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International Journal of Theoretical Physics

Quantum binding of the BCS interaction model

M. Casas, Mauricio Fortes, MANUEL DE LLANO, Antonio Puente, M. A. Solís

Published May 1, 1995• 3 citations
Publisher
DOI
10.1007/bf00671018

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This dissertation presents the theoretical study of two-component fermionic gases. It firstly studies the BCS-BEC crossover for the system of the two-component fermionic gas in optical lattices. If the system were loaded into deep three-dimensional optical lattices, a tight-binding model with the lowest band is applicable. If the band is more than half-filled and by increasing the attractive interactions, then there exists a novel crossover, from a paired BCS superfluid to a BEC of holes, back to the BCS superfluid, and finally to a conventional BEC regime of diatomic molecules. If the band is less than half-filled, the crossover is similar to that without optical lattices. If the band is fully filled, a quantum phase transition from band insulator to BCS-BEC superfluid occurs. Then, this dissertation studies a model describing the one-dimensional Feshbach resonance. Because this model satisfies the Yang-Baxter equations, it was assumed that the Bethe Ansatz could solve this model. However, this dissertation demonstrates that it is not integrable by calculating the chemical potential from the Bethe Ansatz, and by studying the scattering between the bosons. To the researcher's knowledge, this model is the first continuum model that satisfies the Yang-Baxter equations, but is not integrable. After that, this dissertation studies the quenches in the system of paired fermionic superfluids. If the system is slightly driven from its equilibrium state, the asymptotic behaviors of this small oscillation are calculated with both s- and p-wave interactions, and in both the BCS and the BEC regimes, under the mean field collisionless approximation.

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The Hubbard model with occupation-dependent hopping rate exhibits superconductivity in a wide range of parameters within mean-field (BCS) theory. Here we study pair binding energies in this model for small clusters by exact diagonalization of the Hamiltonian. The model is defined by on-site and nearest-neighbor repulsions U and V, and occupation-dependent hopping rate t(n)=t+n\ensuremath{\Delta}t. We present results for one-dimensional chains and for two-dimensional square lattices of sizes 4\ifmmode\times\else\texttimes\fi{}4, 6\ifmmode\times\else\texttimes\fi{}6, and 8\ifmmode\times\else\texttimes\fi{}8. As a function of carrier density n the pair binding energy first increases and then decreases, and vanishes beyond a critical density. BCS results for the pair binding energy are found to be in remarkably good agreement with the exact results. In particular, BCS theory accurately reproduces the range of interaction parameters where pair binding exists in the exact solution. The model is found to exhibit no tendency to phase separation or clustering of more than two particles even for parameters giving rise to strong pair binding, in contrast to other tight-binding models where pairing occurs.
We study the thermodynamics of a two-species Feshbach-resonant atomic Fermi gas in a periodic potential, focusing in a deep optical potential where a tight binding model is applicable. We show that for a more than half-filled band the gas exhibits a reentrant crossover with decreased detuning (increased attractive interaction), from a paired BCS superfluid to a Bose-Einstein condensate (BEC) of molecules of holes, back to the BCS superfluid, and finally to a conventional BEC of diatomic molecules. This behavior is associated with the nonmonotonic dependence of the chemical potential on detuning and the concomitant Cooper-pair or molecular size, larger in the BCS and smaller in the BEC regimes. For a single filled band we find a quantum phase transition from a band insulator to a BCS-BEC superfluid, and map out the corresponding phase diagram.
This dissertation presents the theoretical study of two-component fermionic gases. It firstly studies the BCS-BEC crossover for the system of the two-component fermionic gas in optical lattices. If the system were loaded into deep three-dimensional optical lattices, a tight-binding model with the lowest band is applicable. If the band is more than half-filled and by increasing the attractive interactions, then there exists a novel crossover, from a paired BCS superfluid to a BEC of holes, back to the BCS superfluid, and finally to a conventional BEC regime of diatomic molecules. If the band is less than half-filled, the crossover is similar to that without optical lattices. If the band is fully filled, a quantum phase transition from band insulator to BCS-BEC superfluid occurs. Then, this dissertation studies a model describing the one-dimensional Feshbach resonance. Because this model satisfies the Yang-Baxter equations, it was assumed that the Bethe Ansatz could solve this model. However, this dissertation demonstrates that it is not integrable by calculating the chemical potential from the Bethe Ansatz, and by studying the scattering between the bosons. To the researcher's knowledge, this model is the first continuum model that satisfies the Yang-Baxter equations, but is not integrable. After that, this dissertation studies the quenches in the system of paired fermionic superfluids. If the system is slightly driven from its equilibrium state, the asymptotic behaviors of this small oscillation are calculated with both s- and p-wave interactions, and in both the BCS and the BEC regimes, under the mean field collisionless approximation.