We investigate the uniform spin susceptibility ${\ensuremath{\chi}}_{\mathrm{s}}$ 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 ${\ensuremath{\chi}}_{\mathrm{s}}$ exhibits nonmonotonic temperature dependence in the normal state. In particular, ${\ensuremath{\chi}}_{\mathrm{s}}$ 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 ${\ensuremath{\chi}}_{\mathrm{s}}$ 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.We investigate the uniform spin susceptibility ${\ensuremath{\chi}}_{\mathrm{s}}$ 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 ${\ensuremath{\chi}}_{\mathrm{s}}$ exhibits nonmonotonic temperature dependence in the normal state. In particular, ${\ensuremath{\chi}}_{\mathrm{s}}$ 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 ${\ensuremath{\chi}}_{\mathrm{s}}$ 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.

arXiv:1312.1035Physical Review AMar 12, 201437 citations

Spin-Gap Temperature in the BCS-BEC Crossover Regime of an Ultracold Fermi Gas

Hiroyuki Tajima, Takashi Kashimura, Ryo Hanai +2 more

We investigate magnetic properties of an ultracold Fermi gas in the BCS (Bardeen-CooperSchrieffer)-BEC (Bose-Einstein condensation) crossover region. Using an extended T -matrix theory, we calculate the (pseudo)spin susceptibility χ in the normal state. In the crossover region, we show that the formation of preformed pairs naturally leads to non-monotonic temperature dependence of χ. Using this result, we introduce the spin-gap temperature T ∗ as the temperature at which χ takes a maximum value. In the BCS side, this characteristic temperature is found to be close to the temperature at which the density of states at ω = 0 starts to be suppressed by pairing fluctuations. Since the existence of the pseudogap is still controversial in ultracold Fermi gases, our results would be helpful to resolve this fundamental problem.We investigate magnetic properties of an ultracold Fermi gas in the BCS (Bardeen-CooperSchrieffer)-BEC (Bose-Einstein condensation) crossover region. Using an extended T -matrix theory, we calculate the (pseudo)spin susceptibility χ in the normal state. In the crossover region, we show that the formation of preformed pairs naturally leads to non-monotonic temperature dependence of χ. Using this result, we introduce the spin-gap temperature T ∗ as the temperature at which χ takes a maximum value. In the BCS side, this characteristic temperature is found to be close to the temperature at which the density of states at ω = 0 starts to be suppressed by pairing fluctuations. Since the existence of the pseudogap is still controversial in ultracold Fermi gases, our results would be helpful to resolve this fundamental problem.

Asia Pacific Physics Conference (APPC)Mar 19, 20140 citations

Molecular Fraction of the BEC-BCS Crossover State

Guthrie B. Partridge, Ramsey I. Kamar, Kevin E. Strecker +2 more

Bulletin of the American Physical SocietyMay 19, 20050 citations

T

-matrix approximation, we show that ${\ensuremath{\chi}}_{\mathrm{s}}$ exhibits nonmonotonic temperature dependence in the normal state. In particular, ${\ensuremath{\chi}}_{\mathrm{s}}$ 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 ${\ensuremath{\chi}}_{\mathrm{s}}$ 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.