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Physical Review A

Spin-excited oscillations in two-component fermion condensates

Tomoyuki Maruyama, G. F. Bertsch

Published Jan 12, 2006• 16 citations
Publisher

Abstract

We investigate collective spin excitations in two-component fermion condensates with special consideration of unequal populations of the two components. The frequencies of monopole and dipole modes are calculated using Thomas-Fermi theory and the scaling approximation. As the fermion-fermion coupling is varied, the system shows various phases of the spin configuration. We demonstrate that spin oscillations have more sensitivity to the spin phase structures than the density oscillations.

DOI
10.1103/physreva.73.013610

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We investigate collective spin excitations in two-component fermion condensates with special consideration of unequal populations of the two components. The frequencies of monopole and dipole modes are calculated using Thomas-Fermi theory and the scaling approximation. As the fermion-fermion coupling is varied, the system shows various phases of the spin configuration. We demonstrate that spin oscillations have more sensitivity to the spin phase structures than the density oscillations.
In this paper, we have reviewed a range of theoretical works on collective oscillations in two-component ultracold quantum gases. It is shown that in two-component systems, two key collective modes are generated: a density mode and a spin mode. Using different prototypes of cold bosonic systems as examples, it is shown how these modes at certain parameter values can get converted into Goldstone modes: indicating criticality. Additionally, emergence of roton modes (and roton softening, too) is discussed in the context of two component boson-boson and boson-fermion mixtures, with a particular focus on striped phases. In the fermionic front, it is demonstrated that two-species systems lead to population-imbalanced exotic phases like FFLO and breached pair; and the collective excitations carry very important information about those structures: like the amount of imbalance present, exact pairing structure, etc. Since it is often difficult to resolve such structures in direct experiments, a study of the collective oscillations can prove to be very useful as indirect probes.
We investigate the effect of unequal masses of the Bose and Fermi particles on the collective oscillations of the degenerate boson-fermion mixtures. In particular, we consider the monopole and the quadrupole modes of the oscillations and study their characters and the frequencies by using variational-sum-rule approach. We find that for both the modes there exist critical values of the ratio of boson-fermion masses below and above which the character and frequency of the respective modes are significantly different.
We theoretically investigate the collective modes of imbalanced two-component one-dimensional Fermi gases with attractive interactions. This is done for trapped and untrapped systems both at zero and nonzero temperature, using self-consistent mean-field theory and the random phase approximation. We discuss how the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state can be detected and the periodicity of the associated density modulations determined from its collective mode spectrum. We also investigate the accuracy of the single-mode approximation for low-lying collective excitations in a trap by comparing frequencies obtained via sum rules with frequencies obtained from direct collective mode calculations. It is found that, for collective excitations where the atomic clouds of the two spin-species oscillate largely in phase, the single-mode approximation holds well for a large parameter regime. Finally, we investigate the collective mode spectrum obtained by parametric modulation of the coupling constant.