Electron-Phonon or Hole Superconductivity in $Mg{B}_2$?

Abstract

The BCS electron-phonon mechanism and the unconventional 'hole mechanism' have been proposed as explanations for the high temperature superconductivity observed in $Mg{B}_2$. It is proposed that a critical test of which theory is correct is the dependence of $T_c$ on hole doping: the hole mechanism predicts that $T_c$ will drop rapidly to zero as holes are added, while the electron-phonon mechanism appears to predict increasing $T_c$ for a substantial range of hole doping. Furthermore, the hole mechanism and electron-phonon mechanism differ qualitatively in their predictions of the effect on $T_c$ of change in the $B-B$ distances. We discuss predictions of the hole mechanism for a variety of observables as a function of doping, emphasizing the expected differences and similarities with the electron-phonon explanation. The hole mechanism predicts coherence length and penetration depth to increase and decrease monotonically with hole doping respectively.

Minor changes in wording in view of referee's comments; one curve added to fig. 11; under consideration for publication in Phys.Rev. B

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