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arXiv:1111.1676 · Solid State Communications

Competition between BCS-pairing and “moth-eaten effect” in BEC–BCS crossover

Guojun Zhu, Monique Combescot

Published Nov 5, 2011• 1 citation
arXivHTMLPDF
DOI
10.1016/j.ssc.2011.10.036

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We extend the well-known Cooper’s problem beyond one pair and study how this dilute limit is connected to the many-pair Bardeen-Cooper-Schrieffer (BCS) condensate. We find that, all over from the dilute to the dense regime of pairs, Pauli blocking induces the same “moth-eaten effect” as the one existing for composite boson excitons. This effect makes the average pair binding energy decrease linearly with pair number, bringing it, in the standard BCS configuration, to half the single-pair value. This proves that, at odds with popular understanding, the BCS gap is far larger than the broken pair energy. The increase comes from Pauli blocking between broken and unbroken pairs. Possible link between our result and the crossover between the Bose-Einstein condensate and BCS condensate is also discussed.
Chapter 8 deals with the Bogoliubov approach to the BCS problem. This approach is of fundamental interest because it gives a mathematical support to the BCS ansatz, which was simply proposed by Bardeen, Cooper, and Schrieffer as a trial wave function. This chapter shows that the BCS ansatz corresponds to the ground-state wave function of the one-body Bogoliubov Hamiltonian constructed from the BCS Hamiltonian. This mathematical support is highly welcome because the BCS ansatz contains the questionable idea that composite bosons condense, like elementary bosons, all into the same state, even if the product of N identical electron pair operators substantially differs from that of N single pair operators when N gets large, owing to the moth-eaten effect induced by the Pauli exclusion principle.
We theoretically study the Josephson effect in ultracold Fermi gases, where the two sides of the Josephson junction are independently tuned to different regions of the Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein condensation (BEC) crossover. Using the nonequilibrium Green's function approach combined with the tunnel Hamiltonian formalism and a mean-field approach, we evaluate the DC and AC Josephson currents throughout the entire crossover region. We calculate the DC Josephson current as a function of interaction strength by tuning both sides of the junction synchronously from the BCS to the BEC regimes, and give the asymptotic expression of the current in the deep BCS and BEC limits. We also study the AC Josephson junction through the interaction-asymmetric junction by fixing the interaction in one reservoir and tuning that of the other one. A peak of the tunneling current is found when one side is fixed in the BCS limit and the other side is tuned into the BEC regime, which corresponds to the interaction-biased Riedel peak. Our results indicate the competition between contributions of increasing pair spectral weight and decreasing chemical potential to Josephson tunneling throughout the BCS-BEC crossover, and demonstrate the realization of the Riedel peak in strong-coupling quantum gases.