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Superconducting ThCr 2 Si 2 Structures. Wendy Xu 286G 5/28/10. Superconductivity. Electrical resistivity goes to zero Meissner effect: magnetic field is excluded from superconductor below critical temperature Type I: abrupt sc non -sc transition with field Pure metals - PowerPoint PPT Presentation
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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
-y2) 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