Possible Benefits from Phonon/Spin-Wave Induced Gaps below or above EF for Superconductivity in High-TC Cuprates

Abstract

A phonon of appropriate momentum kF will open a band gap at the Fermi energy EF. The gap within the electronic density-of-states (DOS), N(EF), leads to a gain in electronic energy and a loss of elastic energy because of the gap-generating phonon. A BCS-like simulation shows that the energy gain is larger than the loss for temperatures below a certain transition temperature, TC. Here, it is shown that the energy count can be almost as favorable for gaps a little below or above EF. Such gaps can be generated by auxiliary phonons (or even spin- and charge-density waves) with k-vectors slightly different from kF. Gaps not too far from EF will add to the energy gain at the superconducting transition. In addition, a DOS-peak can appear at EF and thereby increase N(EF) and TC. A dip in the DOS below EF will result for temperatures below TC, which is similar to what often is observed in cuprate superconductors. The roles of spin waves and thermal disorders are discussed.