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Masaki Oshikawa

Publications and source records attributed to Masaki Oshikawa.

At least 109 records · Page 6Linked to original sources
arXiv2006

Stable Skyrmions in spinor condensates

Globally symmetric spinor condensates in free space are argued not to support stable topological defects in either two or three dimensions. In the latter case, however, we show that a topological Skyrmion can be stabilized by forcing it to adopt certain density profiles. A sufficient condition for the existence of Skyrmion solutions in three dimensions is formulated and illustrated in simple examples. Our results pertain to Bose-Einstein condensation in Rb.

cond-mat.supr-con↗
arXiv2006

Systematic Derivation of Order Parameters through Reduced Density Matrices

A systematic method for determining order parameters for quantum many-body systems on lattices is developed by utilizing reduced density matrices. This method allows one to extract the order parameter directly from the wave functions of the degenerate ground states without aid of empirical knowledge, and thus opens a way to explore unknown exotic orders. The applicability of this method is demonstrated numerically or rigorously in models which are considered to exhibit dimer, scalar chiral, and topological orders.

cond-mat.str-el↗
arXiv2006

Saturated Ferromagnetism from Statistical Transmutation in Two Dimensions

The total spin of the ground state is calculated in the U -> infinity Hubbard model with uniform magnetic flux perpendicular to a square lattice, in the absence of Zeeman coupling. It is found that the saturated ferromagnetism emerges in a rather wide region in the space of the flux density ϕand the electron density n_e. In particular, the saturated ferromagnetism at ϕ= n_e is induced by the formation of a spin-1/2 boson, which is a composite of an electron and the unit flux quantum.

cond-mat.str-el↗
arXiv2005

Fractionalization, topological order, and quasiparticle statistics

We argue, based on general principles, that topological order is essential to realize fractionalization in gapped insulating phases in dimensions $d \geq 2$. In $d=2$ with genus $g$, we derive the existence of the minimum topological degeneracy $q^g$ if the charge is fractionalized in unit of $1/q$, irrespective of microscopic model or of effective theory. Furthermore, if the quasiparticle is either boson or fermion, it must be at least $q^{2g}$.

cond-mat.str-el↗
arXiv2005

Junctions of three quantum wires

We study a junction of three quantum wires enclosing a magnetic flux. The wires are modeled as single-channel spinless Tomonaga-Luttinger liquids. This is the simplest problem of a quantum junction between Tomonaga-Luttinger liquids in which Fermi statistics enter in a non-trivial way. We study the problem using a mapping onto the dissipative Hofstadter model, describing a single particle moving on a plane in a magnetic field and a periodic potential coupled to a harmonic oscillator bath. Alternatively we study the problem by identifying boundary conditions corresponding to the low energy fixed points. We obtain a rich phase diagram including a chiral fixed point in which the asymmetric current flow is highly sensitive to the sign of the flux and a phase in which electron pair tunneling dominates. We also study the effects on the conductance tensor of the junction of contacting the three quantum wires to Fermi liquid reservoirs.

cond-mat.str-el↗
arXiv2005

Direct perturbation theory on the shift of Electron Spin Resonance

We formulate a direct and systematic perturbation theory on the shift of the main paramagnetic peak in Electron Spin Resonance, and derive a general expression up to second order. It is applied to one-dimensional XXZ and transverse Ising models in the high field limit, to obtain explicit results including the polarization dependence for arbitrary temperature.

cond-mat.stat-mech↗
arXiv2004

Bose-Einstein Condensation of Magnons in TlCuCl3: Phase diagram and specific heat from a self-consistent Hartee-Fock calculation with a realistic dispersion relation

We extend the self-consistent Hartree-Fock-Popov calculations by Nikuni et al. [Phys. Rev. Lett. 84, 5868 (2000)] concerning the Bose-Eistein condensation of magnons in TlCuCl3 to include a realistic dispersion of the excitations. The result for the critical field as a function of temperature behaves as Hc(T)-Hc(0) T^{3/2} below 2K but deviates from this simple power-law at higher temperature and is in very good agreement with the experimental results. The specific heat is computed as a function of temperature for different values of the magnetic field. It shows a lambda-like shape at the transition and is in good qualitative agreement with the results of Oosawa et al. [Phys. Rev. B 63, 134416 (2001)].

cond-mat.str-el↗
arXiv2004

Coupled $S=1/2$ Heisenberg antiferromagnetic chains in an effective staggered field

We present a systematic study of coupled $S=1/2$ Heisenberg antiferromagnetic chains in an effective staggered field. We investigate several effects of the staggered field in the {\em higher} ({\em two or three}) {\em dimensional} spin system analytically. In particular, in the case where the staggered field and the inter-chain interaction compete with each other, we predict, using mean-field theory, a characteristic phase transition. The spin-wave theory predicts that the behavior of the gaps induced by the staggered field is different between the competitive case and the non-competitive case. When the inter-chain interactions are sufficiently weak, we can improve the mean-field phase diagram by using chain mean-field theory and the analytical results of field theories. The ordered phase region predicted by the chain mean-field theory is substantially smaller than that by the mean-field theory.

cond-mat.str-el↗
arXiv2003

Random Matrix Theory Analysis of Cross Correlations in Financial Markets

We confirm universal behaviors such as eigenvalue distribution and spacings predicted by Random Matrix Theory (RMT) for the cross correlation matrix of the daily stock prices of Tokyo Stock Exchange from 1993 to 2001, which have been reported for New York Stock Exchange in previous studies. It is shown that the random part of the eigenvalue distribution of the cross correlation matrix is stable even when deterministic correlations are present. Some deviations in the small eigenvalue statistics outside the bounds of the universality class of RMT are not completely explained with the deterministic correlations as proposed in previous studies. We study the effect of randomness on deterministic correlations and find that randomness causes a repulsion between deterministic eigenvalues and the random eigenvalues. This is interpreted as a reminiscent of ``level repulsion'' in RMT and explains some deviations from the previous studies observed in the market data. We also study correlated groups of issues in these markets and propose a refined method to identify correlated groups based on RMT. Some characteristic differences between properties of Tokyo Stock Exchange and New York Stock Exchange are found.

cond-mat.stat-mech↗
arXiv2003

Coexistence of d_{x^2-y^2}-Wave Superconductivity and Antiferromagnetism Induced by a Staggered Field

The two-dimensional t-J model in a staggered field is studied by exact diagonalization of small clusters. For the low-hole-density region and a realistic value of J/t, it is found that the presence of a staggered field strengthens the attraction between two holes. With increasing field, the d_{x^2-y^2}-wave superconducting correlations are enhanced while the extended-s-wave ones hardly change. This implies that coexistence of the d_{x^2-y^2}-wave superconducting order and the commensurate antiferromagnetic order occurs in a staggered field.

cond-mat.str-el↗
arXiv2003

Insulator, conductor and commensurability: a topological approach

I discuss a topological relation of the conduction property of a many-particle system on a periodic lattice at zero temperature to the energy spectrum. When the particle number per unit cell is an irreducible fraction $p/q$, an insulator must have $q$ low-lying states of energy $o(1/L)$ in one dimension and of energy $o(1)$ in two dimensions, where $L$ is the linear system size.

cond-mat.str-el↗
arXiv2003

Junctions of three quantum wires and the dissipative Hofstadter model

We study a junction of three quantum wires enclosing a magnetic flux. This is the simplest problem of a quantum junction between Tomonaga-Luttinger liquids in which Fermi statistics enter in a non-trivial way. We present a direct connection between this problem and the dissipative Hofstadter problem, or quantum Brownian motion in two dimensions in a periodic potential and an external magnetic field, which in turn is connected to open string theory in a background electromagnetic field. We find non-trivial fixed points corresponding to a chiral conductance tensor leading to an asymmetric flow of the current.

cond-mat.str-el↗
arXiv2002

Effective charge of a deconfined slave particle: Comment on ``Confinement of Slave Particles in U(1) Gauge Theories of Strongly Interacting Electrons''

I reexamine the Nayak's criticism [Phys. Rev. Lett. 85, 178 (2000); 86, 943 (2001).] on the U(1) gauge theory approach to the strongly correlated systems. Contrary to his conclusion, the arbitrariness of the charge assignment observed by him does not contradict the possibility of a deconfinement. Should the deconfinement occur, the deconfined slave particle carries an effective fractional charge, which is independent of the arbitrary assignment but is rather determined dynamically.

cond-mat.str-el↗
arXiv2002

Electron Spin Resonance in S=1/2 antiferromagnetic chains

A systematic field-theory approach to Electron Spin Resonance (ESR) in the $S=1/2$ quantum antiferromagnetic chain at low temperature $T$ (compared to the exchange coupling $J$) is developed. In particular, effects of a transverse staggered field $h$ and an exchange anisotropy (including a dipolar interaction) $δ$ on the ESR lineshape are discussed. In the lowest order of perturbation theory, the linewidth is given as $\propto Jh^2/T^2$ and $\propto (δ/J)^2 T$, respectively. In the case of a transverse staggered field, the perturbative expansion diverges at lower temperature; non-perturbative effects at very low temperature are discussed using exact results on the sine-Gordon field theory. We also compare our field-theory results with the predictions of Kubo-Tomita theory for the high-temperature regime, and discuss the crossover between the two regimes. It is argued that a naive application of the standard Kubo-Tomita theory to the Dzyaloshinskii-Moriya interaction gives an incorrect result. A rigorous and exact identity on the polarization dependence is derived for certain class of anisotropy, and compared with the field-theory results.

cond-mat.str-el↗
arXiv2000

Boundary Critical Phenomena in SU(3) "Spin" Chains

SU(3)-invariant "spin" chains with a single impurity, such as a modified exchange coupling on one link, are analyzed using boundary conformal field theory techniques. These chains are equivalent to a special case of the "tJV" model, i.e. the t-J model with a nearest neighbour repulsion added. In the continuum limit they are equivalent to two free bosons at a special value of the compactification radii. The SU(3) symmetry, which is made explicit in this formulation, provides insight into the exact solution of a non-trivial boundary critical point found earlier in another formulation of this model as a theory of quantum Brownian motion.

cond-mat↗
arXiv2000

Quantum brownian motion on a triangular lattice and c=2 boundary conformal field theory

We study a single particle diffusing on a triangular lattice and interacting with a heat bath, using boundary conformal field theory (CFT) and exact integrability techniques. We derive a correspondence between the phase diagram of this problem and that recently obtained for the 2 dimensional 3-state Potts model with a boundary. Exact results are obtained on phases with intermediate mobilities. These correspond to non-trivial boundary states in a conformal field theory with 2 free bosons which we explicitly construct for the first time. These conformally invariant boundary conditions are not simply products of Dirichlet and Neumann ones and unlike those trivial boundary conditions, are not invariant under a Heisenberg algebra.

cond-mat.stat-mech↗
arXiv1999

Commensurability, excitation gap and topology in quantum many-particle systems on a periodic lattice

Combined with Laughlin's argument on the quantized Hall conductivity, Lieb-Schultz-Mattis argument is extended to quantum many-particle systems (including quantum spin systems) with a conserved particle number, on a periodic lattice in arbitrary dimensions. Regardless of dimensionality, interaction strength and particle statistics (bose/fermi), a finite excitation gap is possible only when the particle number per unit cell of the groundstate is an integer.

cond-mat.str-el↗