arXiv subjects

Emiko Arahata

Publications and source records attributed to Emiko Arahata.

12 recordsLinked to original sources

arXiv2023

Simulation method for evaporative cooling of trapped Bose gases at finite temperatures

We develop a simulation method for evaporative cooling of trapped Bose-Einstein condensate at finite temperatures using Zaremba-Nikuni-Griffin (ZNG) formalism. ZNG formalism includes the generalized GP equation and a semiclassical kinetic equation for the thermal cloud, which treats the excitations semiclassically within the Hartree Fock approximation. The generalized GP equation includes the mean field due to the thermal cloud and the source term associated with collisions between the condensate and the thermal cloud. Our method is based on the numerical approach developed by Jackson and Zaremba, which simulates the kinetic equation using test particles. A key point of our method is to mimic the evaporative cooling process by eliminating the test particles with high energy. We show that our method successfully describes condensate growth during evaporative cooling. We also numerically simulate vortex lattice formation during evaporative cooling in the presence of the rotating thermal cloud.

cond-mat.quant-gas↗

arXiv2019

Ground-state and Excitation spectra of Bose-Fermi Mixtures in a Three-Dimensional Optical Lattice

Motivated by recent developments in the experimental study of Bose-Fermi mixtures, we investigate ground-state phase diagrams and excitation spectra for Bose-Fermi mixtures in a three-dimensional (3D) optical lattice. The Gutzwiller approximation is used to identify a new phase in which both superfluid bosons and metal fermions coexist. As a useful probe to identify the quantum phases, we also calculate the excitation spectra. In Mott insulator phase, two excitation features appear in the spectra that correspond to particle and hole excitations. In superfluid phase, there are Bogoliubov modes and amplitude modes. In coexisting phase, two gapless dispersive modes are identified, which shift due to interaction between bosons and fermions.

cond-mat.quant-gas↗

arXiv2016

Cooperon condensation and intra-valley pairing states in honeycomb Dirac systems

Motivated by recent developments in the experimental study of superconducting graphene and transition metal dichalcogenides, we investigate superconductivity of the Kane-Mele (KM) model with short-range attractive interactions on the two-dimensional honeycomb lattice. We show that intra-valley spin-triplet pairing arises from nearest-neighbor (NN) attractive interaction and the intrinsic spin-orbit coupling. We demonstrate this in two independent approaches: We study superconducting instability driven by condensation of Cooperons, which are in-gap bound states of two conduction electrons, within the $T$-matrix approximation and also study the superconducting ground state within the mean-field theory. We find that Cooperons with antiparallel spins condense at the $K$ and $K'$ points. This leads to the emergence of an intra-valley spin-triplet pairing state belonging to the irreducible representation A$_1$ of the point group $C_{6v}$. The fact that this pairing state has opposite chirality for $K$ and $K'$ identifies this state as a "helical" valley-triplet state, the valley-analog to the $^3$He-B phase in two dimension. Because of the finite center of mass momentum of Cooper pairs, the pair amplitude in NN bonds exhibits spatial modulation on the length scale of lattice constant, such that this pairing state may be viewed as a pair-density wave state. We find that the pair amplitude spontaneously breaks the translational symmetry and exhibits a $p$-Kekulé pattern. We also discuss the selection rule for pairing states focusing the characteristic band structure of the KM model and the Berry phase effects to the emergence of the intra-valley pairing state.

cond-mat.supr-con↗

arXiv2014

Ginzburg-Landau description of twin boundaries in noncentrosymmetric superconductors

We study theoretically a model for twin boundaries in superconductors with Rashba spin-orbit coupling, which can be relevant to both three-dimensional noncentrosymmetric tetragonal crystals and two-dimensional gated superconductors such as the LaAlO$_3$/SrTiO$_3$ interface. In both cases, the broken inversion symmetry allows for a coexistence of singlet and triplet pairing. Within the framework of a Ginzburg-Landau theory, we identify two $\mathbb{Z}_2$ symmetries that are broken via two consecutive second order phase transitions as the temperature is lowered. We show that a time-reversal symmetry breaking superconducting state nucleates near the twin boundary if singlet and triplet pairing amplitudes are of comparable magnitude. As a consequence, the tendency towards ferromagnetic order is locally increased along with the emergence of spontaneous supercurrents parallel to the twin boundary. Spin currents, which are present in the form of Andreev bound states, are found enhanced in the time-reversal broken phases.

cond-mat.supr-con↗

arXiv2013

DC conductivity in an s-wave superconducting single vortex system

We study dynamics of a two-dimensional s-wave superconductor in the presence of a moving single vortex. Our analysis is based on the quasiclassical theory including the Hall term, generalized by Kita[T. Kita, PRB, 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 Dyson equation, gap equation, Maxwell equations and generalized quasiclassical equation 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.

cond-mat.supr-con↗

arXiv2013

Impurity effects in a vortex core in a chiral p-wave superconductor within the t-matrix approximation

We study the effects of non-magnetic impurity scattering on the Andreev bound states (ABS) in an isolated vortex in a two-dimensional chiral p-wave superconductor 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 vorticity and that coming from chirality have the same (opposite) sign. When the scattering phase-shift $δ_0$ of each impurity is small, we find that impurities affect differently low energy quasiparticle spectrum around the two types of vortex in a way similar to that in the Born limit ($δ_0\rightarrow 0$). For a larger $δ_0(\leq π/2)$ however we find that ABS in the vortex is strongly suppressed by impurities for both types of vortex. We found that there are some correlations between the suppression of ABS near vortex cores and the low energy density of states due to impurity bands in the bulk.

cond-mat.supr-con↗

arXiv2013

Theory of the Two-Particle Emission from Superfluid Fermi Gases in the BCS-BEC Crossover

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 time-dependent 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.

cond-mat.quant-gas↗

arXiv2013

Spin Currents and Spontaneous Magnetization at Twin Boundaries of Noncentrosymmetric Superconductors

Twin boundaries are generic crystalline defects in noncentrosymmetric crystal structures. We study theoretically twin boundaries in time-reversal symmetric noncentrosymmetric superconductors that admit parity-mixed Cooper pairing. Twin boundaries support spin currents as a consequence of this parity mixing. If the singlet and triplet components of the superconducting order parameter are of comparable magnitude, the superconducting state breaks spontaneously the bulk time-reversal symmetry locally near the twin boundary. By self-consistently evaluating the Bogoliubov-de-Gennes equations and the gap functions we find two distinct phases: First, time-reversal symmetry breaking enhances the spin currents but does not lead to chiral supercurrents. A secondary phase transition then triggers a spin magnetization and an orbital supercurrent near the twin boundary.

cond-mat.supr-con↗

arXiv2011

Single-Particle Spectral Density of a Bose Gas in the Two-Fluid Hydrodynamic Regime

In Bose supefluids, the single-particle Green's function can be directly related to the superfluid velocity-velocity correlation function in the hydrodynamic regime. An explicit expression for the single-particle spectral density was originally written down by Hohenberg and Martin in 1965, starting from the two-fluid equations for a superfluid. We give a simple derivation of their results. Using these results, we calculate the relative weights of first and second sound modes in the single-particle spectral density as a function of temperature in a uniform Bose gas. We show that the second sound mode makes a dominant contribution to the single-particle spectrum in relatively high temperature region. We also discuss the possibility of experimental observation of the second sound mode in a Bose gas by photoemission spectroscopy.

cond-mat.quant-gas↗

arXiv2010

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 sound. The second sound is of particular interest as it is a clear signal of a superfluid component. Using Landau's 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 Nozieres and Schmitt-Rink (NSR) for the BCS-BEC crossover. We show that, in a superfluid Fermi gas at unitarity, the second sound pulse is excited with an appreciate amplitude by density perturbations.

cond-mat.quant-gas↗

arXiv2009

Critical Velocity in a Bose Gas in a Moving Optical Lattice at Finite Temperatures

We study the critical velocity of a Bose-condensed gas in a moving one-dimensional (1D) optical lattice potential at finite temperatures. Solving the Gross-Pitaeavskii equation and the Bogoliubov equations, within the Popov approximation, we calculate the Bogoliubov excitations with varying lattice velocity. From the condition of the negative excitation energy, we determine the critical velocity as a function of the lattice depth and the temperature. We find that the critical velocity decreases rapidly with increasing the temperature; this result is consistent with the experimental observations. Moreover, the critical velocity shows a rapid decrease with increasing lattice depth. This tendency is much more significant than in the previous works ignoring the effect of thermal excitations in the radial direction.

cond-mat.other↗

arXiv2007

Damping of Condensate Oscillation of a Trapped Bose Gas in a One-Dimensional Optical Lattice at Finite Temperatures

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.

cond-mat.other↗