Masahiro Takahashi

We have observed and characterized the nonequilibrium spatial dynamics of a two-component Rb-87 Bose-Einstein condensate (BEC) that is controllable switched back and forth between the miscible and immiscible phases of the phase separation transition by changing the internal states of the Rb-87 atoms. The subsequent evolution exhibits large scale oscillations of the spatial structure that involve component mixing and separation. We show that the larger total energy of the miscible system results in a higher oscillation frequency. This investigation introduces a new technique to control the miscibility and the spatial degrees of freedom in atomic BECs.

We investigate the dynamic properties of bouncing and penetration in colliding binary and ternaryBose-Einstein condensates comprised of different Zeeman or hyperfine states of Rb-87. Through the application ofmagnetic field gradient pulses, two-or three-component condensates in an optical trap are spatially separated and then made to collide. The subsequent evolutions are classified into two categories: repeated bouncing motion and mutual penetration after damped bounces. We experimentally observed mutual penetration for immiscible condensates, bouncing between miscible condensates, and domain formation for miscible condensates. From numerical simulations of the Gross-Pitaevskii equation, we find that the penetration time can be tuned by slightly changing the atomic interaction strengths.

We study a system of interacting spinless fermions in one dimension that, in the absence of interactions, reduces to the Kitaev chain [Kitaev, Phys. Usp. 44, 131 (2001)]. In the noninteracting case, a signal of topological order appears as zero-energy modes localized near the edges. We show that the exact ground states can be obtained analytically even in the presence of nearest-neighbor repulsive interactions when the on-site (chemical) potential is tuned to a particular function of the other parameters. As with the noninteracting case, the obtained ground states are twofold degenerate and differ in fermionic parity. We prove the uniqueness of the obtained ground states and show that they can be continuously deformed to the ground states of the noninteracting Kitaev chain without gap closing. We also demonstrate explicitly that there exists a set of operators each of which maps one of the ground states to the other with opposite fermionic parity. These operators can be thought of as an interacting generalization of Majorana edge zero modes.

We examine the possible phase diagram in an H-T plane for Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in a two-band Pauli-limiting superconductor. We here demonstrate that, as a result of the competition of two different modulation lengthscales, the FFLO phase is divided into two phases by the first-order transition: the Q(1)- and Q(2)-FFLO phases at the higher and lower fields. The Q(2)-FFLO phase is further divided by successive first order transitions into an infinite family of FFLO subphases with rational modulation vectors, forming a devil's staircase structure for the field dependences of the modulation vector and paramagnetic moment. The critical magnetic field above which the FFLO is stabilized is lower than that in a single-band superconductor. However, the tricritical Lifshitz point L at T-L is invariant under two-band parameter changes.

Multiband effects on Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of a Pauli-limiting two-band superconductor are studied theoretically in the wide range of parameters, based on the Bogoliubov-de Gennes equation. First, we examine the phase diagrams of two-band systems with a passive band in which the intraband pairing interaction is absent and superconductivity is induced by a Cooper pair tunneling from an active band. The critical field of Bardeen-Cooper-Schrieffer to FFLO states becomes lower than the Lifshitz point with increasing the interband tunneling strength. We also study the thermodynamics of Pauli-limiting two-band superconductors with nonzero intraband pairing interactions. As a consequence of a competing effect between two bands, the FFLO phase is divided into two phases: Q(1)- and Q(2)-FFLO phases. In a particular case, the latter is further subdivided into a family of FFLO states with rational modulation lengths, where the spatial structure of the pair potential is approximately describable with sinusoidal functions with multiple modulation wave numbers. The resultant phase diagram is qualitatively different from that in a single-band superconductor and gives rise to a devil's staircase structure in the field dependence of physical quantities.