Crossover from BCS to Composite Boson (Local Pair) Superconductivity in Quasi-2D systems

TL;DR

This paper investigates the transition from BCS superconductivity to composite boson condensation in quasi-2D systems, revealing how the critical temperature varies with carrier density and coupling strength, highlighting unique quasi-2D effects.

Contribution

It provides a detailed analysis of the BCS to boson crossover in quasi-2D systems, emphasizing the nonmonotonic behavior of critical temperature and the conditions for Bose-Einstein condensation.

Findings

At low density, Tc equals the Bose-condensation temperature of heavy quasi-2D bosons.

At high density, the BCS theory remains valid.

The critical temperature exhibits nonmonotonic dependence on coupling strength.

Abstract

The crossover from cooperative Cooper pairing to independent bound state (composite bosons) formation and condensation in quasi-2D systems is studied. It is shown that at low carrier density the critical superconducting temperature is equal to the temperature of Bose-condensation of ideal quasi-2D Bose-gas with heavy dynamical mass, meanwhile at high densities the BCS result remains valid. The evident nonmonotoneous behaviour of the critical temperature as function of the coupling constant (the energy of the two particle bound state) is a qualitative difference of quasi-2D crossover from 3D one.