Superconducting ThCr2Si2 Structures

Wendy Xu286G

5/28/10

Electrical resistivity goes to zero

Meissner effect: magnetic field is excluded from superconductor below critical temperature

Type I: abrupt scnon-sc transition with field ◦ Pure metals◦ low temperatures and small magnetic fields◦ BCS Theory: Cooper pairs

Type II: scmixed statenon-sc◦ Alloys, intermetallics, ceramics, cuprates◦ Higher temperatures and fields higher currents

Superconductivity

AM2X2

◦ A: alkaline earth or lanthanide◦ M: transition metal◦ X: group 3-6

Variety of bonding & properties◦ Mixed valency e.g. EuNi2P2

◦ Heavy fermion behavior e.g. CeCu2Si2◦ Magnetism e.g. BaFe2As2

◦ Superconductivity e.g. BaFe2As2

ThCr2Si2 structure types

AM2X2 Tetragonal I4/mmm

Layers of edge sharing MX4 tetrahedra separated by planes of A atoms

MX4 almost undistorted w/ strong M-X bonds

X-X interlayer distances varies◦ Changing M from left to right, M-M

distance increases, X-X distance decreases

◦ Changing A from small to big, X-X distance increases

A is an electron donor, and maintains geometry◦ Alkaline earth—almost completely ionized◦ Ln—d shells partially occupied, not

completely ionized

ThCr2Si2 structure

Johrendt et al.J. Solid St. Chem. 130 (1997) 254-265

I4/mmm a=3.464A, c=10.631A (2.3C) LuC NaCl layers alternate w/ Ni2B2 layers

B-C:1.47A, shortB-B: 2.94ALu-C: 2.499A, strong

c expands, a contracts

Ni-Ni (planar): 2.449A, strongshorter than metallic metal (2.5A)

Ni-B: 2.10AB-Ni-B: 108.75, 110.9

Rigid Ni2B2 layers, nearly ideal NiB4

Ln contraction: a axis contracts as size of Ln ion decreases c axis expands, volume contraction small

LuNi2B2C – Tc up to 23K

Siegrist et al.Nature 367 (1994) 254-256

Contribution of all atoms present All five Ni(3d) orbital contributions roughly

equal

Lu(5d) contribution non-negligible◦ doping at this site less favorable than in typical

cuprate sc’s

LuNi2B2C multiband 3D SC

L. F. MattheissPhys. Rev. B 49 (1994) 13279

BaFe2As2 structure

Q. Huang et al.arXiv:0806.2776v29 Jul 2008

At 142K, NMAFM transition accompanies tetragonalorthorhombic structural transition

BaFe2As2 AFM behavior

Q. Huang et al.arXiv:0806.2776v29 Jul 2008

(Ba0.6K0.4)Fe2As2 Tc=38K ◦ Ideal FeAs4

KFe2As2 exists◦ r(Ba2+)=1.42A◦ r(K+)=1.51A

As x=01◦ As-Fe-As gets smaller

Fe(3dx2-y

2) and As(3sp) overlap increases

◦ Fe-Fe gets shorter◦ FeAs4 stretched along c

(Ba1-xKx)Fe2As2

Rotter et al. DOI: 10.1002/anie.200

P=4GPa Tc=35K◦ Lower Tc than doping due to slightly smaller N(EF)

Similarities to doping◦ a lattice parameter trend◦ As-Fe-As converge to 109.5 towards sc region

Modification of Fermi surface by structural distortions more important than charge doping for sc

BaFe2As2 under pressureS. Kimber et al. Nature Mat. 8 (2009) 471-475

Ba(Fe1.9Pt0.1)As2 Tc=25K

All sc structures are tetragonal

Ba(Fe2-xMx)As2

◦ M=Co, Ni(3d), Rh(4d), Pt(5d)◦ a increases, c decreases◦ Similar Tc’s

Regardless of mass, bandwidth, and spin orbit coupling

Ba(Fe2-xPtx)As2

Xiyu Zhu et al.arXiv:1001.4913v3 1 Apr 2010

SC’s w/ ThCr2Si2 structure◦ Intermediate Tc values bridging gap btw pure metal sc’s and high

Tc cuprates

LuNi2B2C Tc=23K◦ Multiband 3D sc

BaFe2As2

◦ K doped Tc=38K

◦ High pressure Tc=35K

◦ Pt doped Tc=25K

Fermi surface very important for sc, but what exactly what leads to sc in these materials are not clear

Summary