Effect of Hydrostatic Pressure on Superconductivity of FeSe Thin Films

The electronic transport in superconducting epitaxial c -axis-oriented FeSe thin films grown on (001)-oriented MgO substrates was investigated. To this end, the in-plane resistivity was measured in dependence on temperature, pressure, and magnetic field. The temperature ranged from 1.2 to 35 K; static magnetic fields with strengths up to 14 T were applied normal to the film surface, i.e. parallel to the FeSe c -axis; and hydrostatic pressure was applied from 0 to 2.7 GPa. Concerning the role of the MgO substrate in the pressure experiments, it is suggested that the substrate mainly reduces the in-plane compressibility of the film in comparison to bulk. The transition to superconductivity shifted to higher temperatures with increasing pressure. The onset critical temperature raised from 11.5 K at zero applied pressure with an initial rate of 2.5 K/GPa to 18.2 K at 2.7 GPa. The pressure-induced increase of the critical temperature was accompanied by a twofold broadening of the transition width. As a counterpart of pressure, the magnetic field shifted the superconducting transition to lower temperature. In addition to pressure, the field also induced a noticeable broadening of the superconductive transition rather than a parallel shift. The positive magnetoresistance at 20 K increased with enhanced pressure and reached 24% at the highest pressure and field. For each applied pressure, the magnetoresistance could be fitted by a Lorentzian function, i.e. it originates from classical Lorentz scattering. The resulting charge carrier mobility increased under pressure suggesting a decreasing collision rate. The upper critical field raised with higher pressure. Its temperature dependence could be fitted by conventional Werthamer-Hohenberg-Helfand theory under the assumption of the Pauli paramagnetic effect that became more pronounced under pressure. The anomalous behaviour of the normalized negative slope of the upper critical field at the critical temperature suggested a change of the Fermi surface above a critical pressure of 2 GPa.