arXiv subjects

Takahiko Miyakawa

Publications and source records attributed to Takahiko Miyakawa.

14 recordsLinked to original sources

arXiv2024

Dynamical properties of Fermi-Fermi mixtures of dipolar and non-dipolar atoms

Dynamical properties of homogeneous Fermi-Fermi mixtures of dipolar and non-dipolar atoms are studied at zero temperature, where dipoles are polarized by an external field. We calculate the density-density correlation functions in a ring-diagram approximation and analyze the pole structure to obtain eigenfrequencies of collective excitations. We first determine stability phase diagrams for the mixtures available in experiments: $^{167}$Er-$^{173}$Yb, $^{167}$Er-$^{6}$Li, $^{161}$Dy-$^{173}$Yb, and $^{161}$Dy-$^{6}$Li systems, and show that the mixtures with larger mass imbalance tend to be more unstable. We then investigate the parameter dependence of an undamped zero sound with an anisotropic real dispersion relation in the stable phase for the $^{161}$Dy-$^{173}$Yb mixture, and the speed of sound exhibits a critical angle of possible propagation with respect to the dipole polarization direction, above which the sound mode disappears in the particle-hole continuum. Since the sound mode is a coherent superposition of density fluctuations of dipolar and non-dipolar atoms, the existence of the sound mode, e.g., the value of the critical angle, is significantly affected by the inter-particle interaction through the density-density correlation between dipolar and non-dipolar atoms. We have also observed such an effect of the inter-particle interaction in the study of a linear response of density fluctuations to an external perturbation.

cond-mat.quant-gas↗

arXiv2024

Two-body problem of impurity atoms in dipolar Fermi gas

The polarized dipolar Fermi gas shows exotic properties at low temperatures, characterized by an axially-deformed Fermi surface and anisotropic single-particle energy, due to the long-range and anisotropic nature of dipole-dipole interaction. In cold-atom experiments such a system has been realized, e.g., in degenerate gas of Er and Dy atoms. In the case that non-dipolar impurity atoms are introduced in such system, they undergoes an induced interaction mediated by the density fluctuations of the background dipolar Fermi gas. We derive the induced interaction potential to the single-loop order of fluctuations and show that it becomes indeed an anisotropic Ruderman-Kittel-Kasuya-Yosida-type potential which preserves the axial symmetry around the polarization axis. We then solve the two-body problem of impurity atoms interacting via the anisotropic potential and figure out the dependence of bound state and scattering properties on the parameters of dipolar Fermi gas.

cond-mat.quant-gas↗

arXiv2021

The ground state of polaron in an ultracold dipolar Fermi gas

An impurity atom immersed in an ultracold atomic Fermi gas can form a quasiparticle, so-called Fermi polaron, due to impurity-fermion interaction. We consider a three-dimensional homogeneous dipolar Fermi gas as a medium, where the interatomic dipole-dipole interaction (DDI) makes the Fermi surface deformed into a spheroidal shape, and, using a Chevy-type variational method, investigate the ground-state properties of the Fermi polaron: the effective mass, the momentum distribution of a particle-hole (p-h) excitation, the drag parameter, and the medium density modification around the impurity. These quantities are shown to exhibit spatial anisotropies in such a way as to reflect the momentum anisotropy of the background dipolar Fermi gas. We have also given numerical results for the polaron properties at the unitarity limit of the impurity-fermion interaction in the case in which the impurity and fermion masses are equal. It has been found that the transverse effective mass and the transverse momentum drag parameter of the polaron both tend to decrease by $ \sim 10\%$ when the DDI strength is raised from $0$ up to around its critical value, while the longitudinal ones exhibit a very weak dependence on the DDI.

cond-mat.quant-gas↗

arXiv2010

Equilibrium Properties of Trapped Dipolar Gases at Finite Temperatures

We study the equilibrium properties of dipolar Bose and Fermi gases at finite temperatures. We recently developed a variational ansatz for the phase-space distribution function of a dipolar Fermi gas at finite temperatures. We extend the variational formalism to a Bose gas and discuss the effect of dipolar interactions with different statistics on the thermal equilibrium, with particular emphasis on the deformation in momentum space. We examine the stability of the system by varying the temperature, trap aspect ratio and the dipole moment. In addition, we discuss how deformation in both real and momentum space can be observed in the high-temperature regime, which is relevant for the current experiments at JILA [Ni et al. Science 322, 231 (2008)] and at University of Tokyo [Aikawa et al. New. J. Phys. 11, 055035 (2009)].

cond-mat.other↗

arXiv2010

Density wave instability in a 2D dipolar Fermi gas

We consider a uniform dipolar Fermi gas in two-dimensions (2D) where the dipole moments of fermions are aligned by an orientable external field. We obtain the ground state of the gas in Hartree-Fock approximation and investigate RPA stability against density fluctuations of finite momentum. It is shown that the density wave instability takes place in a broad region where the system is stable against collapse. We also find that the critical temperature can be a significant fraction of Fermi temperature for a realistic system of polar molecules.

cond-mat.quant-gas↗

arXiv2010

Equilibrium Properties of a Trapped Dipolar Fermion at Finite Temperatures

We study the equilibrium properties of a dipolar Fermi gas at finite temperatures. We introduce a variational ansatz for the phase-space distribution function that can describe the deformation in both real and momentum space. The effect of dipole--dipole interactions on the thermal equilibrium is discussed with particular emphasis on the deformation in momentum space. We examine the stability of the system by varying the temperature, trap aspect ratio, and the dipole moment. In addition, we discuss how the deformation in both real and momentum space can be observed in the high-temperature regime.

cond-mat.other↗

arXiv2008

p-Wave superfluid and phase separation in atomic Bose-Fermi mixture

We consider a system of repulsively interacting Bose-Fermi mixtures of spin polarized uniform atomic gases at zero temperature. We examine possible realization of p-wave superfluidity of fermions due to an effective attractive interaction via density fluctuations of Bose-Einstein condensate within mean-field approximation. We find the ground state of the system by direct energy comparison of p-wave superfluid and phase-separated states, and suggest an occurrence of the p-wave superfluid for a strong boson-fermion interaction regime. We study some signatures in the p-wave superfluid phase, such as anisotropic energy gap and quasi-particle energy in the axial state, that have not been observed in spin unpolarized superfluid of atomic fermions. We also show that a Cooper pair is a tightly bound state like a diatomic molecule in the strong boson-fermion coupling regime and suggest an observable indication of the p-wave superfluid in the real experiment.

cond-mat.supr-con↗

arXiv2007

Phase space deformation of a trapped dipolar Fermi gas

We consider a system of quantum degenerate spin polarized fermions in a harmonic trap at zero temperature, interacting via dipole-dipole forces. We introduce a variational Wigner function to describe the deformation and compression of the Fermi gas in phase space and use it to examine the stability of the system. We emphasize the important roles played by the Fock exchange term of the dipolar interaction which results in a non-spherical Fermi surface.

cond-mat.other↗

arXiv2006

Dissociation dynamics of resonantly coupled bose-fermi mixtures in an optical lattice

We consider the photodissociation of ground-state bosonic molecules trapped in an optical lattice potential into two-component fermionic atoms. The system is assumed to be described by a one-band resonantly-coupled Bose-Fermi Hubbard model. We show that in the strong fermion-fermion interaction limit the dissociation dynamics is governed by a spin-boson lattice Hamiltonian. In the framework of a mean-field analysis based on the Gutzwiller ansatz, we then examine the crossover of the dissociation from a regime of independent single-site dynamics to a regime of cooperative dynamics as the molecular tunneling increases. We also identify two types of self-trapping transitions, a {\it coherent} and an {\it incoherent} one, depending on the ratio of the repulsive molecule-molecule interaction strength to molecular tunneling. Finally, we show that in the limits of weak and strong intersite tunneling the mean-field solutions agree well with the results from the quantum optical Jaynes-Cummings and Tavis-Cummings models, respectively.

cond-mat.other↗

arXiv2005

Anderson Lattice Description of Photoassociation in an Optical Lattice

We consider atomic mixtures of bosons and two-component fermions in an optical lattice potential. We show that if the bosons are in a Mott-insulator state with precisely one atom per lattice, the photoassociation of bosonic and fermionic atoms into heteronuclear fermionic molecules is described by the Anderson Lattice Model. We determine the ground state properties of an inhomogeneous version of that model in the strong atom-molecule coupling regime, including an additional harmonic trap potential. Various spatial structures arise from the interplay between the atom-molecule correlations and the confining potential. Perturbation theory with respect to the tunneling coupling between fermionic atoms shows that anti-ferromagnetic correlations develop around a spin-singlet core of fermionic atoms and molecules.

cond-mat.other↗

arXiv2004

Exact Analysis of Soliton Dynamics in Spinor Bose-Einstein Condensates

We propose an integrable model of a multicomponent spinor Bose-Einstein condensate in one dimension, which allows an exact description of the dynamics of bright solitons with spin degrees of freedom. We consider specifically an atomic condensate in the F=1 hyperfine state confined by an optical dipole trap. When the mean-field interaction is attractive (c_0 < 0) and the spin-exchange interaction of a spinor condensate is ferromagnetic (c_2 < 0), we prove that the system possesses a completely integrable point leading to the existence of multiple bright solitons. By applying results from the inverse scattering method, we analyze a collision law for two-soliton solutions and find that the dynamics can be explained in terms of the spin precession.

cond-mat.stat-mech↗

arXiv2004

Matter-Wave Solitons in an F=1 Spinor Bose-Einstein Condensate

Following our previous work [J. Ieda, T. Miyakawa, M. Wadati, cond-mat/0404569] on a novel integrable model describing soliton dynamics of an F=1 spinor Bose--Einstein condensate, we discuss in detail the properties of the multi-component system with spin-exchange interactions. The exact multiple bright soliton solutions are obtained for the system where the mean-field interaction is attractive (c_0 < 0) and the spin-exchange interaction is ferromagnetic (c_2 < 0). A complete classification of the one-soliton solution with respect to the spin states and an explicit formula of the two-soliton solution are presented. For solitons in polar state, there exists a variety of different shaped solutions including twin peaks. We show that a "singlet pair" density can be used to distinguish those energetically degenerate solitons. We also analyze collisional effects between solitons in the same or different spin state(s) by computing the asymptotic forms of their initial and final states. The result reveals that it is possible to manipulate the spin dynamics by controlling the parameters of colliding solitons.

cond-mat.mes-hall↗

arXiv2004

Boson-Fermion coherence in a spherically symmetric harmonic trap

We consider the photoassociation of a low-density gas of quantum-degenerate trapped fermionic atoms into bosonic molecules in a spherically symmetric harmonic potential. For a dilute system and the photoassociation coupling energy small compared to the level separation of the trap, only those fermions in the single shell with Fermi energy are coupled to the bosonic molecular field. Introducing a collective pseudo-spin operator formalism we show that this system can then be mapped onto the Tavis-Cummings Hamiltonian of quantum optics, with an additional pairing interaction. By exact diagonalization of the Hamiltonian, we examine the ground state and low excitations of the Bose-Fermi system, and study the dynamics of the coherent coupling between atoms and molecules. In a semiclassical description of the system, the pairing interaction between fermions is shown to result in a self-trapping transition in the photoassociation, with a sudden suppression of the coherent oscillations between atoms and molecules. We also show that the full quantum dynamics of the system is dominated by quantum fluctuations in the vicinity of the self-trapping solution.

cond-mat.mes-hall↗

arXiv2004

Peierls instability, periodic Bose-Einstein condensates and density waves in quasi-one-dimensional boson-fermion mixtures of atomic gases

We study the quasi-one-dimensional (Q1D) spin-polarized bose-fermi mixture of atomic gases at zero temperature. Bosonic excitation spectra are calculated in random phase approximation on the ground state with the uniform BEC, and the Peierls instabilities are shown to appear in bosonic collective excitation modes with wave-number $2k_F$ by the coupling between the Bogoliubov-phonon mode of bosonic atoms and the fermion particle-hole excitations. The ground-state properties are calculated in the variational method, and, corresponding to the Peierls instability, the state with a periodic BEC and fermionic density waves with the period $π/k_F$ are shown to have a lower energy than the uniform one. We also briefly discuss the Q1D system confined in a harmonic oscillator (HO) potential and derive the Peierls instability condition for it.

cond-mat.mes-hall↗