American Physical SocietyActa Medica Okayama246999268162010Spin textures in condensates with large dipole moments063623ENJ. A. M.HuhtamakiDepartment of Physics, Okayama UniversityM.TakahashiDepartment of Physics, Okayama UniversityT. P.SimulaDepartment of Physics, Okayama UniversityT.MizushimaDepartment of Physics, Okayama UniversityK.MachidaDepartment of Physics, Okayama University We have solved numerically the ground states of a Bose-Einstein condensate in the presence of dipolar interparticle forces using a semiclassical approach. Our motivation is to model, in particular, the spontaneous spin textures emerging in quantum gases with large dipole moments, such as Cr-52 or Dy condensates, or ultracold gases consisting of polar molecules. For a pancake-shaped harmonic ( optical) potential, we present the ground-state phase diagram spanned by the strength of the nonlinear coupling and dipolar interactions. In an elongated harmonic potential, we observe a helical spin texture. The textures calculated according to the semiclassical model in the absence of external polarizing fields are predominantly analogous to previously reported results for a ferromagnetic F = 1 spinor Bose-Einstein condensate, suggesting that the spin textures arising from the dipolar forces are largely independent of the value of the quantum number F or the origin of the dipolar interactions.No potential conflict of interest relevant to this article was reported.American Physical SocietyActa Medica Okayama1050-29478222010Splitting and oscillation of Majorana zero modes in the p-wave BCS-BEC evolution with plural vortices023624ENT.MizushimaDepartment of Physics, Okayama UniversityK.MachidaDepartment of Physics, Okayama University We investigate how the vortex-vortex separation changes Majorana zero modes in the vicinity of the BCS-BEC (Bose-Einstein condensation) topological phase transition of p-wave resonant Fermi gases. By analytically and numerically solving the Bogoliubov-de Gennes equation for spinless p-wave superfluids with plural vortices, it is demonstrated that the quasiparticle tunneling between neighboring vortices gives rise to the quantum oscillation of the low-lying spectra on the scale of the Fermi wavelength in addition to the exponential splitting. This rapid oscillation, which appears in the weak-coupling regime as a consequence of quantum oscillations of quasiparticle wave functions, disappears in the vicinity of the BCS-BEC topological phase transition. This is understandable from that the wave function of the Majorana zero modes is described by the modified Bessel function in the strong-coupling regime, and thus it becomes spread over the vortex core region. Due to the exponential divergence of the modified Bessel function, the concrete realization of the Majorana zero modes near the topological phase transition requires the neighboring vortices to be separated beyond the length scale defined by the coherence length and the dimensionless coupling constant. All these behaviors are also confirmed by carrying out the full numerical diagonalization of the nonlocal Bogoliubov-de Gennes equation in a two-dimensional geometry. Furthermore, this argument is expanded into the case of three-vortex systems, where a pair of core-bound and edge-bound Majorana states survive at zero-energy state regardless of the vortex separation.No potential conflict of interest relevant to this article was reported.Acta Medica Okayama7262005Vortex structure in spinor f=2 bose-einstein condensatesENW VPogosovRKawateTMizushimaKMachida<p>Extended Gross-Pitaevskii equations for the rotating F=2 condensate in a harmonic trap are solved both numerically and variationally using trial functions for each component of the wave function. Axially symmetric vortex solutions are analyzed and energies of polar and cyclic states are calculated. The equilibrium transitions between different phases with changing of the magnetization are studied. We show that at high magnetization the ground state of the system is determined by interaction in "density" channel, and at low magnetization spin interactions play a dominant role. Although there are five hyperfine states, all the particles are always condensed in one, two, or three states. Two interesting types of vortex structures are also discussed.</p>No potential conflict of interest relevant to this article was reported.Acta Medica Okayama7252005Multibranch Bogoliubov-Bloch spectrum of a cigar-shaped Bose condensate in an optical latticeENTarun KantiGhoshK.Machida<p>We study properties of excited states of an array of weakly coupled quasi-two-dimensional Bose condensates by using the hydrodynamic theory. The spectrum of the axial excited states strongly depends on the coupling among the various discrete radial modes in a given symmetry. By including mode coupling within a given symmetry, the complete excitation spectrum of axial quasiparticles with various discrete radial nodes are presented. A single parameter which determines the strength of the mode coupling is identified. The excitation spectrum in the zero angular momentum sector can be observed by using the Bragg scattering experiments.</p>No potential conflict of interest relevant to this article was reported.Acta Medica Okayama7422006Cyclic phase in f=2 spinor condensate:long-range order, kinks, and roughening transitionENW VPogosovKMachida<p>We study the effect of thermal fluctuations on homogeneous infinite Bose-Einstein condensate with spin F=2 in the cyclic state, when atoms occupy three hyperfine states with m(F)=0,+/- 2. We use both the approach of small-amplitude oscillations and mapping of our model on the sine-Gordon model. We show that thermal fluctuations lead to the existence of the rough phase in one- and two-dimensional systems, when the presence of kinks is favorable. The structure and energy of a single kink are found. We also discuss the effect of thermal fluctuations on spin degrees of freedom in F=1 condensate.</p>No potential conflict of interest relevant to this article was reported.Acta Medica Okayama7422006Thermal fluctuations of vortex clusters in quasi-two-dimensional bose-einstein condensatesENW VPogosovKMachida<p>We study the thermal fluctuations of vortex positions in small vortex clusters in a harmonically trapped rotating Bose-Einstein condensate. It is shown that the order-disorder transition of two-shell clusters occurs via the decoupling of shells with respect to each other. The corresponding "melting" temperature depends strongly on the commensurability between numbers of vortices in shells. We show that melting can be achieved at experimentally attainable parameters and very low temperatures. Also studied is the effect of thermal fluctuations on vortices in an anisotropic trap with small quadrupole deformation. We show that thermal fluctuations lead to the decoupling of a vortex cluster from the pinning potential produced by this deformation. The decoupling temperatures are estimated and strong commensurability effects are revealed.</p>No potential conflict of interest relevant to this article was reported.Acta Medica Okayama9462005Anisotropic diamagnetic response in type-II superconductors with gap and fermi-surface anisotropies ENHAdachiPMiranovicMIchiokaKMachida<p>The effects of anisotropic gap structures on a diamagnetic response are investigated in order to demonstrate that the field-angle-resolved magnetization [M-L (chi)] measurement can be used as a spectroscopic method to detect gap structures. Our microscopic calculation based on the quasiclassical Eilenberger formalism reveals that M-L (chi) in a superconductor with a fourfold gap displays a fourfold oscillation reflecting the gap and Fermi-surface anisotropies, and the sign of this oscillation changes at a field between H-c1 and H-c2. As a prototype of unconventional superconductors, magnetization data for borocarbides are also discussed.</p>
No potential conflict of interest relevant to this article was reported.Acta Medica Okayama7262005Carbon-substitution dependent multiple superconducting gap of MgB2: A sub-meV resolution photoemission studyENSTsudaTYokoyaTKissTShimojimaSShinTTogashiSWatanabeCZhangC TChenSLeeHUchiyamaSTajimaNNakaiKMachida<p>"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg(B1-xCx)(2). Two features corresponding to sigma and pi gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the sigma gap is proportional to T-c while the pi gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed.</p>No potential conflict of interest relevant to this article was reported.Acta Medica Okayama97122006Generic phase diagram of fermion superfluids with population imbalanceENKMachidaTMizushimaMIchioka<p>It is shown by microscopic calculations for trapped imbalanced Fermi superfluids that the gap function always has sign changes, i.e., the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state, up to a critical imbalance P-c, beyond which normal state becomes stable, at temperature T=0. A temperature-versus-pressure phase diagram is constructed, where the BCS state without sign change is stable only at T not equal 0. We reproduce the observed bimodality in the density profile to identify its origin and evaluate P-c as functions of T and the coupling strength. These dependencies match with the recent experiments.</p>No potential conflict of interest relevant to this article was reported.