arXiv subjects

Masaki Oshikawa

Publications and source records attributed to Masaki Oshikawa.

124 records · Page 7Linked to original sources

arXiv1999

Ordered phase and scaling in $Z_n$ models and the three-state antiferromagnetic Potts model in three dimensions

Based on a Renormalization-Group picture of $Z_n$ symmetric models in three dimensions, we derive a scaling law for the $Z_n$ order parameter in the ordered phase. An existing Monte Carlo calculation on the three-state antiferromagnetic Potts model, which has the effective $Z_6$ symmetry, is shown to be consistent with the proposed scaling law. It strongly supports the Renormalization-Group picture that there is a single massive ordered phase, although an apparently rotationally symmetric region in the intermediate temperature was observed numerically.

cond-mat.stat-mech↗

arXiv1999

On the Field-Induced Gap in Cu Benzoate and Other S=1/2 Antiferromagnets

Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu benzoate, discovered an unexpected gap scaling as approximately the 2/3 power of an applied magnetic field. A theory of this gap, based on an effective staggered field, orthogonal to the applied uniform field, resulting from a staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading to a sine-Gordon quantum field theory, has been developed. Here we discuss many aspects of this subject in considerable detail, including a review of the S=1/2 chain in a uniform field, a spin-wave theory analysis of the uniform plus staggered field problem, exact amplitudes for the scaling of gap, staggered susceptibility and staggered magnetization with field or temperature, intensities of soliton and breather peaks in the structure function and field and temperature dependence of the total susceptibility.

cond-mat.str-el↗

arXiv1999

Theory of Low Temperature Electron Spin Resonance in Half-integer Spin Antiferromagnetic Chains

A theory of low temperature (T) electron spin resonance (ESR) in half-integer spin antiferromagnetic chains is developed using field theory methods and avoiding previous approximations. It is compared to experiments on Cu benzoate. Power laws are predicted for the line-width broadening due to various types of anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate Neel order as previously claimed, but to correspond to a sine-Gordon "breather" excitation.

cond-mat.str-el↗

arXiv1999

Voltage fluctuations on a superconductor grain attached to a quantum wire

When a finite superconductor is in contact with a 1D normal conductor, superconducting phase fluctuations lead to power-law response of the normal subsystem. As a result, the charge fluctuations on the superconductor at zero temperature have logarithmic correlator and a 1/ωpower spectrum (1/f-noise). At higher temperatures 1/ωis pushed to the high frequency region, and 1/ω^2 behavior prevails.

cond-mat.supr-con↗

arXiv1998

Boundary Critical Phenomena in the Three-State Potts Model

Boundary critical phenomena are studied in the 3- State Potts model in 2 dimensions using conformal field theory, duality and renormalization group methods. A presumably complete set of boundary conditions is obtained using both fusion and orbifold methods. Besides the previously known free, fixed and mixed boundary conditions a new one is obtained. This illustrates the necessity of considering fusion with operators that don't occur in the bulk spectrum, to obtain all boundary conditions. It is shown that this new boundary condition is dual to the mixed ones. The phase diagram for the quantum chain version of the Potts model is analyzed using duality and renormalization group arguments.

cond-mat↗

arXiv1997

Spontaneous Magnetic Flux and Quantum Noise in a Doubly Connected Mesoscopic SND Junction

We consider spontaneous magnetic flux in a doubly connected s-wave/normal metal/ d-wave superconductor (SND) Josephson junction. The flux magnitude is not quantized and is determined by the parameters of the system. In an isolated system, quantum fluctuations of the superconducting phase take place, leading to quantum flux noise. The possibilities of experimental observation of the effect are discussed.

cond-mat.supr-con↗

arXiv1997

Impurity in a Luttinger liquid away from half-filling: a numerical study

Conformal field theory gives quite detailed predictions for the low energy spectrum and scaling exponents of a massless Luttinger liquid at generic filling in the presence of an impurity. While these predictions were verified for half-filled systems, there was till now no analysis away from this particular filling. Here, we fill in this gap by numerically investigating a quarter-filled system using the density matrix renormalization group technique. Our results confirm conformal field theory predictions, and suggest that they are indeed valid for arbitrary fillings.

cond-mat.str-el↗

arXiv1997

Theory of the field-induced gap in $S=1/2$ antiferromagnetic chains

In a recent neutron-scattering experiment on the quasi-one-dimensional $S=1/2$ antiferromagnet Cu Benzoate, a gap was induced by an applied magnetic field. We argue that the primary mechanism of the gap formation is an effective staggered field due to both the alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction. We explain the dependence of the gap on the applied field, as well as identify several peaks in the structure factor $S(q,ω)$.

cond-mat.str-el↗

arXiv1997

Boundary conformal field theory approach to the two-dimensional critical Ising model with a defect line

We study the critical two-dimensional Ising model with a defect line (altered bond strength along a line) in the continuum limit. By folding the system at the defect line, the problem is mapped to a special case of the critical Ashkin-Teller model, the continuum limit of which is the $Z_2$ orbifold of the free boson, with a boundary. Possible boundary states on the $Z_2$ orbifold theory are explored, and a special case is applied to the Ising defect problem. We find the complete spectrum of boundary operators, exact two-point correlation functions and the universal term in the free energy of the defect line for arbitrary strength of the defect. We also find a new universality class of defect lines. It is conjectured that we have found all the possible universality classes of defect lines in the Ising model. Relative stabilities among the defect universality classes are discussed.

cond-mat.stat-mech↗

arXiv1997

Non-perturbative approach to Luttinger's theorem in one dimension

The Lieb-Schultz-Mattis theorem for spin chains is generalized to a wide range of models of interacting electrons and localized spins in one-dimensional lattice. The existence of a low-energy state is generally proved except for special commensurate fillings where a gap may occur. Moreover, the crystal momentum of the constructed low-energy state is $2k_F$, where $k_F$ is the Fermi momentum of the non-interacting model, corresponding to Luttinger's theorem. For the Kondo lattice model, our result implies that $k_F$ must be calculated by regarding the localized spins as additional electrons.

cond-mat.str-el↗

arXiv1997

Magnetization plateaus in spin chains: ``Haldane gap'' for half-integer spins

We discuss zero-temperature quantum spin chains in a uniform magnetic field, with axial symmetry. For integer or half-integer spin, $S$, the magnetization curve can have plateaus and we argue that the magnetization per site $m$ is topologically quantized as $q (S - m)= integer$ at the plateaus, where $q$ is the period of the groundstate. We also discuss conditions for the presence of the plateau at those quantized values. For $S=3/2$ and $m=1/2$, we study several models and find two distinct types of massive phases at the plateau. One of them is argued to be a ``Haldane gap phase'' for half-integer $S$.

cond-mat.str-el↗

arXiv1996

Defect Lines in the Ising Model and Boundary States on Orbifolds

Critical phenomena in the two-dimensional Ising model with a defect line are studied using boundary conformal field theory on the $c=1$ orbifold. Novel features of the boundary states arising from the orbifold structure, including continuously varying boundary critical exponents, are elucidated. New features of the Ising defect problem are obtained including a novel universality class of defect lines and the universal boundary to bulk crossover of the spin correlation function.

hep-th↗

arXiv1996

Hidden Order and Dimerization Transition in $S=2$ Chains

We study ground state properties of the $S=2$ quantum antiferromagnetic chain with a bond alternation \[ H = \sum_{j} [ 1 + δ(-1)^j ] \mbox{\boldmath $S$}_{j} \cdot \mbox{\boldmath $S$}_{j+1} \] by a Quantum Monte Carlo calculation. We find that the hidden $Z_2 \times Z_2$ symmetry is broken for $0.3 < |δ| < 0.5$ while it is unbroken in the other regions. This confirms the successive dimerization transitions first predicted by Affleck and Haldane. Our result shows that these transitions can be understood in terms of the hidden $Z_2 \times Z_2$ symmetry breaking, as was discussed using the Valence-Bond-Solid states. Furthermore, we find that the behavior of the generalized string correlation is qualitatively very similar to that in the Valence-Bond-Solid states, including the location of zeroes as a function of the angle parameter.

cond-mat↗

arXiv1995

Chiral Anomaly and Spin Gap in One-Dimensional Interacting Fermions

Semiclassical approach has been developed for the one-dimensional interacting fermion systems. Starting from the incommensurate spin density wave (SDW) mean field state for the repulsive Hubbard model in 1D, the non-Abelian bosonized Lagrangian describing the spin-charge separation is obtained. The Berry phase term is derived from the chiral anomaly, and we obtain the massless Tomonaga-Luttinger liquid in the single chain case while the spin gap opens in the double-chain system. This approach offers a new method to identify the strong-coupling fixed point, and its relation to the Abelian bosonization formalism is discussed on the spin gap state. The generalization to higher dimensions is also discussed.

cond-mat↗

arXiv1994

Quantized Hall conductivity of Bloch electrons: topology and the Dirac fermion

We consider the Hall conductivity of two-dimensional non-interacting Bloch electrons when the magnetic flux per unit cell is a rational number $p/q$ where $p$ and $q$ are mutually coprime. We present a counter-example for the naive expectation that the Hall conductivity carried by a band is given by treating gap minima as Dirac fermions. Instead of the above expectation, we show that the {\em change\/} of the Hall conductivity at a gap-closing phenomenon is given by the Dirac fermion argument. Comparing with the Diophantine equation, our result implies that a band-gap closes at $q$ points simultaneously. Furthermore, we show that the dispersion relation is $q$-fold degenerate in the magnetic Brillouin zone.

cond-mat↗

arXiv1994

Fractal Structure in Two-Dimensional Quantum Regge Calculus

We study the fractal structure of the surface in two-dimensional quantum Regge calculus by performing Monte Carlo simulation with up to 200,000 triangles. The result can be compared with the universal scaling function obtained analytically in the continuum limit of dynamical triangulation, which provides us with a definite criterion whether Regge calculus serves as a proper regularization of quantum gravity. When the scale-invariant measure is taken as the measure of the link-length integration, we observe the correct scaling behavior in the data for the type of loop attached to a baby universe. The data seem to converge to the universal scaling function as the number of triangles is increased. The data for the type of loop attached to the mother universe, on the other hand, shows no scaling behavior up to the present size.

hep-lat↗