Atypical BCS-BEC crossover induced by quantum-size effects

TL;DR

This paper reveals a many-body quantum-size effect causing an atypical BCS-BEC crossover in a confined ultracold Fermi gas, driven by size quantization and subband formation, differing from single-particle explanations.

Contribution

It uncovers a novel many-body quantum-size effect leading to an atypical BCS-BEC crossover in a confined Fermi gas, emphasizing the role of subband physics.

Findings

Quantum-size oscillations affect many-body properties.

Subband crossings induce BCS-BEC crossover in specific subbands.

The condensate is a mixture of BCS and BEC components depending on subband energies.

Abstract

Quantum-size oscillations of the basic physical characteristics of a confined fermionic condensate are a well-known phenomenon. Its conventional understanding is based on the single-particle physics, whereby the oscillations follow the size-dependent changes in the single-particle density of states. Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which demonstrates an important many-body aspect of the quantum-size effects, overlooked previously. The many-body physics is revealed in the atypical crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the Bose-Einstein condensate (BEC) induced by the size quantization of the particle motion. Quantized perpendicular spectrum results in the formation of single-particle subbands (shells) so that the aggregate fermionic condensate becomes a coherent mixture of subband condensates. Each time when the lower edge of a subband crosses the chemical potential, the BCS-BEC crossover is approached in this subband, and the aggregate condensate contains both the BCS and BEC-like components.