We have investigated superconductivity and structure in a gold-antimony-tellurium ternary alloy system (Au x Sb 1−x−y Te y ) synthesized under high pressure. We found that Au 0.125 Sb 0.75 Te 0.125 and AuSb 6 Te when crystallized in simple cubic structures (α-Po type), which are reported to be semiconductors above 20 K, are superconductors with a transition temperature (T c ) of 6.7 K. The structure and T c are mapped on an Au-Sb-Te triangular diagram. A β-Po-type rhombohedral structure appears before the crystal structure changes from As type (Sb) to α-Po type. Superconductivity is observed in the β-Po-type structure as well as in the α-Po-type structure. T c increases with decreasing Te concentration toward the Te-free Au-Sb end of the diagram. A T c of 8.1 K was achieved for the Au-Sb alloy with a typical composition of Au 0.15 Sb 0.85 . This T c value is the highest among materials with the α-Po-type structure under ambient pressure. Our Au 0.125 Sb 0.75 Te 0.125 sample exhibits a weak metallic behavior in resistivity. The discrepancy in the normal state resistivity behaviors between the previous study and ours must originate from a difference in the number of lattice defects in the samples.
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm -1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm -1 in the linewidth is observed. These precursor effects at T 0 ( ~ 2T c ) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å ) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K ( ~ 7 % ) indicates strong spin-phonon coupling in this superconductor.
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