Above : Transition temperature versus hydrostatic pressure for selected Fe-based superconductors.

1. Hydrostatic Pressure Studies on Fe-based SuperconductorsJames S. Schilling, Washington University, DMR 1104742 Since the discovery of the Fe-based high-temperature superconductors in 2008, a large number of high-pressure experiments have been carried out which can potentially reveal the nature of the superconducting state. In the most cases Tc is found to initially increase with pressure, as in two recent studies on LaFePO using less than hydrostatic pressure media. Helium gas affords fully hydrostatic pressure. In the figure is seen that to 0.8 GPa (8,000 atm) pressure, Tc for both single- and polycrystalline LaFePO decreases rapidly under He-gas pressure. These measurements, coupled with uniaxial pressure studies, give important information on what makes the Fe-based superconductors tick. The misleading results of the earlier experiments likely come from shear stresses generated by the non-hydrostatic pressure media. Above: Transition temperature versus hydrostatic pressure for selected Fe-based superconductors.

2. Hydrostatic Pressure Studies on Fe-based Superconductors James S. Schilling, Washington University, DMR 1104742 Strangely, in the new Fe-based high-temperature superconductors, subjecting the sample to strong shear strains (squashing it) appears to strengthen the superconductivity! This is illustrated in the figure. In the upper figure the transition temperature Tc to superconductivity for LaFePO is seen to initially increase rapidly if the sample is squashed between two diamond anvils. But in the lower figure, using helium gas to press on the sample hydrostatically shifts the superconducting transition to lower temperatures. This strange behavior must be thoroughly understood before these promising Fe-based materials can be used in applications. In collaboration with a group in Karlsruhe, Germany, the Schilling group studying in detail the dependence of Tc on hydrostatic and direction-dependent pressure for Fe-based materials. Upper. Tc versus pressure (1 GPa = 10,000 atm) for LaFePO (no pressure medium). Lower. Superconducting transition in magnetic susceptibility versus temperature at three pressures (He-gas pressure medium).