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One of the features of the unconventional state in iron-based superconductors is possibility to transform to the s++ state with the increase of the nonmagnetic disorder. Detection of such a transition would prove the existence of the state. Here we study the temperature dependence of the London magnetic penetration depth within the two-band model for the and s++ superconductors. By solving Eliashberg equations accounting for the spin-fluctuation mediated pairing and nonmagnetic impurities in the T-matrix approximation, we have derived a set of specific signatures of the transition: (1) sharp change in the behavior of the penetration depth λL as a function of the impurity scattering rate at low temperatures; (2) before the transition, the slope of increases as a function of temperature, and after the transition this value decreases; (3) the sharp jump in the inverse square of the penetration depth as a function of the impurity scattering rate, , at the transition; (4) change from the single-gap behavior in the vicinity of the transition to the two-gap behavior upon increase of the impurity scattering rate in the superfluid density ρs(T). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Based on the self-consistent T-matrix approximation (SCTMA), the Mott insulator – Bose-glass phase transition of one-dimensional noninteracting bosons subject to binary disorder is considered. The results obtained differ essentially from the conventional case of box distribution of the disorder. The Mott insulator – Bose-glass transition is found to exist at arbitrary strength of the impurities. The single particle density of states is calculated within the frame of SCTMA, numerically, and (for infinite disorder strength) analytically. A good agreement is reported among all three methods. We speculate that certain types of the interaction may lead to the Bose-glass – superfluid transition absent in our theory. [ABSTRACT FROM AUTHOR]

We investigate the superfluid phase transition in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas with mass imbalance. In the presence of mass imbalance, it is known that the strong-coupling Gaussian fluctuation theory, as well as the ordinary non-self-consistent T-matrix theory, that have been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically give double valued superfluid phase transition temperature T in the crossover region. In our previous paper (R. Hanai et al., J. Low Temp. Phys. 171:389, ), this difficulty was shown to be eliminated by an extended T-matrix approximation (ETMA). However, it was also found that this improved theory still gives vanishing T in the BCS regime, when the mass imbalance ratio remarkably deviates from unity. In this paper, further extending ETMA to include higher order pairing fluctuations to the fully self-consistent T-matrix level, we clarify whether the vanishing T obtained in ETMA is an intrinsic phenomenon or an artifact of this approximation. We show that the self-consistent T-matrix theory always gives a finite T, even in the region where ETMA predicts the absence of superfluid instability. The key to the recovery of T is found to be a consistent treatment with respect to the Fermi surface sizes of the light mass and heavy mass components, which ETMA lacks in. Since Fermi condensates with mass imbalance have been recently discussed in various systems, such as a K-Li Fermi gas, exciton-polariton condensate, as well as color superconductivity, our results would be useful in constructing a reliable strong-coupling theory to examine physical properties of these novel Fermi superfluids. [ABSTRACT FROM AUTHOR]

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 Li Fermi gas. [ABSTRACT FROM AUTHOR]

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 K-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. [ABSTRACT FROM AUTHOR]

We study cold fermions in 2D that are interacting through the s-wave contact potential, using the path-integral formalism. The phase transitions are investigated using the joint solution of the gap and number equations. In the latter ones, the amplitude and phase fluctuations are taken into account within the t-matrix NSR-type formalism. It is shown that convergent solutions for the fermion density in 2D can be obtained even in the lowest-order (quadratic) approximation for fluctuations with a proper determination of the spectral functions. This approach leads, in particular, to a non-coherent quasicondensate which exists between the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature below which the system is superfluid and a transition temperature T above which the quasicondensate is destroyed. [ABSTRACT FROM AUTHOR]

Abstract  We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature T c including pairing fluctuations within a T-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature T * at which the pseudogap structure in DOS disappears. We also introduce another temperature T ** at which the BCS-like double-peak structure disappears in the spectral weight. While one finds T *>T ** in the BCS regime, T ** becomes higher than T * in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction. [ABSTRACT FROM AUTHOR]

Studies the critical current of superfluid [sup 3]He confined to a thin channel between realistic rough surfaces using quasiclassical techniques by applying the t-matrix boundary conditions of Rainier. Rainier's conditions leading directly to a simple stable and comprehensive algorithm for solving the quasiclassical equations presented in detail.

• Obtaining the shear viscosity using the self-energy and the T-matrix method in the Kubo formula. • The effect of the p x -wave anisotropic pairing interaction in the transport properties. • The obtained viscous relaxation rate using the Kubo formula for a p x -wave Fermi superfluid gas differs from the s -wave one. The p -wave superfluid state is a promising spin-triplet and non s -wave pairing state in an ultracold Fermi gas. In this work we study the low-temperature shear viscosity of a one-component p -wave superfluid Fermi gas, by means of Kubo formalism. Our study is done in the strong-coupling limit where Fermi superfluid reduces into a system of composite bosons. Taking into account p x -wave Cooper channel in the self-energy, the viscous relaxation rates are determined. The relaxation rates related to these interactions are calculated as a function of temperature. Their temperature dependence is different from the s -wave superfluid Fermi gas, and this is due to the anisotropic pairing interaction in the p -wave superfluid. Our results contribute to understand how this anisotropy affects transport properties of this unconventional superfluid Fermi gas in low temperature limit. [ABSTRACT FROM AUTHOR]