Selected topics related to the transport and superconductivity in boron-doped diamond

Abstract

This contribution deals with a few topics closely related to the superconductivity in the heavily boron-doped diamond which are, in our opinion, not properly treated in the current literature. Attention is paid especially to the classification of metallic and insulating state, selection of pairing mechanism, limits of weak coupling approximation and to the influence of granularity on the superconducting transition.

Recent advances in superconductivity of covalent superconductors

“…The second theory, by Mares et al [24], is based on spin-flip pairing (attractive spin-spin interaction) of holes weakly localized near the Fermi level. This model was developed to explain the results on nanocrystalline diamond films.…”

Superconductivity in covalent semiconductors

“…Resistance behavior at T > T indicates whether material is metal-like ( R/ T > 0) or semiconductor-like ( R/ T < 0). This test is however primitive because carrier mobility varies with temperature and because R/ T does not unambiguously define metal or semiconductor [22].…”

“…It provides holes into the valence band with a thermal activation energy of 0.37 eV [39], and thus can not sustain conductivity at low temperatures. The latter is achieved through the heavy doping resulting in such strong overlap of acceptor wavefunctions that not only an impurity band is formed, but it also merges with the valence band, making diamond a quasi-metal [22,43]. Such overlap would be impeded if some boron atoms were present in forms other than substitutitional acceptor, and the different values of T for a given boron concentration ( Fig.…”

“…The second model by J. J. Mares [22] is based on spin-flip-driven pairing (attractive spin-spin interaction) of holes weakly localized in the vicinity of the Fermi level. It explains the superconductivity in diamond in general, and especially in nanocrystalline diamond films.…”

“…The last model is the phonon-mediated pairing mechanism by Bardeen, Cooper and Schrieffer (BCS), advocated by M. Cardona and others [43]. Recent detailed measurements of T shift upon 11 B/ 10 B or 13 C/ 12 C substitutions [27] might favor the BCS model; however, both Mares [22] and Baskaran [25] argue this result is inconclusive. Fig.…”