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arXiv:1703.00585 · Journal of the Physical Society of Japan

Specific Heat and Effects of Uniaxial Anisotropy of ap-Wave Pairing Interaction in a Strongly Interacting Ultracold Fermi Gas

Daisuke Inotani, Pieter van Wyk, Yoji Ohashi

Published Mar 3, 2017• 2 citations
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Abstract

We investigate the specific heat $C_V$ at constant volume and effects of uniaxial anisotropy of a $p$-wave attractive interaction in the normal state of an ultracold Fermi gas. Within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we evaluate this thermodynamic quantity as a function of temperature, in the whole interaction regime. While the uniaxial anisotropy is not crucial for $C_V$ in the weak-coupling regime, $C_V$ is found to be sensitive to the uniaxial anisotropy in the strong-coupling regime. This originates from the population imbalance among $p_i$-wave molecules ($i=x,y,z$), indicating that the specific heat is a useful observable to see which kinds of $p$-wave molecules dominantly exist in the strong-coupling regime when the $p$-wave interaction has uniaxial anisotropy. Using this strong point, we classify the strong-coupling regime into some characteristic regions. Since a $p$-wave pairing interaction with uniaxial anisotropy has been discovered in a $^{40}$K Fermi gas, our results would be useful in considering strong-coupling properties of a $p$-wave interacting Fermi gas, when the interaction is uniaxially anisotropic.

DOI
10.7566/jpsj.86.044301

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We investigate the specific heat CVC_VCV​ at constant volume and effects of uniaxial anisotropy of a ppp-wave attractive interaction in the normal state of an ultracold Fermi gas. Within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we evaluate this thermodynamic quantity as a function of temperature, in the whole interaction regime. While the uniaxial anisotropy is not crucial for CVC_VCV​ in the weak-coupling regime, CVC_VCV​ is found to be sensitive to the uniaxial anisotropy in the strong-coupling regime. This originates from the population imbalance among pip_ipi​-wave molecules (i=x,y,zi=x,y,zi=x,y,z), indicating that the specific heat is a useful observable to see which kinds of ppp-wave molecules dominantly exist in the strong-coupling regime when the ppp-wave interaction has uniaxial anisotropy. Using this strong point, we classify the strong-coupling regime into some characteristic regions. Since a ppp-wave pairing interaction with uniaxial anisotropy has been discovered in a 40^{40}40K Fermi gas, our results would be useful in considering strong-coupling properties of a ppp-wave interacting Fermi gas, when the interaction is uniaxially anisotropic.
We theoretically investigate the specific heat CVC_VCV​ at constant volume in the normal state of a ppp-wave interacting Fermi gas. Including fluctuations in the ppp-wave Cooper channel within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we clarify how CVC_VCV​ as a function of temperature varies, as one moves from the weak-coupling regime to the strong-coupling limit. In the weak-coupling regime, CVC_VCV​ is shown to be enhanced by ppp-wave pairing fluctuations, near the superfluid phase transition temperature TcT_{\rm c}Tc​. Similar enhancement of CV(T≃Tc)C_V(T\simeq T_{\rm c})CV​(T≃Tc​) is also obtained in the strong-coupling regime, which, however, reflects that system is close an ideal Bose gas of ppp-wave two-body bound molecules. Using these results, we classify the normal state into (1) the normal Fermi gas regime, (2) the ppp-wave molecular Bose gas regime, and (3) the region between the two, where ppp-wave pairing fluctuations are dominant. Since the current experiments can only access the normal phase of a ppp-wave interacting Fermi gas, our results would be useful for experiments to understand strong-coupling properties of this Fermi system above TcT_{\rm c}Tc​.
We investigate the specific heat CVC_VCV​ at constant volume in the normal state of a ppp-wave interacting Fermi gas. Including ppp-wave pairing fluctuations within the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that, in the weak-coupling side, CVC_VCV​ exhibits a dip-hump behavior as a function of the temperature. While the dip is associated with the pseudogap phenomenon near TcT_{\rm c}Tc​, the hump structure is found to come from the suppression of Fermi quasiparticle scattering into a ppp-wave molecular state in the Fermi degenerate regime. Since the latter phenomenon does not occur in the ordinary sss-wave interacting Fermi gas, it may be viewed as a characteristic phenomenon associated with a ppp-wave pairing interaction.
We theoretically investigate the specific heat at constant volume C V in the BCS(Bardeen-Cooper-Schrieffer)-BEC(Bose-Einstein-condensation)-crossover regime of an ultracold Fermi gas, below the superfluid phase transition temperature T c . Within the strong-coupling framework developed by Nozières and Schmitt-Rink, we show that the temperature dependence of C V drastically changes as one passes through the crossover region, and is sensitive to strong fluctuations in the Cooper channel near the unitarity limit. We also compare our results to a recent experiment on a 6 Li unitary Fermi gas. Since fluctuation effects are a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how the fermionic BCS superfluid changes into BEC with increasing the interaction strength, from the viewpoint of specific heat.
We investigate the specific heat at constant volume CV{C}_{V}CV​ in the Bardeen-Cooper-Schrieffer--Bose-Einstein-condensate (BCS-BEC)-crossover regime of an ultracold Fermi gas above the superfluid phase transition temperature Tc{T}_{\mathrm{c}}Tc​. Within the framework of the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink, we show that this thermodynamic quantity is sensitive to the stability of preformed Cooper pairs. That is, while ${C}_{V}(T\ensuremath{\gtrsim}{T}_{\mathrm{c}})$ in the unitary regime is remarkably enhanced by metastable preformed Cooper pairs or pairing fluctuations, it is well described by that of an ideal Bose gas of long-lived stable molecules in the strong-coupling BEC regime. Using these results, we identify the region where the system may be viewed as an almost ideal Bose gas of stable pairs, as well as the pseudogap regime where the system is dominated by metastable preformed Cooper pairs, in the phase diagram of an ultracold Fermi gas with respect to the strength of a pairing interaction and the temperature. We also show that the calculated specific heat agrees with the recent experiment on a 6Li^{6}\text{Li}6Li unitary Fermi gas. Since the formation of preformed Cooper pairs is a crucial key in the BCS-BEC-crossover phenomenon, our results would be helpful in considering how fluctuating preformed Cooper pairs appear in a Fermi gas to eventually become stable as one passes through the BCS-BEC-crossover region.
We investigate theoretically strong-coupling and finite-temperature effects on the ppp-wave contacts, as well as the asymptotic behavior of the momentum distribution in the large-momentum region in a one-component Fermi gas with a tunable ppp-wave interaction. Including ppp-wave pairing fluctuations within a strong-coupling theory, we calculate the ppp-wave contacts above the superfluid transition temperature Tc{T}_{\mathrm{c}}Tc​ from the adiabatic energy relations. We show that, while the ppp-wave contact related to the scattering volume monotonically increases with increasing interaction strength, the one related to the effective range depends nonmonotonically on interaction strength and its sign changes in the intermediate-coupling regime. The nonmonotonic interaction dependence of these quantities is shown to originate from the competition between the increase of the cutoff momentum and the decrease of the coupling constant of the ppp-wave interaction with increasing effective range. We also analyze the asymptotic form of the momentum distribution in the large-momentum region. In contrast to the conventional sss-wave case, we show that the asymptotic behavior cannot be completely described by only the ppp-wave contacts, and the extra terms, which are not related to the thermodynamic properties, appear. Furthermore, in the high-temperature region, we find that the extra terms dominate the subleading term of the large-momentum distribution. We also directly compare our results with the recent experimental measurement by including the effects of a harmonic trap potential within the local density approximation. We show that, while our model qualitatively explains the dependence on the interaction strength of the ppp-wave contacts, there are some quantitative disagreements between our theory and recent experiment; for example, the peak position of the ppp-wave contact related to the effective range. Since the ppp-wave contacts connect the microscopic properties to the thermodynamic properties of the system, our results would be helpful for further understanding many-body properties of this anisotropic interacting Fermi gas in the normal phase.
We investigate single-particle properties of a one-component Fermi gas with a tunable p-wave interaction. Including pairing fluctuations associated with this anisotropic interaction within a TTT-matrix theory, we calculate the single-particle density of states, as well as the spectral weight, above the superfluid transition temperature TcT_{\rm c}Tc​. Starting from the weak-coupling regime, we show that the so-called pseudogap first develops in these quantities with increasing the interaction strength. However, when the interaction becomes strong to some extent, the pseudogap becomes obscure to eventually disappear in the strong-coupling regime. This non-monotonic interaction dependence is quite different from the case of an s-wave interaction, where the pseudogap simply develops with increasing the interaction strength. The difference between the two cases is shown to originate from the momentum dependence of the p-wave interaction, which vanishes in the low momentum limit. We also identify the pseudogap regime in the phase diagram with respect to the temperature and the p-wave interaction strength. Since the pseudogap is a precursor phenomenon of the superfluid phase transition, our results would be useful for the research toward the realization of p-wave superfluid Fermi gases.