The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Yoji Ohashi - One of the best experts on this subject based on the ideXlab platform.
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superfluid properties of an ultracold fermi gas with an orbital feshbach resonance in the bcs bec Crossover Region
Physical Review A, 2021Co-Authors: Taro Kamihori, Daichi Kagamihara, Yoji OhashiAbstract:We theoretically investigate the superfluid properties of a two-band gas of $^{173}\mathrm{Yb}$ Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC Crossover Region, we include superfluid fluctuations caused by interband and intraband pairing interactions associated with OFR by extending the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, the effects of an experimentally inaccessible deep bound state are removed to model a real $^{173}\mathrm{Yb}$ Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced interband interaction. In addition to these superfluid quantities, we also examine collective modes, such as the Goldstone mode, the Schmid (Higgs) mode, and Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multiband superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC Crossover Region.
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nonequilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
Physical Review A, 2020Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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kovtun son starinets conjecture and effects of mass imbalance in the normal state of an ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Daichi Kagamihara, Yoji OhashiAbstract:We theoretically assess the conjecture proposed by Kovtun, Son, and Starinets, stating that the ratio $\eta/s$ of the shear viscosity $\eta$ to the entropy density $s$ has the lower bound as $\eta/s\ge\hbar/(4\pi k_{\mathrm{B}})$. In the normal state of a mass-imbalanced ultracold Fermi gas, consistently including strong-coupling corrections to both $\eta$ and $s$ within the self-consistent $T$-matrix approximation, we evaluate $\eta/s$ over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) Crossover Region, in the presence of mass imbalance. We find that $\eta/s$ achieves the minimum value $4.5\times \hbar/(4\pi k_{\mathrm{B}})$, not at the unitarity, but slightly in the BEC regime, $(k_{\mathrm{F}}a_s)^{-1}\simeq 0.4>0$ (where $a_s$ is the $s$-wave scattering length, and $k_{\mathrm{F}}$ is the Fermi momentum). In contract to the previous expectation, we find that this lower bound is almost independent of mass imbalance: Our results predict that all the mass-balanced $^6$Li-$^6$Li and $^{40}$K-$^{40}$K mixtures and the mass-imbalanced $^{40}$K-$^{161}$Dy mixture give almost the same lower bound of $\eta/s$. We also point out that the two quantum phenomena, Pauli blocking and bound-state formation, are crucial keys for the lower bound of $\eta/s$.
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non equilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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specific heat and effects of strong pairing fluctuations in a superfluid fermi atom gas in the bcs bec Crossover Region
28th International Conference on Low Temperature Physics LT 2018, 2018Co-Authors: P Van Wyk, Daisuke Inotani, Yoji OhashiAbstract: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 Nozieres 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 6Li 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.
Daichi Kagamihara - One of the best experts on this subject based on the ideXlab platform.
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superfluid properties of an ultracold fermi gas with an orbital feshbach resonance in the bcs bec Crossover Region
Physical Review A, 2021Co-Authors: Taro Kamihori, Daichi Kagamihara, Yoji OhashiAbstract:We theoretically investigate the superfluid properties of a two-band gas of $^{173}\mathrm{Yb}$ Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC Crossover Region, we include superfluid fluctuations caused by interband and intraband pairing interactions associated with OFR by extending the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, the effects of an experimentally inaccessible deep bound state are removed to model a real $^{173}\mathrm{Yb}$ Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced interband interaction. In addition to these superfluid quantities, we also examine collective modes, such as the Goldstone mode, the Schmid (Higgs) mode, and Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multiband superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC Crossover Region.
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nonequilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
Physical Review A, 2020Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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kovtun son starinets conjecture and effects of mass imbalance in the normal state of an ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Daichi Kagamihara, Yoji OhashiAbstract:We theoretically assess the conjecture proposed by Kovtun, Son, and Starinets, stating that the ratio $\eta/s$ of the shear viscosity $\eta$ to the entropy density $s$ has the lower bound as $\eta/s\ge\hbar/(4\pi k_{\mathrm{B}})$. In the normal state of a mass-imbalanced ultracold Fermi gas, consistently including strong-coupling corrections to both $\eta$ and $s$ within the self-consistent $T$-matrix approximation, we evaluate $\eta/s$ over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) Crossover Region, in the presence of mass imbalance. We find that $\eta/s$ achieves the minimum value $4.5\times \hbar/(4\pi k_{\mathrm{B}})$, not at the unitarity, but slightly in the BEC regime, $(k_{\mathrm{F}}a_s)^{-1}\simeq 0.4>0$ (where $a_s$ is the $s$-wave scattering length, and $k_{\mathrm{F}}$ is the Fermi momentum). In contract to the previous expectation, we find that this lower bound is almost independent of mass imbalance: Our results predict that all the mass-balanced $^6$Li-$^6$Li and $^{40}$K-$^{40}$K mixtures and the mass-imbalanced $^{40}$K-$^{161}$Dy mixture give almost the same lower bound of $\eta/s$. We also point out that the two quantum phenomena, Pauli blocking and bound-state formation, are crucial keys for the lower bound of $\eta/s$.
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non equilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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zero temperature properties of a strongly interacting superfluid fermi gas in the bcs bec Crossover Region
Journal of Low Temperature Physics, 2017Co-Authors: Hiroyuki Tajima, P Van Wyk, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Munekazu Horikoshi, Yoji OhashiAbstract:We investigate thermodynamic properties and effects of quantum fluctuations in the Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) Crossover Region of a superfluid Fermi gas in the low-temperature limit. Including strong-coupling corrections within the framework of an extended T-matrix approximation, we numerically compute the isothermal compressibility $$\chi _n$$ . While quantum fluctuation effects on $$\chi _n$$ in the strong-coupling BEC regime are explained by the quantum depletion due to a repulsive interaction between tightly bound molecules, effects of self-energy shift on the Fermi chemical potential are found to enhance $$\chi _n$$ in the weak-coupling BCS Region. We also show that the calculated $$\chi _n$$ agrees well with the recent experiment on a $$^6$$ Li Fermi gas done from the weak-coupling Region to the unitarity limit. Our result would be useful for the study of many-body quantum corrections in the BCS–BEC Crossover Region of a strongly interacting Fermi superfluid.
Ryo Hanai - One of the best experts on this subject based on the ideXlab platform.
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nonequilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
Physical Review A, 2020Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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non equilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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dynamical instability of a driven dissipative electron hole condensate in the bcs bec Crossover Region
Physical Review B, 2017Co-Authors: Ryo Hanai, P B Littlewood, Yoji OhashiAbstract:We present a stability analysis on a driven-dissipative electron-hole condensate in the BCS (Bardeen-Cooper-Schrieffer)--BEC (Bose-Einstein condensation) Crossover Region. Extending the combined BCS-Leggett theory with the generalized random phase approximation to the nonequilibrium case by employing the Keldysh formalism, we show that the pumping and decay of carriers causes a depairing effect on excitons. This phenomenon gives rise to an attractive interaction between excitons in the BEC regime, as well as a supercurrent that anomalously flows antiparallel to $\mathbf{\ensuremath{\nabla}}\ensuremath{\theta}(\mathbit{r})$ [where $\ensuremath{\theta}(\mathbit{r})$ is the phase of the condensate] in the BCS regime, both leading to dynamical instabilities of an exciton BEC. Our results suggest that a substantial Region of the exciton-BEC phase in the phase diagram (in terms of the interaction strength and the decay rate) is unstable.
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zero temperature properties of a strongly interacting superfluid fermi gas in the bcs bec Crossover Region
Journal of Low Temperature Physics, 2017Co-Authors: Hiroyuki Tajima, P Van Wyk, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Munekazu Horikoshi, Yoji OhashiAbstract:We investigate thermodynamic properties and effects of quantum fluctuations in the Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) Crossover Region of a superfluid Fermi gas in the low-temperature limit. Including strong-coupling corrections within the framework of an extended T-matrix approximation, we numerically compute the isothermal compressibility $$\chi _n$$ . While quantum fluctuation effects on $$\chi _n$$ in the strong-coupling BEC regime are explained by the quantum depletion due to a repulsive interaction between tightly bound molecules, effects of self-energy shift on the Fermi chemical potential are found to enhance $$\chi _n$$ in the weak-coupling BCS Region. We also show that the calculated $$\chi _n$$ agrees well with the recent experiment on a $$^6$$ Li Fermi gas done from the weak-coupling Region to the unitarity limit. Our result would be useful for the study of many-body quantum corrections in the BCS–BEC Crossover Region of a strongly interacting Fermi superfluid.
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pseudogap phenomena near the bkt transition of a two dimensional ultracold fermi gas in the Crossover Region
arXiv: Quantum Gases, 2016Co-Authors: Morio Matsumoto, Ryo Hanai, Daisuke Inotani, Yoji OhashiAbstract:We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal phase. In the three-dimensional case, it has been shown that the so-called pseudogap phenomena can be well described by a (non-self-consistent) $T$-matrix approximation (TMA). In the two-dimensional case, while this strong coupling theory can explain the pseudogap phenomenon in the strong-coupling regime, it unphysically gives large pseudogap size in the Crossover Region, as well as in the weak-coupling regime. We show that this difficulty can be overcome when one improve TMA to include higher order pairing fluctuations within the framework of a self-consistent $T$-matrix approximation (SCTMA). The essence of this improvement is also explained. Since the observation of the BKT transition has recently been reported in a two-dimensional $^6$Li Fermi gas, our results would be useful for the study of strong-coupling physics associated with this quasi-long-range order.
Philipp Werner - One of the best experts on this subject based on the ideXlab platform.
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out of time ordered correlators of the hubbard model sachdev ye kitaev strange metal in the spin freezing Crossover Region
Physical Review B, 2019Co-Authors: Naoto Tsuji, Philipp WernerAbstract:The Sachdev-Ye-Kitaev (SYK) model describes a strange metal that shows peculiar non-Fermi liquid properties without quasiparticles. It exhibits a maximally chaotic behavior characterized by out-of-time-ordered correlators (OTOCs), and is expected to be a holographic dual to black holes. Recently, a striking similarity between the SYK model and the Hund-coupling induced spin-freezing Crossover in multi-orbital Hubbard models has been pointed out. To further explore this connection, we study OTOCs for fermionic Hubbard models, which are prototypical models for strongly correlated electrons in solids. We introduce an imaginary-time four-point correlation function with an appropriate time ordering, which by means of the spectral representation and the out-of-time-order fluctuation-dissipation theorem can be analytically continued to real-time OTOCs. Based on this approach, we numerically evaluate real-time OTOCs for Hubbard models in the thermodynamic limit, using the dynamical mean-field theory in combination with a numerically exact continuous-time Monte Carlo impurity solver. The results for the single-orbital model show that a certain spin-related OTOC captures local moment formation in the vicinity of the metal-insulator transition, while the self-energy does not show SYK-like non-Fermi liquid behavior. On the other hand, for the two- and three-orbital models with nonzero Hund coupling we find that the OTOC exhibits a rapid damping at short times and an approximate power-law decay at longer times in the spin-freezing Crossover regime characterized by fluctuating local moments and a non-Fermi liquid self-energy $\Sigma(\omega) \sim \sqrt{\omega}$. These results are in a good agreement with the behavior of the SYK model, providing firm evidence for the close relation between the spin-freezing Crossover physics of multi-orbital Hubbard models and the SYK strange metal.
Daisuke Inotani - One of the best experts on this subject based on the ideXlab platform.
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nonequilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
Physical Review A, 2020Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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non equilibrium strong coupling theory for a driven dissipative ultracold fermi gas in the bcs bec Crossover Region
arXiv: Quantum Gases, 2019Co-Authors: Taira Kawamura, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Yoji OhashiAbstract:We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC Crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.
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specific heat and effects of strong pairing fluctuations in a superfluid fermi atom gas in the bcs bec Crossover Region
28th International Conference on Low Temperature Physics LT 2018, 2018Co-Authors: P Van Wyk, Daisuke Inotani, Yoji OhashiAbstract: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 Nozieres 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 6Li 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.
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zero temperature properties of a strongly interacting superfluid fermi gas in the bcs bec Crossover Region
Journal of Low Temperature Physics, 2017Co-Authors: Hiroyuki Tajima, P Van Wyk, Ryo Hanai, Daichi Kagamihara, Daisuke Inotani, Munekazu Horikoshi, Yoji OhashiAbstract:We investigate thermodynamic properties and effects of quantum fluctuations in the Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) Crossover Region of a superfluid Fermi gas in the low-temperature limit. Including strong-coupling corrections within the framework of an extended T-matrix approximation, we numerically compute the isothermal compressibility $$\chi _n$$ . While quantum fluctuation effects on $$\chi _n$$ in the strong-coupling BEC regime are explained by the quantum depletion due to a repulsive interaction between tightly bound molecules, effects of self-energy shift on the Fermi chemical potential are found to enhance $$\chi _n$$ in the weak-coupling BCS Region. We also show that the calculated $$\chi _n$$ agrees well with the recent experiment on a $$^6$$ Li Fermi gas done from the weak-coupling Region to the unitarity limit. Our result would be useful for the study of many-body quantum corrections in the BCS–BEC Crossover Region of a strongly interacting Fermi superfluid.
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pseudogap phenomena near the bkt transition of a two dimensional ultracold fermi gas in the Crossover Region
arXiv: Quantum Gases, 2016Co-Authors: Morio Matsumoto, Ryo Hanai, Daisuke Inotani, Yoji OhashiAbstract:We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal phase. In the three-dimensional case, it has been shown that the so-called pseudogap phenomena can be well described by a (non-self-consistent) $T$-matrix approximation (TMA). In the two-dimensional case, while this strong coupling theory can explain the pseudogap phenomenon in the strong-coupling regime, it unphysically gives large pseudogap size in the Crossover Region, as well as in the weak-coupling regime. We show that this difficulty can be overcome when one improve TMA to include higher order pairing fluctuations within the framework of a self-consistent $T$-matrix approximation (SCTMA). The essence of this improvement is also explained. Since the observation of the BKT transition has recently been reported in a two-dimensional $^6$Li Fermi gas, our results would be useful for the study of strong-coupling physics associated with this quasi-long-range order.