Possible T_C Superconducting Enhancement in Q2D Materials by Incommensurate Structural Phase Transition

Abstract

It is shown that low dimensional electron systems in a deformable lattice are unstable against a lattice modulation at the Fermi wave vector only when there is half-filling of the conductive band (no doping: n = 0). When there is doping (n ≠ 0), the phase transition is metal-semimetal, the gap appears at a wave vector that is different from the Fermi-wave vector and an incommensurate structural phase transition takes place. This phase transition plays an important role in the formation of electron (or hole) bags, in the considerable increase of carrier DOS near the Fermi level and in the change of Cooper pair wave character that may lead to enhancement of superconducting T_C in Quasi-two dimensional (Q2D) materials. The start of our study is that when the doping n is different from zero the nesting vector is different from (π,π) and defined by minimizing the energy of the system. By the method of Green 's functions for the charge density wave and the Cooper pair we found a system of equations describing the phase.transition properties of Q2D materials, including superconductivity. A rough estimate of T_C gives agreement to experiments.