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Superconducting ThCr 2 Si 2 Structures

Explore the fascinating world of superconductivity with structures like ThCr2Si2, BaFe2As2, and LuNi2B2C. Discover the unique characteristics, bondings, and properties of these materials, along with their superconducting behavior and transitions at different temperatures and pressures.

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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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