Uniform spin susceptibility and spin-gap phenomenon in the BCS-BEC crossover regime of an ultracold Fermi gas

Abstract

We investigate the uniform spin susceptibility ${\chi }_{\mathrm{s}}$ in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Including pairing fluctuations within the framework of an extended $T$-matrix approximation, we show that ${\chi }_{\mathrm{s}}$ exhibits non-monotonic temperature dependence in the normal state. In particular, ${\chi }_{\mathrm{s}}$ is suppressed near the superfluid phase transition temperature $T_{\mathrm{c}}$ due to strong pairing fluctuations. To characterize this anomalous behavior, we introduce the spin-gap temperature $T_{\mathrm{s}}$ as the temperature at which ${\chi }_{\mathrm{s}}$ takes a maximum value. Determining $T_{\mathrm{s}}$ in the whole BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas in terms of the temperature and the strength of a pairing interaction. We also clarify how the spin-gap phenomenon is related to the pseudogap phenomenon appearing in the single-particle density of states. Our results indicate that an ultracold Fermi gas in the BCS-BEC crossover region is a very useful system to examine the pseudogap phenomenon and the spin-gap phenomenon in a unified manner.

19 pages, 9 figures

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• Theory of ultracold Fermi gases
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• Emergence of a pseudogap in the BCS-BEC crossover
• Magic gap ratio for optimally robust fermionic condensation and its implications for high-Tc superconductivity
• The pseudogap regime in the unitary Fermi gas
• Specific heat and effects of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Fermi gas
• Pairing and the spin susceptibility of the polarized unitary Fermi gas in the normal phase
• Mesoscopic spin transport between strongly interacting Fermi gases
• Strong-coupling corrections to spin susceptibility in the BCS-BEC crossover regime of a superfluid Fermi gas
• Hetero pairing and component-dependent pseudogap phenomena in an ultracold Fermi gas with mass imbalance
• Precursor of Superfluidity in a Strongly Interacting Fermi Gas with Negative Effective Range
• QCD-like phase diagram with Efimov trimers and Cooper pairs in resonantly interacting SU(3) Fermi gases
• Spin Susceptibility Above the Superfluid Onset in Ultracold Fermi Gases
• Strong-Coupling and Finite Temperature Effects on $p$-wave Contacts
• Spin susceptibility and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas
• Increasing superconducting $T_c$ by layering in the attractive Hubbard model
• Isothermal compressibility and effects of multi-body molecular interactions in a strongly interacting ultracold Fermi gas
• Spin-dipole mode in a trapped Fermi gas near unitarity
• Specific Heat and Effects of Uniaxial Anisotropy of a $p$-wave Pairing Interaction in a Strongly Interacting Ultracold Fermi Gas
• A lattice pairing-field approach to ultracold Fermi gases
• Strong coupling effects on specific heat in the BCS-BEC crossover
• Spin susceptibility and effects of inhomogeneous strong pairing fluctuations in a trapped ultracold Fermi gas