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Search Results: 1 - 10 of 4264 matches for " Takashi Kashimura "
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Pseudogap phenomenon and effects of population imbalance in the normal state of a unitary Fermi gas
Takashi Kashimura,Ryota Watanabe,Yoji Ohashi
Physics , 2013, DOI: 10.1103/PhysRevA.89.013618
Abstract: We investigate strong-coupling corrections to single-particle excitations in the normal state of a spin-polarized unitary Fermi gas. Within the framework of an extended T-matrix approximation, we calculate the single-particle density of states, as well as the single-particle spectral weight, to show that the so-called pseudogap phenomenon gradually disappears with increasing the magnitude of an effective magnetic field. In the highly spin-polarized regime, the calculated spin-polarization rate as a function of the effective magnetic field agrees well with the recent experiment on a 6Li Fermi gas. Although this experiment has been considered to be incompatible with the existence of the pseudogap in an unpolarized Fermi gas, our result clarifies that the observed spin-polarization rate in the highly spin-polarized regime and the pseudogap in the unpolarized limit can be explained in a consistent manner, when one correctly includes effects of population imbalance on single-particle excitations. Since it is a crucial issue to clarify whether the pseudogap exists or not in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas, our results would be useful for the understanding of this strongly interacting fermion system.
Superfluid/ferromagnet/superfluid-junction and $π$-phase in a superfluid Fermi gas at finite temperatures
Takashi Kashimura,Shunji Tsuchiya,Yoji Ohashi
Physics , 2011,
Abstract: We investigate the stability of $\pi$-phase in a polarized superfluid Fermi gas ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms in the hyperfine state described by pseudospin-$\sigma$). In our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A {\bf 82}, 033617 (2010)], we showed that excess atoms ($\Delta N=N_\uparrow-N_\downarrow$) localized around a potential barrier embedded in the system induces the $\pi$-phase at T=0, where the phase of superfluid order parameter differ by $\pi$ across the junction. In this paper, we extend our previous work to include temperature effects within the mean-field theory. We show that the $\pi$-phase is stable even at finite temperatures, although transition from the $\pi$-phase to 0-phase eventually occurs at a certain temperature. Our results indicate that the $\pi$-phase is experimentally accessible in cold Fermi gases.
Superfluid/ferromagnet/superfluid junction and $π$-phase in a superfluid Fermi gas
Takashi Kashimura,Shunji Tsuchiya,Yoji Ohashi
Physics , 2010, DOI: 10.1103/PhysRevA.82.033617
Abstract: We investigate the possibility of superfluid/ferromagnet/superfluid (SFS)-junction in a superfluid Fermi gas. To examine this possibility in a simple manner, we consider an attractive Hubbard model at $T=0$ within the mean-field theory. When a potential barrier is embedded in a superfluid Fermi gas with population imbalance ($N_\uparrow>N_\downarrow$, where $N_\sigma$ is the number of atoms with pseudospin $\sigma=\uparrow,\downarrow$), this barrier is shown to be {\it magnetized} in the sense that excess $\uparrow$-spin atoms are localized around it. The resulting superfluid Fermi gas is spatially divided into two by this {\it ferromagnet}, so that one obtains a junction similar to the superconductor/ferromagnet/superconductor-junction discussed in superconductivity. Indeed, we show that the so-called $\pi$-phase, which is a typical phenomenon in the SFS-junction, is realized, where the superfluid order parameter changes its sign across the junction. Our results would be useful for the study of magnetic effects on fermion superfluidity using an ultracold Fermi gas.
$π$-junction and spontaneous current state in a superfluid Fermi gas
Takashi Kashimura,Shunji Tsuchiya,Yoji Ohashi
Physics , 2011, DOI: 10.1103/PhysRevA.84.013609
Abstract: We discuss an idea to realize a spontaneous current in a superfluid Fermi gas. When a polarized Fermi superfluid ($N_\up > N_\dwn$, where $N_\sigma$ is the number of atoms in the hyperfine state described by pseudospin $\sigma=\uparrow, \downarrow$.) is loaded onto a ring-shaped trap with a weak potential barrier, some of excess atoms ($\Delta N=N_\uparrow-N_\downarrow$) are localized around the barrier. As shown in our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A \textbf{82}, 033617 (2010)], this polarized potential barrier works as a $\pi$-junction in the sense that the superfluid order parameter changes its sign across the barrier. Because of this, the phase of the superfluid order parameter outside the junction is shown to be twisted by $\pi$ along the ring, which naturally leads to a circulating supercurrent. While the ordinary supercurrent state is obtained as a metastable state, this spontaneous current state is shown to be more stable than the case with no current. Our results indicate that localized excess atoms would be useful for the manipulation of the superfluid order parameter in cold Fermi gases.
Spin susceptibility and fluctuation corrections in the BCS-BEC crossover regime of an ultracold Fermi gas
Takashi Kashimura,Ryota Watanabe,Yoji Ohashi
Physics , 2012, DOI: 10.1103/PhysRevA.86.043622
Abstract: We investigate magnetic properties and effects of pairing fluctuations in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Recently, Liu and Hu, and Parish, pointed out that the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink (NSR), which has been extensively used to successfully clarify various physical properties of cold Fermi gases, unphysically gives negative spin susceptibility in the BCS-BEC crossover region. The same problem is found to also exist in the ordinary non-self-consistent T-matrix approximation. In this paper, we clarify that this serious problem comes from incomplete treatment in term of pseudogap phenomena originating from strong pairing fluctuations, as well as effects of spin fluctuations on the spin susceptibility. Including these two key issues, we construct an extended T-matrix theory which can overcome this problem. The resulting positive spin susceptibility agrees well with the recent experiment on a 6Li Fermi gas done by Sanner and co-workers. We also apply our theory to a polarized Fermi gas to examine the superfluid phase transition temperature Tc, as a function of the polarization rate. Since the spin susceptibility is an important physical quantity, especially in singlet Fermi superfluids, our results would be useful in considering how singlet pairs appear above and below Tc in the BCS-BEC crossover regime of cold Fermi gases.
Magnetic properties and strong-coupling corrections in an ultracold Fermi gas with population imbalance
Takashi Kashimura,Ryota Watanabe,Yoji Ohashi
Physics , 2012, DOI: 10.1007/s10909-012-0724-2
Abstract: We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by Nozieres and Schmitt-Rink (which is frequently referred to as the NSR theory, or Gaussian fluctuation theory) is known to give unphysical results in the BCS-BEC crossover region. We point out that this problem comes from how to treat pseudogap effects originating from pairing fluctuations and many-body corrections to the spin susceptibility. We also clarify how to overcome this problem by including higher order fluctuations beyond the ordinary T-matrix theory. Calculated spin susceptibility based on our extended T-matrix theory agrees well with the recent experiment on a 6Li Fermi gas.
Excitation properties and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas
Ryo Hanai,Takashi Kashimura,Ryota Watanabe,Daisuke Inotani,Yoji Ohashi
Physics , 2013, DOI: 10.1103/PhysRevA.88.053621
Abstract: We investigate single-particle properties of a mass-imbalanced Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. In the presence of mass imbalance, we point out that the ordinary $T$-matrix approximation, which has been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically gives a double-valued solution in terms of the superfluid phase transition temperature $T_{\rm c}$ in the crossover region. To overcome this serious problem, we include higher order strong-coupling corrections beyond the $T$-matrix level. Using this extended $T$-matrix theory, we calculate single-particle excitations in the normal state above $T_{\rm c}$. The so-called pseudogap phenomena originating from pairing fluctuations are shown to be different between the light mass component and heavy mass component, which becomes more remarkable at higher temperatures. Since Fermi condensates with hetero-Cooper pairs have recently been discussed in various fields, such as exciton (polariton) condensates, as well as color superconductivity, our results would be useful for the further development of Fermi superfluid physics, beyond the conventional superfluid state with homo-Cooper pairs.
Superfluid phase transition and strong-coupling effects in an ultracold Fermi gas with mass imbalance
Ryo Hanai,Takashi Kashimura,Ryota Watanabe,Daisuke Inotani,Yoji Ohashi
Physics , 2012, DOI: 10.1007/s10909-012-0738-9
Abstract: We investigate the superfluid phase transition and effects of mass imbalance in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an cold Fermi gas. We point out that the Gaussian fluctuation theory developed by Nozi\`eres and Schmitt-Rink and the $T$-matrix theory, that are now widely used to study strong-coupling physics of cold Fermi gases, give unphysical results in the presence of mass imbalance. To overcome this problem, we extend the $T$-matrix theory to include higher-order pairing fluctuations. Using this, we examine how the mass imbalance affects the superfluid phase transition. Since the mass imbalance is an important key in various Fermi superfluids, such as $^{40}$K-$^6$Li Fermi gas mixture, exciton condensate, and color superconductivity in a dense quark matter, our results would be useful for the study of these recently developing superfluid systems.
Standard imsets for undirected and chain graphical models
Takuya Kashimura,Akimichi Takemura
Statistics , 2011, DOI: 10.3150/14-BEJ611
Abstract: We derive standard imsets for undirected graphical models and chain graphical models. Standard imsets for undirected graphical models are described in terms of minimal triangulations for maximal prime subgraphs of the undirected graphs. For describing standard imsets for chain graphical models, we first define a triangulation of a chain graph. We then use the triangulation to generalize our results for the undirected graphs to chain graphs.
Clinical and Radiological Features in the Cases with Cryptogenic Brain Abscess in Association with Patent Foramen Ovale: A Case Report and Review of the Literature  [PDF]
Masayuki Sugie, Takahiro Jimi, Yojiro Kashimura, Hiroo Ichikawa
International Journal of Clinical Medicine (IJCM) , 2014, DOI: 10.4236/ijcm.2014.521178
Abstract:
Brain abscesses are commonly associated with cranial trauma, a contiguous focus of infection, or hematogenous spread from a distant focus. However, no predisposing factors are identified in approximately 4% of the cases, being recognized as a cryptogenic brain abscess (CBA). Here we report a patient with a CBA in the left occipital lobe presumably caused by a periodontal disease. The patient displayed a patent foramen ovale (PFO), through which a spontaneous right-to-left shunt was revealed with transesophageal echocardiography. A literature review indicated that in contrast to cases of general brain abscesses, patients with CBA were older and mostly had dental disorders represented by periodontal diseases and a large PFO. In these patients, the abscess was located predominantly in the posterior circulation area, and their prognosis was worse than that of general cases. Consequently, we emphasize the significance of screening for PFO in cases of advanced age with CBA in the posterior circulation region. Furthermore, to avoid neurological sequelae, we suggest immediate surgical drainage with antibiotic administration and maintenance of oral hygiene.
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