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Superconducting ThCr 2 Si 2 Structures

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

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Superconducting ThCr 2 Si 2 Structures

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  1. Superconducting ThCr2Si2 Structures Wendy Xu 286G 5/28/10

  2. 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 • low temperatures and small magnetic fields • BCS Theory: Cooper pairs • Type II: scmixedstatenon-sc • Alloys, intermetallics, ceramics, cuprates • Higher temperatures and fields higher currents

  3. ThCr2Si2 structure types • 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

  4. ThCr2Si2 structure • AM2X2 Tetragonal I4/mmm • Layers of edge sharing MX4tetrahedra 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 Johrendt et al. J. Solid St. Chem. 130 (1997) 254-265

  5. LuNi2B2C – Tc up to 23K • I4/mmm • a=3.464A, c=10.631A (2.3C) • LuCNaCl layers alternate w/ Ni2B2 layers B-C:1.47A, short B-B: 2.94A Lu-C: 2.499A, strong c expands, a contracts Ni-Ni (planar): 2.449A, strong shorter than metallic metal (2.5A) Ni-B: 2.10A B-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 Siegrist et al. Nature 367 (1994) 254-256

  6. LuNi2B2C multiband 3D SC • 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 L. F. Mattheiss Phys. Rev. B 49 (1994) 13279

  7. BaFe2As2 structure Q. Huang et al. arXiv:0806.2776v2 9 Jul 2008

  8. BaFe2As2 AFM behavior • At 142K, NMAFM transition accompanies tetragonalorthorhombic structural transition Q. Huang et al. arXiv:0806.2776v2 9 Jul 2008

  9. (Ba1-xKx)Fe2As2 • (Ba0.6K0.4)Fe2As2Tc=38K • Ideal FeAs4 • KFe2As2 exists • r(Ba2+)=1.42A • r(K+)=1.51A • As x=01 • As-Fe-As gets smaller • Fe(3dx2-y2) and As(3sp) overlap increases • Fe-Fe gets shorter • FeAs4 stretched along c Rotter et al. DOI: 10.1002/anie.200

  10. S. Kimber et al. Nature Mat. 8 (2009) 471-475 BaFe2As2 under pressure • P=4GPa Tc=35K • Lower Tcthan 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

  11. Ba(Fe2-xPtx)As2 • 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 Xiyu Zhu et al. arXiv:1001.4913v3 1 Apr 2010

  12. Summary • SC’s w/ ThCr2Si2 structure • Intermediate Tc values bridging gap btw pure metal sc’s and high Tccuprates • 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

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