Specific heat versus field for LiFe1−xCuxAs

Abstract

LiFeAs is one of the new class of iron superconductors with a bulk in the 15–17 K range. We report on the specific heat characterization of single crystal material prepared by self-flux growth techniques with significantly improved properties, including a much decreased residual gamma, γr (≡C/T as T → 0), in the superconducting state. Thus, in contrast to previous explanations of a finite γr in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual γ in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields, to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe0.98Cu0.02As, which has the reduced Tc of ≈9 K and an Hc2 of 15 T. Interestingly, although presumably both LiFeAs and LiFe0.98Cu0.02As are nodeless, they clearly show a non-linear dependence of the electronic density of states (∝ specific heat γ) at the Fermi energy in the mixed state with the applied field, similar to the Volovik effect for nodal superconductors. However, rather than indicating nodal behavior, the satisfactory comparison with a recent theory for γ(H) for a superconductor with two isotropic gaps in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in a magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor.