Pressure dependence of upper critical fields in FeSe single crystals

Abstract

We investigate the pressure dependence of the upper critical fields (mu H-0(c2)) for FeSe single crystals with pressure up to 2.57 GPa. The superconducting (SC) properties show a disparate behavior across a critical pressure where the pressure-induced antiferromagnetic phase coexists with superconductivity. The magnetoresistance for H//ab and H//c is very different: for H//c, magnetic field induces and enhances a hump in the resistivity close to the T-c for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured mu H-0(c2) for FeSe samples is smaller than the orbital limited upper critical field (H-c2(orb)) estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (alpha) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of mu H-0(c2)(T). Interestingly, the alpha value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the mu H-0(c2)(0) for the FeSe single crystals is found to be almost similar to that of T-c(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature.