Wendy Xu 286G 5/28/10

 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  mixed state  non-sc ◦ Alloys, intermetallics, ceramics, cuprates ◦ Higher temperatures and fields  higher currents

 AM 2 X 2 ◦ A: alkaline earth or lanthanide ◦ M: transition metal ◦ X: group 3-6  Variety of bonding & properties ◦ Mixed valency e.g. EuNi 2 P 2 ◦ Heavy fermion behavior e.g. CeCu 2 Si 2 ◦ Magnetism e.g. BaFe 2 As 2 ◦ Superconductivity e.g. BaFe 2 As 2

 AM 2 X 2 Tetragonal I4/mmm  Layers of edge sharing MX 4 tetrahedra separated by planes of A atoms  MX 4 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)

 I4/mmm  a=3.464A, c=10.631A (2.3C)  LuC NaCl layers alternate w/ Ni 2 B 2 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: , Rigid Ni 2 B 2 layers, nearly ideal NiB 4  Ln contraction: a axis contracts as size of Ln ion decreases  c axis expands, volume contraction small Siegrist et al. Nature 367 (1994)

 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

Q. Huang et al. arXiv: v2 9 Jul 2008

 At 142K, NM  AFM transition accompanies tetragonal  orthorhombic structural transition Q. Huang et al. arXiv: v2 9 Jul 2008

 (Ba 0.6 K 0.4 )Fe 2 As 2 T c =38K ◦ Ideal FeAs 4  KFe 2 As 2 exists ◦ r(Ba 2+ )=1.42A ◦ r(K + )=1.51A  As x=0  1 ◦ As-Fe-As gets smaller  Fe(3d x 2 -y 2 ) and As(3sp) overlap increases ◦ Fe-Fe gets shorter ◦ FeAs 4 stretched along c Rotter et al. DOI: /anie.200

 P=4GPa T c =35K ◦ Lower T c than doping due to slightly smaller N(E F )  Similarities to doping ◦ a lattice parameter trend ◦ As-Fe-As converge to towards sc region  Modification of Fermi surface by structural distortions more important than charge doping for sc S. Kimber et al. Nature Mat. 8 (2009)

 Ba(Fe 1.9 Pt 0.1 )As 2 T c =25K  All sc structures are tetragonal  Ba(Fe 2-x M x )As 2 ◦ M=Co, Ni(3d), Rh(4d), Pt(5d) ◦ a increases, c decreases ◦ Similar T c ’s  Regardless of mass, bandwidth, and spin orbit coupling Xiyu Zhu et al. arXiv: v3 1 Apr 2010

 SC’s w/ ThCr 2 Si 2 structure ◦ Intermediate T c values bridging gap btw pure metal sc’s and high T c cuprates  LuNi 2 B 2 C T c =23K ◦ Multiband 3D sc  BaFe 2 As 2 ◦ K doped T c =38K ◦ High pressure T c =35K ◦ Pt doped T c =25K  Fermi surface very important for sc, but what exactly what leads to sc in these materials are not clear