BCS-BEC crossover-like phenomena driven by quantum-size effects in   quasi-one-dimensional fermionic condensates
  Quantum confinement is known to influence fermionic condensates, resulting in quantum-size oscillations of superfluid/superconducting properties. Here we show that the impact of quantum-size effects is even more dramatic. Under realistic conditions, a significant phase-space reconfiguration induced by quantum-size effects opens a quasi-molecule channel in the fermionic pairing so that the condensed pairs exhibit features typical of a molecular state. As an illustration we consider a quasi-one-dimensional fermionic condensate, as realized, e.g., in cigar-shaped atomic Fermi gases or superconducting quantum wires. In this case the transverse quantization of the particle motion favors pairing through a coherent superposition of quantum channels that are formed due to the grouping of single-particle levels into a series of well distinguished subbands. Whenever the bottom of a subband approaches the Fermi level, the longitudinal spatial distribution of fermions in a condensed pair becomes strongly localized within the corresponding quantum channel. The fermionic pairs in this channel resemble molecules with bosonic character. 
A. A. Shanenko, M. D. Croitoru, A. V. Vagov, V. M. Axt, A. Perali, and   F. M. Peeters
cond-mat.quant-gas