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arXiv:1710.01479 · Journal of the Physical Society of Japan

Pseudogap Regime of a Two-dimensional Uniform Fermi Gas

Morio Matsumoto, Ryo Hanai, Daisuke Inotani, Yoji Ohashi

Published Dec 19, 2017• 3 citations
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Abstract

We investigate pseudogap phenomena in a two-dimensional Fermi gas. Including pairing fluctuations within a self-consistent $T$-matrix approximation, we determine the pseudogap temperature $T^*$ below which a dip appears in the density of states $ρ(ω)$ around the Fermi level. Evaluating $T^*$, we identify the pseudogap region in the phase diagram of this system. We find that, while the observed BKT (Berezinskii-Kosterlitz-Thouless) transition temperature $T^{\rm exp}_{\rm BKT}$ in a $^6$Li Fermi gas is in the pseudogap regime, the detailed pseudogap structure in $ρ(ω)$ at $T^{\rm exp}_{\rm BKT}$ still differs from a fully-gapped one, indicating the importance of amplitude fluctuations in the Cooper channel there. Since the observed $T^{\rm exp}_{\rm BKT}$ in the weak-coupling regime cannot be explained by the recent BKT theory which only includes phase fluctuations, our results may provide a hint about how to improve this BKT theory. Although $ρ(ω)$ has not been measured in this system, we show that the assessment of our results is still possible by using the observable Tan's contact.

DOI
10.7566/jpsj.87.014301

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We investigate pseudogap phenomena in a two-dimensional Fermi gas. Including pairing fluctuations within a self-consistent TTT-matrix approximation, we determine the pseudogap temperature T∗T^*T∗ below which a dip appears in the density of states ρ(ω)ρ(ω)ρ(ω) around the Fermi level. Evaluating T∗T^*T∗, we identify the pseudogap region in the phase diagram of this system. We find that, while the observed BKT (Berezinskii-Kosterlitz-Thouless) transition temperature TBKTexpT^{\rm exp}_{\rm BKT}TBKTexp​ in a 6^66Li Fermi gas is in the pseudogap regime, the detailed pseudogap structure in ρ(ω)ρ(ω)ρ(ω) at TBKTexpT^{\rm exp}_{\rm BKT}TBKTexp​ still differs from a fully-gapped one, indicating the importance of amplitude fluctuations in the Cooper channel there. Since the observed TBKTexpT^{\rm exp}_{\rm BKT}TBKTexp​ in the weak-coupling regime cannot be explained by the recent BKT theory which only includes phase fluctuations, our results may provide a hint about how to improve this BKT theory. Although ρ(ω)ρ(ω)ρ(ω) has not been measured in this system, we show that the assessment of our results is still possible by using the observable Tan's contact.
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We investigate single-particle excitations and strong-coupling effects in a twodimensional Fermi gas. Including pairing fluctuations within a Gaussian fluctuation theory, we calculate the density of states ρ ( ω ) near the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature T BKT . Near T BKT , we show that superfluid fluctuations induce a pseudogap in ρ (ω). The pseudogap structure is very similar to the BCS superfluid density of states, although the superfluid order parameter is absent in the present two-dimensional case. Since a twodimensional 40 K Fermi gas has recently been realized, our results would contribute to the understanding of single-particle properties near the BKT instability.
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We investigate strong-coupling properties of a unitary Fermi gas consisting of two different species with different masses. Including pairing fluctuations within the self-consistent T -matrix approximation, we calculate the single-particle density of states in the normal state. We show that the pseudogap phenomenon, which is characterized by a dip structure in the density of states around ω = 0, appears more remarkably in the light mass component than in the heavy mass component. As a result, the pseudogap temperature, which is determined as the temperature at which the pseudogap disappears in the density of states, is higher in the former than in the latter. We also find that this different pseudogap temperatures lead to the existence of two kinds of pseudogap regions. That is, one is the ordinary pseudogap regime where the pseudogap appears in both the component, and the other case is that the light mass component only exhibits the pseudogap phenomenon. As the origin of these component- dependent pseudogap phenomena, we point out the importance of different Fermi temperatures between the two components. Since the formation of hetero-Cooper pairs and their condensation are expected in various systems, such as a 6 Li- 40 K Fermi gas mixture, an exciton (polariton) gas, as well as color superconductivity, our results would be useful for the understanding of strong-coupling properties of these novel Fermi condensates.
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