We investigate the pressure dependence of the upper critical fields ({ \mu}$_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 { \mu}$_0$$H$$_{ c2}$ for FeSe samples is smaller than the orbital limited upper critical field ($H$$^{ orb}$$_{ c2}$) estimated by the Werthamer Helfand and Hohenberg (WHH) model, the Maki parameter ({ \alpha}) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of { \mu}$_0$$H$$_{ 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}$_0$$H$$_{ c2}$(0)s 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.
Categories: cond-mat.supr-con