Specific Heat vs Field of LiFe1-xCuxAs

TL;DR

This study investigates the specific heat of high-quality LiFeAs and Cu-doped LiFeAs superconductors, revealing nodeless multiband behavior and the influence of sample quality on residual gamma, with implications for understanding their superconducting gap structure.

Contribution

It demonstrates that residual gamma in LiFeAs is due to sample quality and confirms nodeless multiband superconductivity through specific heat measurements under magnetic fields.

Findings

Residual gamma depends on sample quality.

LiFeAs shows non-linear gamma(H) similar to nodal superconductors.

Data aligns with theory for nodeless two-gap superconductors.

Abstract

LiFeAs is one of the new class of iron superconductors with a bulk onset Tc in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared from self-flux growth techniques with significantly improved properties, including a much decreased residual gamma (proportional to C/T as T->0) in the superconducting state. Thus, in contrast to previous explanations of a finite residual gamma in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual gamma 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 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 (proportional to the specific heat gamma) at the Fermi energy in the mixed state with applied field similar to the Volovik effect for nodal superconductors. However, rather than nodal behavior, the satisfactory comparison with a recent theory for gamma(H) for a two isotropic gap superconductor in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor.