Pressure dependence of upper critical fields in FeSe single crystals

We investigate the pressure dependence of the upper critical fields (μ ₀ H _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 μ ₀ H _c2 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 μ ₀ H _c2(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 μ ₀ H _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.