Akira Iyo

Superconductivity above 50 K in LnFeAsO1-y (Ln = Nd, Sm, Gd, Tb, and Dy) Synthesized by High-Pressure Technique

We have succeeded in synthesizing single-phase polycrystalline samples of oxygen-deficient oxypnictide superconductors, Ln FeAsO 1- y ( Ln : lanthanide elements) with Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, and Dy using high-pressure technique. It is found out that the synthesis pressure is a key parameter for synthesizing samples, in particular for the heavier Ln s, such as Tb and Dy. The lattice parameters systematically decrease with the atomic number of Ln , reflecting the shrinkage of Ln ionic radius. For the lighter Ln s (La, Ce, Pr, Nd), T c increases monotonously with decreasing the lattice parameters from 26 K for Ln = La to 54 K for Ln = Nd, then stays at the constant value around 53 K for the heavier counterpart (Nd, Sm, Gd, Tb, and Dy). The results suggest the intimate relationship between the crystal structural parameters and the superconductivity, as well as the possible existence of the inherent maximum T c , which is located around 50 K in the Ln FeAsO based materials.

Electronic Structure of Novel Superconductor AuSb6Te

Some elements show valence-skipping behavior, e.g. Bi takes 3+ and 5+ valences but scarcely takes 4+ valence. This means that the effective interaction Ueff=2E(Bi 4+ )-E(Bi 3+ )-E(Bi 5+ ) is negative. This situation naturally leads to the so-called negative-U model[1], and the superconductivity in (Ba,K)BiO3 and Tl:PbTe was attributed to this mechanism[2,3]. Varma has firstly tackled this problem and consecutive works are revealing the mechanism of this valence-skipping behavior [2,4,5]. Apart from the quantitative estimation of |Ueff|, this concept may be useful to search new superconductors. From this concept we have found a new superconductor AuSb6Te[6]. This compound was known to be a semiconductor[7], but our research has clarified that this compound is a superconductor with Tc=6.7K. In order to investigate the mechanism of the superconductivity in this new material, it is necessary to know the precise band structure. In this paper, we have performed an ab-initio band structure calculation for AuSb6Te. Section 2 describes the framework of our calculation. The results and discussions are developed in §3. A brief summary is shown in §4.