Pauli-limiting effects in the upper critical fields of a clean LiFeAs single crystal

Abstract

We have investigated the temperature dependence of the upper critical field ${H}_{\mathrm{c}2}$($T$ ) in a LiFeAs single crystal by direct measurements of resistivity under static magnetic fields up to 36 T. We find in the case of a magnetic field $H$ along the $ab$ plane that ${H}_{\mathrm{c}2}^{ab}$(0) $=$ 30 T is clearly lower than the orbital limiting field ${H}_{\mathrm{c}2}^{\mathrm{orb},ab}$(0) $=$ 39.6 T estimated by the |${\mathit{dH}}_{\mathrm{c}2}^{ab}$/$\mathit{dT}$ |${}_{Tc}$, suggesting the presence of both Pauli- and orbital-limiting effects in the pair breaking process. The best fit of ${H}_{\mathrm{c}2}^{ab}$($T$ ) to the Werthamer--Helfand--Hohenberg formula results in the Maki parameter \ensuremath{\alpha} $=$ 0.9 and negligible spin-orbit scattering constant (${\ensuremath{\lambda}}_{\mathrm{so}}$ $=$ 0.0). On the other hand, for $H$ along the $c$ axis, ${H}_{\mathrm{c}2}^{c}$($T$ ) increases linearly down to our lowest temperature of 0.8 K, which can be explained by the multiband effects. The anisotropy ratio ${H}_{\mathrm{c}2}^{ab}$($T$ )/${H}_{\mathrm{c}2}^{c}$($T$ ) is 3 near ${T}_{\mathrm{c}}$ and systematically decreases upon lowering temperature to become 1.3 at zero temperature. A comparative overview of the behavior of ${H}_{\mathrm{c}2}^{ab}$($T$ ) in various Fe-based superconductors shows that, similar to LiFeAs, the calculated ${H}_{\mathrm{c}2}^{\mathrm{orb},ab}$(0) is generally much larger than the measured ${H}_{\mathrm{c}2}^{ab}$(0) and thus finite \ensuremath{\alpha} values ranging from \ensuremath{\sim}0.4 to 3 are necessary to describe the low temperature ${H}_{\mathrm{c}2}^{ab}$($T$ ) behaviors. Moreover, LiFeAs is found to have the smallest |${\mathit{dH}}_{\mathrm{c}2}^{ab}$/$\mathit{dT}$ |${}_{Tc}$ values, indicating that LiFeAs is one of the cleanest Fe-based superconductors with a finite Maki parameter. We also discuss the implications of multiband effects and spin-orbit scattering based on the finding that the estimated Pauli-limiting field is generally much larger than the BCS prediction in the Fe-based superconductors.