New Explanation of the Excitation Spectra of Conventional Superconductors

Abstract

It is shown that, in the conventional superconductors, there exist in addition to the discontinuous excitation spectrum of the BCS-theory, continuous (gap-less) excitation spectra due to bosons. Since above and below the transition temperature, T_SC, the heat capacity of the superconducting elements exhibits power functions of absolute temperature, T = 0 is the only critical point. The superconducting transitions therefore are all within the fairly large critical range at the critical point T = 0. Typical of a critical range is short-range order. At T_SC, the type of short-range order changes. While above T_SC, the type of short-range order is not quite clear, the short-range ordered units below T_SC are the Cooper-pairs. The observed power functions of absolute temperature result from the fact that the generally gap-less excitation spectra of the ballistically propagating bosons are given by simple power functions of wave-vector. As we know from Renormalization Group (RG) theory, in the vicinity of a critical point, including the critical point T = 0, the atomistic interactions are excluded from the dynamics. The gapped excitation spectrum of the Cooper-pairs is a typical atomistic excitation that is not relevant for the boson-defined zero-field heat capacity. It can reasonably be assumed that the relevant bosons below T_SC are identical with the bosons giving rise to the universal linear-in-T electronic heat capacity above T_SC, as it is typical for all metals. Ordering of these bosons at T_SC entails the formation of Cooper-pairs. Arguments will be given that the bosons have to be identified with electric quadrupole radiation, generated by the non-spherical charge distributions in the soft zones between the metal atoms.