroelectricity and charge ordering in 1D (TMTTF)2X
Dielectric relaxation
in 1D and 2D molecular solids
Monceau et al., Phys. Rev. Lett. 86, 4080 (2001)
Nad et al., JPCM12, 435 (2000)
Nad et al., EPL73, 567 (2006)
Nad et al., PRB62, 1753 (2000)
Ferroelectricity and charge ordering in 2D α-(BEDT-TTF)2I3Ivek et al., PRL104, 206406 (2010)
Ivek et al., PRB83, 165128 (2011)
Ivek et al., PRB86, 245125 (2012)
• Ferroelectricity and QSL: κ-(BEDT-TTF)2Cu2(CN)3 ?
Lunkenheimer et al., Nat. Mater. 11, 775 (2012)
Tomic et al., JPCM 25, 436004 (2013)
Abdel-Jawad et al., PRB 82, 125119 (2010)
Pinteric et al., submitted to PRB (2014),
arXiv: 1407.6252v1
2D κκκκ-(BEDT-TTF)2Cu2(CN)3
Shimizu et al., PRL (2003) ; PRB (2006)
Keszmarki et al., PRB (2006)
Kandpal et al., PRL (2009)
Elsaesser et al., PRB(2012)
• ET molecules form face-to-face dimers• One hole per ET dimer site• ET dimers arranged in triangles• Strong frustration on triangular lattice• Dimer Mott insulator close to the MIT• No magnetic order down to 20 mK despite J/kB ≈ 250 K→ Quantum spin liquid
Anomalous charge and spin response, 6 K < T < 50 K
• Anomalies of uniaxial expansivities → phase transition at 6 K in which charge degrees of freedom may be involved
Manna et al., PRL(2010)
• 13C-NMR spectral line broadening; a broad peak of (T1T)-1
Kawamoto et al., PRB(2004)
Shimizu et al, PRLB(2006)
20 60 T(K)
Anomalous charge response, 6 K < T < 50 K
• Relaxor type of dielectric response for E ll a*-axis• Phase transition at 6K: FE Curie and relaxor-like freezing of dipoles on ET dimers?• Dipole moments: origin ?• Internal heterogeneity: origin ?
Poirier et al., PRB (2012)
• Anomaly in microwave dielectric constant: 6-10 K
• Broad band at 30 cm-1
Itoh et al., PRL(2013)
Abdel-Jawad et al., PRB (2010)
20 40 T(K)
Theory:• Quantum electric dipoles on dimers• Dipolar-spin coupling• Paired electron crystal – frustration induced CD
Experiments:
Symmetry breaking: Noo Structural changes: noo Charge disproportionation: no
Hotta PRB(2010), Crystals (2012),
Naka et Ishihara,JPSJpn (2010, 2013)
Li et al., JPCM (2010);
Dayal et al., PRB (2011)
Are there electric dipoles in κκκκ-(ET)2Cu2(CN)3 ?
Sedelmeier et al., PRB 86, 245103 (2012)
BEDT-TTF dimers
One hole/dimer CD
Sedelmeier et al., PRB 86, 245103 (2012)
• ν27 does not split upon cooling• Charge imbalance of more than 0.5% is excluded
0
50
100
150
200
0
50
100
150
200
250
1380 1400 1420 1440 1460 1480 15000
50
100
150
200
Conductivity (Ω
-1cm
-1)
12 K
30 K
50 K
100 K
150 K
200 K
300 Kκ-Cl
250 K
12 K
30 K
50 K
100 K
300 Kκ-CN
12 K
100 K
200 K
Frequency (cm-1)
300 Kκ-BrE ⊥ plane
Are there electric dipoles in κκκκ-(ET)2Cu2(CN)3 ?
C
C
C
C
Position/splitting of antisymmetric C = C stretching mode depends on ET charge:
ν27(ρ) = 1398+140(1-ρ) cm-1
Infrared vibrational spectroscopy
No electric dipoles on ET dimers but
fast temporal fluctuations ~ 1011 Hz possible
ρ ≈ 0.5 e
ρ ≈ 0.8 e
ρ ≈ 0.2 e
Ivek et al., PRB83838383, 165128 (2011)
Dressel et al., Crystals 2222, 528 (2012)
2D α-(BEDT-TTF)2I3
• Ferroelectric phase transition due to charge orderingin which (ET) sites become non-equivalent• Net dipole moment evidenced by time-resolved PP optical second-harmonic
Yamamoto et al., JPSJpn (2008)
Yamamoto et al., Appl.Phys.Lett.(2010)
Pinteric et al., submitted to PRB (2014),
arXiv: 1407.6252v1In- and out-of-plane ε
Dielectric response: Relaxor-like
Dielectric mode: Cole-Cole function
• τ0 increases faster than ρ(dc)• Anomalously broad loss peak→Cooperative motion and glassy freezing within ET planes
Dielectric response; dependence on disorder
In- and out-of-plane ε
At Tb :• Anomaly in τ, ε´• Dip in 1-αMean relaxation time:• At high T: Arrheniusgradual slowing downbehavior• Tg (τ = 100 s): 7 – 15 K• At low T: tunneling
• Tb , Tg depend onlevel of disorder
Pinteric et al., submitted to PRB (2014),
arXiv: 1407.6252v1Sample synthesis: sA, sB
Other physical properties: dependence on
disorder
•Microwave dielectric constant• Specific heat• Thermal conductivity
Poirier et al., PRB (2012)
S.Yamashita et al., Nature (2008)
M.Yamashita et al., Nature Physics (20089
Monceau et al., Phys. Rev. Lett. 86, 4080 (2001)
Nad et al., JPCM12, 435 (2000)
Nad et al., EPL73, 567 (2006)
Nad et al., PRB62, 1753 (2000)
Relaxor ferroelectricity• Strong dispersion• Internal heterogeneity
• ε´, τ: huge peak• No dispersion away from TCO
Dielectric response in (TMTTF)2X : dependence on
disorder
• Ferroelectric phase transitiondue to charge ordering• Charge disproportionation
Curie-like peak
dc transport properties; dependence on disorder
• dc transport properties within ET planes: VRH for d=2
Emil Tafra:Poster PII-15 Tuesday 19Aug
•Tcross: crossover from nearest-neighbor hopping to VRHcorrelated with ordering of ethylene groups
• To , Tcross depend on level of disorder
Pinteric et al., submitted to PRB (2014)
Proposal: • Disorder originates in the anion layers
Origin of disorder
Pinteric et al., submitted to PRB (2014),
arXiv: 1407.6252v1
Supporting evidences:
• Structural refinements at RT and 100 K• Symmetries P2_1/c (with the inversion center of CN groups) and P2_1 (without the inversion center) characterize the crystal structure with comparably high agreement factors
• DFT calculations on 4[Cu2(CN)3)]-4 fragment
• Fluctuations of CN at ICs → variations of angles in the Cu-C-N-Cu bonds
• Anion network: Cu ions triangularly coordinated to CN groups
Origin of disorder
Proposal: • Disorder originates in the anion layer
Pinteric et al., submitted to PRB (2014),
arXiv: 1407.6252v1
• Fluctuations of CN at ICs → variations of angles in the Cu-C-N-Cu bonds• As T decreases → fluctuations become hindered• Inversion symmetry breaking at local scale• Local order with two different Cu triangular configurations• Anti-phase boundary between two neighboring domains
Origin of dielectric response
2) Cooperatively relaxing charge defects
• Random potential created by CN disorder in the anion subystem is mapped on the ET layer via hydrogen bonds• It changes the charge distribution and causes domain boundaries• Charge defects within these boundaries which can respond to ac electric field
Anomalous charge response, 6 K < T < 50 K :• Microwave and terahertz range
1) Fluctuating intradimer electric dipoles:
Poirier et al., PRB (2012)
Itoh et al., PRL(2013), PRB(2013)
Softening of the collective mode associated with intradimer excitation possible
Naka et Ishihara JPSJ(2013)
Is it possible that the dielectric relaxation at low-frequences is also a fingerprint of fluctuating intra-dimer electric dipoles?
Pinteric et al., submitted to PRB (2014),arXiv: 1407.6252v1
Conclusions
kappa-(BEDT-TTF)2Cu2(CN)3
• Anomalous dielectric response and VRH in dc limit within molecular planes due to charge defects in boundaries between frustration-limited domains
• Random potential and domain structure in BEDT-TTF subsystem originates in the anion network
• Important role of the cation-anion coupling
Byczuk et al.,PRL94, 056404 (2005)
Aguiar, Dobrosavljevic et al., PRL102, 156402 (2009)
• Disordered strongly correlated system close to the Mott-Anderson phase transition
Foury-Leylekian, Pouget et al., PRB82, 134116 (2010)
Alemany, Pouget and Canadell, PRB85, 195118 (2012)
Pouget et al., PSSB249, 937 (2012)
