Emiko Arahata

Propagation of second sound in a superfluid Fermi gas in the unitary limit

We study sound propagation in a uniform superfluid gas of Fermi atoms in the unitary limit. The existence of normal and superfluid components leads to appearance of two sound modes in the collisional regime, referred to as first and second sounds. The second sound is of particular interest as it is a clear signal of a superfluid component. Using Landaus two-fluid hydrodynamic theory, we calculate hydrodynamic sound velocities and these weights in the density response function. The latter is used to calculate the response to a sudden modification of the external potential generating pulse propagation. The amplitude of a pulse which is proportional to the weight in the response function is calculated, the basis of the approach of Nozi\`eres and Schmitt-Rink for the BCS-BEC. We show that, in a superfluid Fermi gas at unitarity, the second-sound pulse is excited with an appreciate amplitude by density perturbations.

DC Conductivity in an s-Wave Superconducting Single Vortex System

We study dynamics of a two-dimensional

Damping of condensate oscillations of a trapped Bose gas in a one-dimensional optical lattice at finite temperatures

s

Propagation of first and second sound in a highly elongated trapped Bose-condensed gas at finite temperatures

s-wave superconductor in the presence of a moving single vortex on the basis of the quasiclassical theory generalized by Kita (Phys Rev B 64:054503, 2001). We numerically calculate the linear response of a moving single vortex driven by a dc external current in a self-consistent way, in the sense that the gap equation, Maxwell equations and generalized quasiclassical equation with the impurity self-energy (self-consistent Born approximation) are solved simultaneously. We obtain Hall conductivity induced by vortex motion using the generalized quasiclassical equation, while we confirm that it vanishes in the conventional quasiclassical equation.

Critical velocity in a Bose gas in a moving optical lattice at finite temperatures

We study the effects of non-magnetic impurity scattering on the Andreev bound states (ABS) in an isolated vortex in two-dimensional chiral p-wave superconductors numerically. We incorporate the impurity scattering effects into the quasiclassical Eilenberger formulation through the self-consistent t-matrix approximation. Within this scheme, we calculate the local density of states (LDOS) around two types of vortices: “parallel” (“anti-parallel”) vortex where the phase winding of the pair-potential coming from the vorticity and that coming from the chirality have the same (opposite) sign.When the scattering phase-shift δ0 of each impurity is small, we find that the impurities affect differently the spectra of quasiparticles localized around the two types of vortices in a way similar to that in the Born limit (δ0→0). For a larger δ0(≲π/2), ABS in the vortex is strongly suppressed by the impurities for both types of vortices. We find that there are some correlations between the suppression of ABS near vortex cores and the low energy density of states due to the impurity bands in the bulk.

Temperature dependence of critical velocity in a Bose gas in a moving optical lattice

We study damping of a dipole oscillation in a Bose-Condensed gas in a combined cigar-shaped harmonic trap and one-dimensional (1D) optical lattice potential at finite temperatures. In order to include the effect of thermal excitations in the radial direction, we derive a quasi-1D model of the Gross-Pitaeavskii equation and the Bogoliubov equations. We use the Popov approximation to calculate the temperature dependence of the condensate fraction with varying lattice depth. We then calculate the Landau damping rate of a dipole oscillation as a function of the lattice depth and temperature. The damping rate increases with increasing lattice depth, which is consistent with experimental observations. The magnitude of the damping rate is in reasonable agreement with experimental data. We also find that the damping rate has a strong temperature dependence, showing a sharp increase with increasing temperature. Finally, we emphasize the importance of the radial thermal excitations in both equilibrium properties and the Landau damping.

Bose-Condensed Gases in a 1D Optical Lattice at Finite Temperatures

We present a theory of the emission of fermion pairs from a superfluid Fermi gas induced by a photon absorption. In the solid state physics, this type of process is called double photo-emission (DPE). The spectrum of the induced two-particle current (or DPE current) provides a direct insight into the pair-correlation of condensate fermion pairs. We develop a general formalism for two-particle current induced by DPE by treating the coupling of two Fermi gases with the timedependent perturbation theory. This formalism is used to calculate energy distributions of DPE current from the superfluid Fermi gas in the BCS-BEC crossover at T = 0. We show that the DPE current has distinct contributions of the condensed pair components and uncorrelated pair states. We also calculate the angular dependence of DPE current in the BCS-BEC crossover. The DPE current of the tightly-bound molecules in the BEC regime is found to be quite deferent from that of the weakly-bound Cooper pairs.

Effect of Born and unitary impurity scattering on the Kramer-Pesch shrinkage of a vortex core in a chiral p-wave superconductor

We study sound propagation in Bose-condensed gases in a highly-elongated harmonic trap at finite temperatures. This problem is studied within the framework of Zaremba-Nikuni-Griffin (ZNG) formalism, which consistent of a generalized Gross-Pitaevskii (GP) equation for the condensate and the kinetic equation for a thermal cloud. We extend the ZNG formalism to deal with a highly-anisotropic trap potential, and use it to simulate sound propagation using the trap parameters corresponding to the experiment on sound pulse propagation at finite temperature. We focus on the high-density two-fluid hydrodynamic regime, and explore the possibility of observing first and second sound pulse propagation. The results of numerical simulation are compared with an analyitical results derived from linearized ZNG hydrodynamic equations. We show that the second sound mode makes a dominant contribution to condensate motion in relatively high temperature, while the first sound mode makes an appreciable contribution.