Pressure dependence of upper critical fields in FeSe single crystals

Abstract

We investigate the pressure dependence of the upper critical fields (μ0Hc2) 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 Tc for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured μ0Hc2 for FeSe samples is smaller than the orbital limited upper critical field estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (α) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of μ0Hc2(T). Interestingly, the α value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the μ0Hc2(0) for the FeSe single crystals is found to be almost similar to that of Tc(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature.