Thermodynamics of Eliashberg theory in the weak-coupling limit

The weak-coupling limits of the gap and critical temperature computed within Eliashberg theory surprisingly deviate from the BCS theory predictions by a factor of 1/√e. Interestingly, however, the ratio of these two quantities agrees for both theories. Motivated by this result, here we investigate the weak-coupling thermodynamics of Eliashberg theory, with a central focus on the free energy, specific heat, and the critical magnetic field. In particular, we numerically calculate the difference between the superconducting and normal-state specific heats, and we find that this quantity differs from its BCS counterpart by a factor of 1/√e, for all temperatures below Tc. We find that the dimensionless ratio of the specific-heat discontinuity to the normal-state specific heat reduces to the BCS prediction given by ΔCV(Tc)/CV,n(Tc)≈1.43. This gives further evidence to the expectation that all dimensionless ratios tend to their ``universal values'' in the weak-coupling limit.