Superconductivity at ca. 40 K in FeSe under high pressure
Dr. Sergei Medvedev (Anorganische Chemie, Mainz)

Superconductivity was discovered recently in iron arsenides and FeSe. The arsenides display superconducting transition temperatures as high as 55 K and share a number of general features with high-T_c cuprates. Also the binary FeSe show superconductivity under ambient pressure up to 10 K and up to 27 K under modest pressure. In Fe_1+δSe the superconductivity is extremely sensitive to stoichiometry. As a significant difference to the arsenides, it was observed that Fe_1+δSe is not magnetically ordered down to low temperatures. Here we will discuss the phase diagram of β-Fe_1.01Se as a function of pressure. A maximal superconducting transition temperature was found at 37 K under a pressure of 9 GPa. On a first view the phase diagram and the resistivity curve look very similar compared to the iron arsenides. As a function of pressure β-Fe_1.01Se shows a transition from a Fermi liquid like transition with a linear resistivity as a function of temperature to a more complex behavior. Under pressure around 15 GPa metallic tetragonal phase transforms into the semiconducting phase having hexagonal structure of NiAs-type.