Coexistence of giant Cooper pairs with a bosonic condensate and anomalous behavior of energy gaps in the BCS-BEC crossover of a two-band superfluid Fermi gas

Abstract

We investigate Bardeen-Cooper-Schrieffer (BCS) − Bose-Einstein condensation (BEC) crossover in a two-band superfluid Fermi gas with an energy shift between the bands. When the intraband coupling in the cold (first) band is fixed as weak, we find that in the case of vanishing interband interaction and in the strong-coupling limit of the hot (second) band the system undergoes a transition to a single-component configuration with the full suppression of the first energy gap and with the full redistribution of particles between bands. For non-vanishing interband interaction we reveal the non-monotonic dependence of the energy gap in the first band vs intraband coupling in the second band with the presence of a hump. In the case of weak interband coupling the system shows a significant amplification of the intrapair correlation length of the condensate in the first band in the strong-coupling regime of the second band, which clearly indicates the coexistence of giant Cooper pairs and a bosonic condensate even for nonzero temperatures. This can lead to a non-monotonic temperature dependence of the second energy gap with a peak. Here predicted coexistence of the giant Cooper pairs and bosonic molecules can be verified by means of the visualization of vortex cores in the two-component atomic condensates as well as in some iron-based superconductors.PACS numbers:

Effects of Pair-Hopping Coupling on Properties of Multi-Band Iron-Based Superconductors

“…The unconventional behavior found in the present work can be observed in many experiments and are in a good agreement with the predictions of the Ginzburg-Landau theory [44]. More recently, similar results have been also found in the case of the two-band superfluid system in the BCS-BEC crossover regime [142].…”

A Dual-Species Bose-Einstein Condensate with Attractive Interspecies Interactions

“…Most of these phenomena are accessible thanks to the ability to control the sign and strength of interactions, opening the way to the study of unconventional matter phases. For instance, in two-component Fermi gases, by tuning the interparticle scattering length close to a Feshbach resonance, it has been possible to explore the Bose-Einstein condensate (BEC)-BCS crossover [18], and in multicomponent Fermi systems, even more exotic scenarios have been discussed which are relevant for high-Tc superconductivity [19][20][21][22][23]. On the other hand, in bosonic mixtures, Feshbach resonances have been recently exploited to produce and study novel quantum phases, arising by competing interaction forces.…”

Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band Superfluid/Superconductor

“…This indicates that all the particles in both bands form molecular condensates in the strong-coupling limit. Although the nonzero pair-exchange couplingṼ 12 does not give qualitative effects on T c in this regime [(k F,2 a 22 ) −1 > ∼ 1], the coexistence of large Cooper pairs and small molecules has been discussed within the mean-field [21], NSR [38], and TMA [39,40] approaches. With increasing V 12 , the two-band system undergoes the BCS-BEC crossover even in the case of weak intraband couplings [(k F,2 a 22 ) −1 < ∼ 1].…”

“…Thanks to these experimental progress, the two-band BCS-BEC crossover theory has been of particular interest in condensed matter and ultracold atomic physics [12][13][14][15][16][17][18][19][20][21][22]. In particular, the FeSe superconductors involve a multiband configuration which plays a crucial role for the BCS-BEC crossover.…”