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New superconductors

Published on 9 November 2022

Superconducting phase diagram of FeSe under high pressure.

The discovery of high Tc superconductivity in 1986 inspired great hopes for applications. More than 20 years later this has clearly not occurred. Today we understand better some ingredients that make high critical temperatures: competing phases, anisotropy, low carrier density… we also realize that these do not necessarily make a system suitable for applications.

Fundamental studies to identify the microscopic mechanisms in high Tc and other unconventional superconductors are essential, however if a new breakthrough occurs it will probably be from the discovery of a new class of material. In this context the iron based superconductors, first discovered in 2008, are an exciting development. Critical temperatures of over 50K are achieved for the first time in a system not containing copper. As it is usually easier to understand simple systems we have studied the binary system FeSe. This is a superconductor with a relatively low critical temperature (8K) barely higher than lead ! However with the application of pressure we have found that Tc increases up to 36K, clearly putting this system in the family of high temperature superconductivity. If the mechanism that allows Tc to increase from 8K to 36K can be identified it should provide strong indications on where to look for even higher temperatures. The challenges now are to produce high quality single crystals, and to correlate detailed low temperature measurements of the macroscopic properties with studies of the microscopic excitations in the system by neutron or Xray spectroscopy.

Furthermore we are interested in:

• iron pnictides and related compounds
• superconductivity in Pyrochlore Oxides
• spin ladder compounds of the Sr14Cu24O41
• family new Ce and U based heavy fermion superconductors