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Masaki Oshikawa

Publications and source records attributed to Masaki Oshikawa.

9 recordsLinked to original sources
PubMed2006

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 87Rb.

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PubMed2006

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 > or = 2. In d = 2 with genus g, we derive the existence of the minimum topological degeneracy q(g) if the charge is fractionalized in units of 1/q, irrespective of microscopic model or effective theory. Furthermore, if the quasiparticle is either boson or fermion, it must be at least q(2g).

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PubMed2006

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 the 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 that are considered to exhibit dimer, scalar chiral, and topological orders.

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PubMed2006

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 phi and the electron density ne. In particular, the saturated ferromagnetism at phi=ne is induced by the formation of a spin-1/2 boson, which is a composite of an electron and the unit flux quantum.

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PubMed2005

Exact analysis of ESR shift in the spin-1/2 Heisenberg antiferromagnetic chain.

A systematic perturbation theory is developed for the ESR shift and is applied to the spin-1/2 Heisenberg antiferromagnetic chain with a general anisotropic exchange interaction. Using the Bethe ansatz technique, the resonance shift is obtained exactly for the whole range of temperature and magnetic field in the first order of the anisotropy. The obtained g shift strongly depends on magnetic fields at low temperature, showing a significant deviation from the previous classical result.

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PubMed2004

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.

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PubMed2003

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 nontrivial 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 nontrivial fixed points corresponding to a chiral conductance tensor leading to an asymmetric flow of the current.

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PubMed2003

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.

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