A huge enhancement of the superconducting transition temperature T c was observed in a tetragonal FeSe superconductor under high pressure. The onset temperature was as high as 27 K at 1.48 GPa and the pressure coefficient showed an extremely high value of 9.1 K/GPa. The upper critical field H c2 was estimated to be ~ 72 T at 1.48 GPa. Because of the high H c2 , the FeSe system can be applied to superconducting wire rods.Since the great discovery of cuprate high-T c superconductors, over 20 years have passed. Recently many researchers have made much effort focused on non-cuprate, new superconductors such as MgB 2 with a T c of 39 K. 1,2,3

We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature T c was observed with the substitution of S or Te for Se; the T c increased with S substitution by up to 20 %, and also increased with Te substitution up to 75 %. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine T c in the FeSe system.

We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.Comment: 13 pages, 10 figure

Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.Comment: 15 pages, 5 figures, ISS200

A micro-pressure cell has been developed for measurements of specific heat and magnetization by using a commercial SQUID magnetometer. This small piston-cylinder device can be used up to 2 GPa. In this paper, we report on the efficiency of our micro-cell and present some results of the pressure effect on the magnetic and thermal properties of CeAg.