研究者詳細 - 川口　由紀

論文 - 川口　由紀

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Valley Edelstein effect in monolayer transition-metal dichalcogenides 査読有り国際共著

Taguchi, K; Zhou, BT; Kawaguchi, Y; Tanaka, Y; Law, KT

PHYSICAL REVIEW B 98 巻 ( 3 ) 2018年7月

Local density of states in two-dimensional topological superconductors under a magnetic field: Signature of an exterior Majorana bound state 査読有り

Suzuki, SI; Kawaguchi, Y; Tanaka, Y

PHYSICAL REVIEW B 97 巻 ( 14 ) 2018年4月

Geometrically frustrated coarsening dynamics in spinor Bose-Fermi mixtures 査読有り

Phuc, NT; Momoi, T; Furukawa, S; Kawaguchi, Y; Fukuhara, T; Ueda, M

PHYSICAL REVIEW A 95 巻 ( 1 ) 2017年1月

Spin and charge transport induced by a twisted light beam on the surface of a topological insulator 査読有り

Kunitaka Shintani, Katsuhisa Taguchi, Yukio Tanaka, Yuki Kawaguchi

PHYSICAL REVIEW B 93 巻 ( 19 ) 2016年5月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

We theoretically study spin and charge transport induced by a twisted light beam irradiated on a disordered surface of a doped three-dimensional topological insulator (TI). We find that various types of spin vortices are imprinted on the surface of the TI depending on the spin and orbital angular momentum of the incident light. The key mechanism for the appearance of the unconventional spin structure is the spin-momentum locking in the surface state of the TI. Besides, the diffusive transport of electrons under an inhomogeneous electric field causes a gradient of the charge density, which then induces nonlocal charge current and spin density as well as the spin current. We discuss the relation between these quantities within the linear response to the applied electric field using the Keldysh-Green's function method.

DOI： 10.1103/PhysRevB.93.195415

Web of Science

Spin Hall effect in a spinor dipolar Bose-Einstein condensate 査読有り

Toshiyuki Oshima, Yuki Kawaguchi

PHYSICAL REVIEW A 93 巻 ( 5 ) 2016年5月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

We theoretically show that the spin Hall effect arises in a Bose-Einstein condensate (BEC) of neutral atoms interacting via the magnetic dipole-dipole interactions (MDDIs). Since the MDDI couples the total spin angular momentum and the relative orbital angular momentum of two colliding atoms, it works as a spin-orbit coupling. Thus, when we prepare a BEC in a magnetic sublevel m=0, thermally and quantum-mechanically excited atoms in the m=1 and -1 states feel the Lorentz-like forces in the opposite directions. This is the origin for the emergence of the spin Hall effect. We define the mass-current and spin-current operators from the equations of continuity and calculate the spin Hall conductivity from the off-diagonal current-current correlation function within the Bogoliubov approximation. We find that the correction of the current operators due to the MDDI significantly contributes to the spin Hall conductivity. A possible experimental situation is also discussed.

DOI： 10.1103/PhysRevA.93.053605

Web of Science

Skyrmionic magnetization configurations at chiral magnet/ferromagnet heterostructures 査読有り

Yuki Kawaguchi, Yukio Tanaka, and Naoto Nagaosa

PHYSICAL REVIEW B 93 巻 ( 6 ) 2016年2月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（学術雑誌）

We consider magnetization configurations at chiral magnet (CM)/ferromagnet (FM) heterostructures. In the CM, magnetic skyrmions and spin helices emerge due to the Dzyaloshinskii-Moriya interaction, which then penetrate into the adjacent FM. However, because the nonuniform magnetization structures are energetically unfavorable in the FM, the penetrated magnetization structures are deformed, resulting in exotic three-dimensional configurations, such as skyrmion cones, sideways skyrmions, and twisted helices and skyrmions. We discuss the stability of possible magnetization configurations at the CM/FM and CM/FM/CM hybrid structures within the framework of the variational method, and find that various magnetization configurations appear in the ground state, some of which cause nontrivial emergent magnetic field.

DOI： 10.1103/PhysRevB.93.064416

Web of Science

Controlling and probing non-abelian emergent gauge potentials in spinor Bose-Fermi mixtures 査読有り

Nguyen Thanh Phuc, Gen Tatara, Yuki Kawaguchi, Masahito Ueda

NATURE COMMUNICATIONS 6 巻 2015年9月

記述言語：英語掲載種別：研究論文（学術雑誌）

Gauge fields, typified by the electromagnetic field, often appear as emergent phenomena due to geometrical properties of a curved Hilbert subspace, and provide a key mechanism for understanding such exotic phenomena as the anomalous and topological Hall effects. Non-abelian gauge potentials serve as a source of non-singular magnetic monopoles. Here we show that unlike conventional solid materials, the non-abelianness of emergent gauge potentials in spinor Bose-Fermi atomic mixtures can be continuously varied by changing the relative particle-number densities of bosons and fermions. The non-abelian feature is captured by an explicit dependence of the measurable spin current density of fermions in the mixture on the variable coupling constant. Spinor mixtures also provide us with a method to coherently and spontaneously generate a pure spin current without relying on the spin Hall effect. Such a spin current is expected to have potential applications in the new generation of atomtronic devices.

DOI： 10.1038/ncomms9135

Web of Science

Coarsening dynamics driven by vortex-antivortex annihilation in ferromagnetic Bose-Einstein condensates 査読有り

Kazue Kudo, Yuki Kawaguchi

PHYSICAL REVIEW A 91 巻 ( 5 ) 2015年5月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

In ferromagnetic Bose-Einstein condensates (BECs), the quadratic Zeeman effect controls magnetic anisotropy, which affects magnetic domain pattern formation. While the longitudinal magnetization is dominant (similar to the Ising model) for a negative quadratic Zeeman energy, the transverse magnetization is dominant (similar to the XY model) for a positive one. When the quadratic Zeeman energy is positive, the coarsening dynamics is driven by vortex-antivortex annihilation in the same way as the XY model. However, due to a superfluid flow of atoms, there exist several combinations of vortex-antivortex pairs in ferromagnetic BECs, which makes the coarsening dynamics more complicated than that of the XY model. We propose a revised domain growth law, which is based on the growth law of the two-dimensional XY model, for a two-dimensional ferromagnetic BEC with a positive quadratic Zeeman energy.

DOI： 10.1103/PhysRevA.91.053609

Web of Science

Quantum Mass Acquisition in Spinor Bose-Einstein Condensates 査読有り

Nguyen Thanh Phuc, Yuki Kawaguchi, and Masahito Ueda

PHYSICAL REVIEW LETTERS 113 巻 ( 23 ) 2014年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

Quantum mass acquisition, in which a massless (quasi)particle becomes massive due to quantum corrections, is predicted to occur in several subfields of physics. However, its experimental observation remains elusive since the emergent energy gap is too small. We show that a spinor Bose-Einstein condensate is an excellent candidate for the observation of such a peculiar phenomenon as the energy gap turns out to be 2 orders of magnitude larger than the zero-point energy. This extraordinarily large energy gap is a consequence of the dynamical instability. The propagation velocity of the resultant massive excitation mode is found to be decreased by the quantum corrections as opposed to phonons.

DOI： 10.1103/PhysRevLett.113.230401

Web of Science

Goldstone-mode instability leading to fragmentation in a spinor Bose-Einstein condensate 査読有り

Yuki Kawaguchi

PHYSICAL REVIEW A 89 巻 ( 3 ) 2014年3月

担当区分：筆頭著者,　最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

We apply the number-conserving Bogoliubov theory to spinor Bose-Einstein condensates and show that the Goldstone magnon causes instability leading to fragmentation. Unlike the dynamical instability, where modes with complex eigenfrequencies grow exponentially, here the zero-energy mode exhibits algebraic growth. We also point out that a small fraction of thermally excited atoms enhances the fragmentation dynamics.

DOI： 10.1103/PhysRevA.89.033627

Web of Science

Fluctuation-induced and symmetry-prohibited metastabilities in spinor Bose-Einstein condensates 査読有り

Nguyen Thanh Phuc, Yuki Kawaguchi, Masahito Ueda

PHYSICAL REVIEW A 88 巻 ( 4 ) 2013年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

Spinor Bose-Einstein condensates provide a unique example in which the Bogoliubov theory fails to describe the metastability associated with first-order quantum phase transitions. This problem is resolved by developing the spinor Beliaev theory which takes account of quantum fluctuations of the condensate. It is these fluctuations that generate terms of higher than the fourth order in the order-parameter field which are needed for the first-order phase transitions. Besides the conventional first-order phase transitions which are accompanied by metastable states, we find a class of first-order phase transitions which are not accompanied by metastable states. The absence of metastability in these phase transitions holds to all orders of approximation since the metastability is prohibited by the symmetry of the Hamiltonian at the phase boundary. Finally, the possibility of macroscopic quantum tunneling from a metastable state to the ground state is discussed.

DOI： 10.1103/PhysRevA.88.043629

Web of Science

Kibble-Zurek mechanism in a trapped ferromagnetic Bose-Einstein condensate 査読有り

Hiroki Saito, Yuki Kawaguchi, Masahito Ueda

JOURNAL OF PHYSICS-CONDENSED MATTER 25 巻 ( 40 ) 2013年10月

Magnetic domain growth in a ferromagnetic Bose-Einstein condensate: Effects of current 査読有り

Kazue Kudo, Yuki Kawaguchi

PHYSICAL REVIEW A 88 巻 ( 1 ) 2013年7月

Beliaev theory of spinor Bose-Einstein condensates 査読有り

Nguyen Thanh Phuc, Yuki Kawaguchi, Masahito Ueda

ANNALS OF PHYSICS 328 巻頁： 158 - 219 2013年1月

Symmetry classification of spin-orbit-coupled spinor Bose-Einstein condensates 査読有り国際共著

Z. F. Xu, Y. Kawaguchi, L. You, M. Ueda

PHYSICAL REVIEW A 86 巻 ( 3 ) 2012年9月

Topological classification of vortex-core structures of spin-1 Bose-Einstein condensates 査読有り

Shingo Kobayashi, Yuki Kawaguchi, Muneto Nitta, Masahito Ueda

PHYSICAL REVIEW A 86 巻 ( 2 ) 2012年8月

Finite-temperature phase diagram of a spin-1 Bose gas 査読有り国際共著

Yuki Kawaguchi, Nguyen Thanh Phuc, P. Blair Blakie

PHYSICAL REVIEW A 85 巻 ( 5 ) 2012年5月

Abe homotopy classification of topological excitations under the topological influence of vortices 査読有り

Shingo Kobayashi, Michikazu Kobayashi, Yuki Kawaguchi, Muneto Nitta, Masahito Ueda

NUCLEAR PHYSICS B 856 巻 ( 2 ) 頁： 577 - 606 2012年3月

Symmetry classification of spinor Bose-Einstein condensates 査読有り

Yuki Kawaguchi, Masahito Ueda

PHYSICAL REVIEW A 84 巻 ( 5 ) 2011年11月

Effects of thermal and quantum fluctuations on the phase diagram of a spin-1 Rb-87 Bose-Einstein condensate 査読有り

Nguyen Thanh Phuc, Yuki Kawaguchi, Masahito Ueda

PHYSICAL REVIEW A 84 巻 ( 4 ) 2011年10月

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