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

We have investigated the temperature dependence of the upper critical field Hc2(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 Hc2ᵃᵇ(0) $=$ 30 T is clearly lower than the orbital limiting field Hc2orb,ab(0) $=$ 39.6 T estimated by the |dHc2ᵃᵇ/dT |Tc, suggesting the presence of both Pauli- and orbital-limiting effects in the pair breaking process. The best fit of Hc2ᵃᵇ(T ) to the Werthamer--Helfand--Hohenberg formula results in the Maki parameter {α} $=$ 0.9 and negligible spin-orbit scattering constant (λₛₒ $=$ 0.0). On the other hand, for H along the c axis, Hc2ᶜ(T ) increases linearly down to our lowest temperature of 0.8 K, which can be explained by the multiband effects. The anisotropy ratio Hc2ᵃᵇ(T )/Hc2ᶜ(T ) is 3 near Tc and systematically decreases upon lowering temperature to become 1.3 at zero temperature. A comparative overview of the behavior of Hc2ᵃᵇ(T ) in various Fe-based superconductors shows that, similar to LiFeAs, the calculated Hc2orb,ab(0) is generally much larger than the measured Hc2ᵃᵇ(0) and thus finite {α} values ranging from {~}0.4 to 3 are necessary to describe the low temperature Hc2ᵃᵇ(T ) behaviors. Moreover, LiFeAs is found to have the smallest |dHc2ᵃᵇ/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.