arXiv subjects

N. Kawakami

Publications and source records attributed to N. Kawakami.

At least 37 records · Page 2Linked to original sources

arXiv1998

Breakdown of the Mott insulator: Exact solution of an asymmetric Hubbard model

The breakdown of the Mott insulator is studied when the dissipative tunneling into the environment is introduced to the system. By exactly solving the one-dimensional asymmetric Hubbard model, we show how such a breakdown of the Mott insulator occurs. As the effect of the tunneling is increased, the Hubbard gap is monotonically decreased and finally disappears, resulting in the insulator-metal transition. We discuss the origin of this quantum phase transition in comparison with other non-Hermitian systems recently studied.

cond-mat.stat-mech↗

arXiv1998

Universal conductance in quantum wires in the presence of Umklapp scattering

The effects of Umklapp scattering on the zero-temperature conductance in one-dimensional quantum wires are reexamined by taking into account both the screening of external potential and the non-uniform chemical potential shift due to electron-electron interaction. It is shown that in the case away from half-filling the conductance is given by the universal value, $2e^2/h$, even in the presence of Umklapp scattering, owing to these renormalization effects of external potential. The conclusion is in accordance with the recent claim obtained for the system with non-interacting leads being attached to a quantum wire.

cond-mat.mes-hall↗

arXiv1998

Spectral flow of non-hermitian Heisenberg spin chain with complex twist

We investigate the spectral flow of the integrable non-hermitian Heisenberg spin chain under boundary conditions with complex twist angle. It is shown that the period of the spectral flow is $4π$ up to a certain critical imaginary twist, beyond which the period jumps successively to higher values. We argue that this phenomenon caused by non-hermitian properties of the system is closely related to the metal-insulator transition caused by non-hermitian hoppings for the one-dimensional insulator.

cond-mat.stat-mech↗

arXiv1997

Mixed-Spin Ladders and Plaquette Spin Chains

We investigate low-energy properties of a generalized spin ladder model with both of the spin alternation and the bond alternation, which allows us to systematically study not only ladder systems but also alternating spin chains. By exploiting non-linear $σ$ model techniques we study the model with particular emphasis on the competition between gapful and gapless states. Our approach turns out to provide a more consistent semi-classical description of alternating spin chains than that in the previous work. We also study a closely related model, i.e., a spin chain with plaquette structure, and show that frustration causes little effect on its low-energy properties so far as the strength of frustration is weaker than a certain critical value.

cond-mat.str-el↗

arXiv1997

Critical Properties of Spectral Functions for the 1D Anisotropic t-J Models with an Energy Gap

We exactly calculate the momentum-dependent critical exponents for spectral functions in the one-dimensional anisotropic t-J models with a gap either in the spin or charge excitation spectrum. Our approach is based on the Bethe ansatz technique combined with finite-size scaling techniques in conformal field theory. It is found that the spectral functions show a power-law singularity, which occurs at frequencies determined by the dispersion of a massive spin (or charge) excitation.We discuss how the nontrivial contribution of a massive excitation controls the singular behavior in optical response functions.

cond-mat.str-el↗

arXiv1997

Alternating-Spin Ladders

We investigate a two-leg spin ladder system composed of alternating-spin chains with two-different kind of spins. The fixed point properties are discussed by using spin-wave analysis and non-linear sigma model techniques. The model contains various massive phases, reflecting the interplay between the bond-alternation and the spin-alternation.

cond-mat.stat-mech↗

arXiv1997

Low-Energy Properties of Regularly Depleted Spin Ladders

We investigate a model for the regularly depleted two-leg spin ladder systems. By using Lieb-Schultz-Mattis theorem, it is rigorously shown that this model realizes massless excitations or, alternatively, a degenerate ground state, although the original spin ladder system has a spin gap and a unique ground state. The ground state of the depleted model is either a spin singlet or partially ferromagnetic reflecting topological properties of the depleted sites. In order to show that the low-energy excitations are indeed massless, we proceed our analysis in two different ways by resorting to effective field theories. We first investigate an effective weak-coupling model in terms of renormalization group methods. Although the tendency to massless spin excitations is seen in the strong coupling regime, it turns out that the model is still massive for any finite coupling, implying that a conventional weak-coupling approach is not efficient to describe massless modes in our model. To overcome this difficulty, we further study low-energy properties of the depleted spin model by mapping on the non-linear sigma model, and confirm that the massless spin excitation indeed occur.

cond-mat.stat-mech↗

arXiv1997

Alternating spin chains with singlet ground states

We investigate low-energy properties of the alternating spin chain model composed of spin $s_1$ and $s_2$ with a singlet ground state. After examining the spin-wave spectrum in detail, we map low-energy spin excitations to the O(3) non-linear sigma model in order to take into account quantum fluctuations. Analyzing the topological term in the resulting sigma model, we discuss how the massless or massive excitations are developed, especially according to the topological nature of the alternating spin system.

cond-mat.stat-mech↗

arXiv1997

Spin Chains with Periodic Array of Impurities

We investigate the spin chain model composed of periodic array of two kinds of spins $S_1$ and $S_2$, which allows us to study the spin chains with impurities as well as the alternating spin chains in a unified fashion. By using the Lieb-Shultz-Mattis theorem, we first study the model rigorously, and then by mapping it to the non-linear sigma model, we extensively investigate low-energy properties with particular emphasis on the competition between the massive and massless phases.

cond-mat.stat-mech↗

arXiv1996

SU(ν) Generalization of Twisted Haldane-Shastry Model

The SU($ν$) generalized Haldane-Shastry spin chain with $1/r^2$ interaction is studied with twisted boundary conditions. The exact wavefunctions of Jastrow type are obtained for every rational value of the twist angle in unit of $2π$. The spectral flow of the ground state is then discussed as a function of the twist angle. By resorting to the motif picture in the Bethe ansatz method, we show that the period of the spectral flow is $ν$, which is determined by the statistical interaction in exclusion statistics.

cond-mat.stat-mech↗

arXiv1996

Spectral flow in the supersymmetric $t$-$J$ model with a $1/r^2$ interaction

The spectral flow in the supersymmetric {\it t-J} model with $1/r^2$ interaction is studied by analyzing the exact spectrum with twisted boundary conditions. The spectral flows for the charge and spin sectors are shown to nicely fit in with the motif picture in the asymptotic Bethe ansatz. Although fractional exclusion statistics for the spin sector clearly shows up in the period of the spectral flow at half filling, such a property is generally hidden once any number of holes are doped, because the commensurability condition in the motif is not met in the metallic phase.

cond-mat↗

arXiv1996

Adiabatic Ground-State Properties of Spin Chains with Twisted Boundary Conditions

We study the Heisenberg spin chain with twisted boundary conditions, focusing on the adiabatic flow of the energy spectrum as a function of the twist angle. In terms of effective field theory for the nearest-neighbor model, we show that the period 2 (in unit $2π$) obtained by Sutherland and Shastry arises from irrelevant perturbations around the massless fixed point, and that this period may be rather general for one-dimensional interacting lattice models at half filling. In contrast, the period for the Haldane-Shastry spin model with $1/r^2$ interaction has a different and unique origin for the period, namely, it reflects fractional statistics in Haldane's sense.

cond-mat↗

arXiv1996

Exact solution and spectral flow for twisted Haldane-Shastry model

The exact solution of the spin chain model with $1/r^2$ exchange is found for twisted boundary conditions. The spectrum thus obtained can be reproduced by the asymptotic Bethe ansatz. The spectral flow of each eigenstate is determined exactly as a function of the twist angle. We find that the period $4π$ for the ground state nicely fits in with the notion of fractional exclusion statistics.

cond-mat↗

arXiv1995

Exact Critical Properties of the Multi-Component Interacting Fermion Model with Boundaries

Exact critical properties of the one-dimensional SU($N$) interacting fermion model with open boundaries are studied by using the Bethe ansatz method. We derive the surface critical exponents of various correlation functions using boundary conformal field theory. They are classified into two types, i.e. the exponents for the chiral SU($N$) Tomonaga-Luttinger liquid and those related to the orthogonality catastrophe. We discuss a possible application of the results to the photoemission (absorption) in the edge state of the fractional quantum Hall effect.

cond-mat↗

arXiv1995

Exact Finite-Size Spectra in the Kondo Problem and Boundary Conformal Field Theory

The exact finite-size spectra for several quantum impurity models related to the Kondo problem are obtained from the Bethe ansatz solutions. Using the finite-size scaling in boundary conformal field theory, we determine various surface critical exponents from the exact spectrum, which accord with those obtained by Affleck and Ludwig with Kac-Moody fusion rules. Some applications to critical phenomena observed in connection with the orthogonality catastrophe are also discussed.

cond-mat↗

arXiv1995

Spin-$S$ generalization of fractional exclusion statistics

We study fractional exclusion statistics for quantum systems with SU(2) symmetry (arbitrary spin $S$), by generalizing the thermodynamic equations with squeezed strings proposed by Ha and Haldane. The bare hole distributions as well as the statistical interaction defined by an infinite-dimensional matrix specify the universality class. It is shown that the system is described by the level-$2S$ WZW model and has a close relationship to non-abelian fractional quantum Hall states. As a low-energy effective theory, the sector of {\it massless} Z$_{2S}$ parafermions is extracted, whose statistical interaction is given by a finite-dimensional matrix.

cond-mat↗

arXiv1995

Exact Critical Exponents for Pseudo-Particles in the Kondo Problem

Exact critical exponents of the Green functions for pseudo-fermions and slave bosons in the SU($N$) Anderson model with $U\rightarrow\infty$ are obtained by using the Bethe ansatz solution and boundary conformal field theory. They are evaluated exactly for mixed valence systems and Kondo systems with crystalline fields. The results agree with the prediction of Menge and Müller-Hartmann, which coincide with those of the X-ray problem. Some implication of our results in one-dimensional chiral systems is also discussed.

cond-mat↗

arXiv1995

Weak-Coupling Approach to Hole-Doped S=1 Haldane Systems

As a weak-coupling analogue of hole-doped $S=1$ Haldane systems, we study two models for coupled chains via Hund coupling; coupled Hubbard chains, and a Hubbard chain coupled with an $S=1/2$ Heisenberg chain. The fixed point properties of these models are investigated by using bosonization and renormalization group methods. The effect of randomness on these fixed points is also studied. It is found that the presence of the disorder parameter inherent in the Haldane state in the former model suppresses the Anderson localization for weak randomness, and stabilizes the Tomonaga-Luttinger liquid state with the spin gap.

cond-mat↗