Spatial emergence of Off-Diagonal Long-Range Order throughout the BCS-BEC crossover

TL;DR

This paper investigates how Off-Diagonal Long-Range Order (ODLRO) develops spatially across the BCS-BEC crossover in superfluid fermionic systems, revealing temperature-dependent changes in the two-particle density matrix's spatial behavior.

Contribution

It provides a systematic diagrammatic analysis including pairing fluctuations and GMB correction to study the spatial emergence of ODLRO throughout the BCS-BEC crossover.

Findings

Condensate density varies with coupling and temperature.

Spatial dependence shifts from power-law to exponential with temperature.

Results limit the minimum size for superfluid correlations in finite systems.

Abstract

In a superfluid system, Off-Diagonal Long-Range Order (ODLRO) is expected to be exhibited in the appropriate reduced density matrices when the relevant particles (either bosons or fermion pairs) are considered to recede sufficiently far apart from each other. This concept is usually exploited to identify the value of the condensate density, without explicit concern, however, on the spatial range over which this asymptotic condition can effectively be achieved. Here, based on a diagrammatic approach that includes beyond-mean-field pairing fluctuations in the broken-symmetry phase at the level of the $t$-matrix also with the inclusion of the Gorkov-Melik-Barkhudarov (GMB) correction, we present a systematic study of the two-particle reduced density matrix for a superfluid fermionic system undergoing the BCS-BEC crossover, when the entities to recede far apart from each other evolve with continuity from largely overlapping Cooper pairs in the BCS limit to dilute composite bosons in the BEC limit. By this approach, we not only provide the coupling and temperature dependence of the condensate density at the level of our diagrammatic approach which includes the GMB correction, but we also obtain the evolution of the spatial dependence of the two-particle reduced density matrix, from a power-law at low temperature to an exponential dependence at high temperature in the superfluid phase, when the inter-particle coupling spans the BCS-BEC crossover. Our results put limitations on the minimum spatial extent of a finite-size system for which superfluid correlations can effectively be established.