Pseudogap phase formation in the crossover from Bose-Einstein condensation to BCS superconductivity in low dimensional systems

TL;DR

This paper investigates the formation of the pseudogap phase in low-dimensional systems during the crossover from Bose-Einstein condensation to BCS superconductivity, revealing a new phase above the superconducting state.

Contribution

It introduces a microscopic phase diagram for 2D metals using a modulus-phase approach, identifying a pseudogap phase linked to neutral fermion order.

Findings

Discovery of a new phase above the BKT superconducting phase.

Persistence of the phase diagram with weak 3D effects.

Identification of the pseudogap phase with neutral fermion order.

Abstract

A phase diagram for a 2D metal with variable carrier density has been studied using the modulus-phase representation for the order parameter in a fully microscopic treatment. This amounts to splitting the degrees of freedom into neutral fermion and charged boson degrees of freedom. Although true long range order is forbidden in two dimensions, long range order for the neutral fermions is possible since this does not violate any continuous symmetry. The phase fluctuations associated with the charged degrees of freedom destroy long range order in the full system as expected. The presence of the neutral order parameter gives rise to new features in the superconducting condensate formation in low dimensional systems. The resulting phase diagram contains a new phase which lies above the superconducting (here Berezinskii-Kosterlitz-Thouless) phase and below the normal (Fermi-liquid) phase. We identify this phase with the pseudogap phase observed in underdoped high-$T_{c}$ superconducting compounds above their critical temperature. We also find that the phase diagram persists even in the presence of weak 3-dimensionalisation.