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 H-c2(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-c2(ab) (0) = 30 T is clearly lower than the orbital limiting field H-c2(orb,ab) (0) = 39.6 T estimated by the vertical bar dH(c2)(ab)/dT vertical bar(Tc), suggesting the presence of both Pauli- and orbital-limiting effects in the pair breaking process. The best fit of H-c2(ab) (T) to the Werthamer-Helfand-Hohenberg formula results in the Maki parameter alpha = 0.9 and negligible spin-orbit scattering constant (lambda(so) = 0.0). On the other hand, for H along the c axis, H-c2(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-c2(ab) (T)/H-c2(c) (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 H-c2(ab) (T) in various Fe-based superconductors shows that, similar to LiFeAs, the calculated H-c2(orb,ab) (0) is generally much larger than the measured H-c2(ab)(0) and thus finite a values ranging from similar to 0.4 to 3 are necessary to describe the low temperature H-c2(ab) (T) behaviors. Moreover, LiFeAs is found to have the smallest vertical bar dH(c2)(ab)/dT vertical bar(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.