Published papers

We investigate the possibility of an unconventional spin density wave (SDW) in the attractive Hubbard model with finite spin polarization. We show that pairing and density fluctuations induce a transverse d-wave SDW near the half-filling. This novel SDW is related to the d-wave superfluidity induced by antiferromagnetic spin fluctuations, in the sense that they are connected with each other through Shibas attraction-repulsion transformation. Our results predict the d-wave SDW in real systems, such as cold Fermi atom gases with a population imbalance and compounds involving valence skipper elements. © 2009 The Physical Society of Japan.

We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by Nozières 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 approximation. Calculated spin susceptibility based on our extended T -matrix approximation agrees well with the recent experiment on a 6Li Fermi gas. © Springer Science+Business Media, LLC 2012.

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 Nozières 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 40K-6Li 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. © Springer Science+Business Media, LLC 2012.

We investigate pseudogap phenomena originating from pairing fluctuations in the BCS-BEC crossover regime of a two-dimensional Fermi gas in a harmonic trap. Including pairing fluctuations within a T -matrix theory and effects of a trap within the local density approximation, we calculate the local density of states (LDOS) at the superfluid phase transition temperature Tc. In the weak-coupling regime, we show that the pseudogap already appears in LDOS around the trap center. The spatial region where the pseudogap can be seen in LDOS becomes wider for a strong pairing interaction. We also discuss how the pseudogap affects the spectrum of the photoemission-type experiment developed by JILA group. © Springer Science+Business Media, LLC 2012.

We investigate superfluid properties and strong-coupling effects in a onecomponent Fermi gas with an anisotropic p-wave interaction. Within the framework of the Gaussian fluctuation theory, we determine the superfluid transition temperature Tc, as well as the temperature T0 at which the phase transition from the px-wave pairing state to the px + ipy-wave state occurs below Tc. We also show that while the anisotropy of the p-wave interaction enhances Tc in the strong-coupling regime, it suppresses T0. © Springer Science+Business Media, LLC 2012.

© 2016 American Physical Society. We investigate the center-of-mass momentum distribution nQ of Cooper pairs and their first-order correlation function g1(r) in a strongly interacting two-dimensional Fermi gas. Recently, the Berezinskii-Kosterlitz-Thouless (BKT) transition was reported in a two-dimensional Li6 Fermi gas, based on (1) the observations of anomalous enhancement of nQ=0 [Ries, Phys. Rev. Lett. 114, 230401 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.230401], as well as (2) a power-law behavior of g1(r) [Murthy, Phys. Rev. Lett. 115, 010401 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.010401]. However, including pairing fluctuations within a T-matrix approximation (TMA), we show that these results can still be explained as strong-coupling properties of a normal-state two-dimensional Fermi gas. Our results indicate the importance of further experimental observations, to definitely confirm the realization of the BKT transition in this system. Since the BKT transition has been realized in a two-dimensional ultracold Bose gas, our results would be useful for the achievement of this quasi-long-range order in an ultracold Fermi gas.

© 2016 American Physical Society. We theoretically investigate the uniform spin susceptibility χ in the superfluid phase of an ultracold Fermi gas in the region of the Bardeen-Cooper-Schrieffer-Bose-Einstein-condensate (BCS-BEC) crossover. In our previous paper [H. Tajima, Phys. Rev. A 89, 033617 (2014)PLRAAN1050-294710.1103/PhysRevA.89.033617], including pairing fluctuations within an extended T-matrix approximation (ETMA), we showed that strong pairing fluctuations cause the so-called spin-gap phenomenon, where χ is anomalously suppressed even in the normal state near the superfluid phase transition temperature Tc. In this paper, we extend this work to the superfluid phase below Tc, to clarify how this many-body phenomenon is affected by the superfluid order. From the comparison of the ETMA χ with the Yosida function describing the spin susceptibility in a weak-coupling BCS superfluid, we identify the region where pairing fluctuations crucially affect this magnetic quantity below Tc in the phase diagram with respect to the strength of a pairing interaction and the temperature. This spin-gap regime is found to be consistent with the previous pseudogap regime determined from the pseudogapped density of states. We also compare our results with a recent experiment on a Li6 Fermi gas. Since the spin susceptibility is sensitive to the formation of spin-singlet preformed pairs, our results would be useful for the study of pseudogap physics in an ultracold Fermi gas on the viewpoint of the spin degrees of freedom.