  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.
