arXiv subjects

Yoshio Kuramoto

Publications and source records attributed to Yoshio Kuramoto.

At least 73 records · Page 4Linked to original sources

arXiv2001

Electron Addition Spectrum in the Supersymmetric t-J Model with Inverse-Square Interaction

The electron addition spectrum A^+(k,omega) is obtained analytically for the one-dimensional (1D) supersymmetric t-J model with 1/r^2 interaction. The result is obtained first for a small-sized system and its validity is checked against the numerical calculation. Then the general expression is found which is valid for arbitrary size of the system. The thermodynamic limit of A^+(k,omega) has a simple analytic form with contributions from one spinon, one holon and one antiholon all of which obey fractional statistics. The upper edge of A^+(k,omega) in the (k,omega) plane includes a delta-function peak which reduces to that of the single-electron band in the low-density limit.

cond-mat.str-el↗

arXiv2000

Dynamical Properties of the 1/r^2-Type Supersymmetric t-J Model in a Magnetic Field: Manifestation of Spin-Charge Separation

Quasi-particle picture in a magnetic field is pursued for dynamical spin and charge correlation functions of the one-dimensional supersymmetric t-J model with inverse-square interaction. With use of exact diagonalization and the asymptotic Bethe-ansatz equations for finite systems, excitation contents of relevant excited states are identified which are valid in the thermodynamic limit. The excitation contents are composed of spinons, antispinons, holons and antiholons obeying fractional statistics. Both longitudinal and transverse components of the dynamical spin structure factor are independent of the electron density in the region where only quasi-particles with spin degrees of freedom (spinons and antispinons) contribute. The dynamical charge structure factor does not depend on the spin-polarization density in the region where only quasi-particles with charge (holons and antiholons) are excited. These features indicate the strong spin-charge separation in dynamics, reflecting the high symmetry of the model.

cond-mat.str-el↗

arXiv2000

Transition between Compressible and Incompressible States in Infinite-Layer Fractional Quantum Hall Systems

Possible phase transitions between incompressible quantum Hall states and compressible three-dimensional states are discussed for infinite-layer electron systems in strong magnetic field. By variational Monte Carlo calculation, relative stability of some trial states is studied. If the inter-layer distance is large enough, the Laughlin state is stabilized for the Landau-level filling ν=1/3 of each layer. When the inter-layer distance is comparable to the magnetic length, the Laughlin state becomes unstable against a different fractional quantum Hall state with inter-layer correlation, and/or against a three-dimensional compressible state. It is discussed how the quantum phase transition between them can be controlled in actual systems.

cond-mat.mes-hall↗

arXiv2000

Octupole Moment as a Hidden Order Parameter in Orbitally Degenerate f-Electron Systems

Possibility of a novel pseudo-scalar (octupole) order is studied theoretically for orbitally degenerate systems with strong spin-orbit coupling such as Ce$_x$La$_{1-x}$B$_6$. It is discussed that coexistence of an octupole order parameter and antiferromagnetic fluctuation should lead to drastic softening of the elastic constant by a mode-mixing effect. Nonlinear coupling between dipole, quadrupole and octupole fluctuations is taken into account in terms of a Ginzburg-Landau-type functional which is derived microscopically through path integral.

cond-mat.str-el↗

arXiv1999

Exact Dynamics of the SU(K) Haldane-Shastry Model

The dynamical structure factor $S(q,ω)$ of the SU(K) (K=2,3,4) Haldane-Shastry model is derived exactly at zero temperature for arbitrary size of the system. The result is interpreted in terms of free quasi-particles which are generalization of spinons in the SU(2) case; the excited states relevant to $S(q,ω)$ consist of K quasi-particles each of which is characterized by a set of K-1 quantum numbers. Near the boundaries of the region where $S(q,ω)$ is nonzero, $S(q,ω)$ shows the power-law singularity. It is found that the divergent singularity occurs only in the lowest edges starting from $(q,ω) = (0,0)$ toward positive and negative q. The analytic result is checked numerically for finite systems via exact diagonalization and recursion methods.

cond-mat.str-el↗

arXiv1999

Exact dynamical structure factor of the degenerate Haldane-Shastry model

The dynamical structure factor $S(q,ω)$ of the K-component (K = 2,3,4) spin chain with the 1/r^2 exchange is derived exactly at zero temperature for arbitrary size of the system. The result is interpreted in terms of a free quasi-particle picture which is generalization of the spinon picture in the SU(2) case; the excited states consist of K quasi-particles each of which is characterized by a set of K-1 quantum numbers. Divergent singularities of $S(q,ω)$ at the spectral edges are derived analytically. The analytic result is checked numerically for finite systems.

cond-mat.str-el↗

arXiv1999

Crossover between Fermi Liquid and non-Fermi Liquid in Orbitally Degenerate Kondo Systems

Entanglement of spin and orbital Kondo effect is investigated on the basis of a Kondo-type exchange model with twofold orbital degeneracy. By using Wilson's numerical renormalization-group method, we examine dynamical and thermal properties respecting the difference in time-reversal property of multipole operators. In the presence of particle-hole symmetry, the model has a new non-Fermi-liquid fixed point with a fractional entropy. The spectral intensity of the quadrupole susceptibility diverges in the zero-frequency limit, while the dipole susceptibility shows a Fermi-liquid-like behavior. This is understood by mapping to the two-channel Kondo model, in which the dipole moment is mapped onto the operators with the scaling dimension $Δ_m=1$, while the quadrupole moment onto the operators with another scaling dimension $Δ_e=1/2$. Even for a fairly particle-hole asymmetric case with the Fermi-liquid ground state, the non-Fermi-liquid behavior has significant influences in electric and thermal properties.

cond-mat.str-el↗

arXiv1999

Exact Charge Dynamics in the Supersymmetric t-J Model with Inverse-Square Interaction

Dynamical charge structure factor $N(Q,ω)$ with $Q$ smaller than the Fermi wave number is derived analytically for the one-dimensional supersymmetric t-J model with $1/r^2$ interaction. Strong spin-charge separation in dynamics is found. $N(Q,ω)$ with given electron density does not depend on the spin polarization for arbitrary size of the system. In the thermodynamic limit, only two holons and one antiholon contribute to $N(Q,ω)$. These results together with generalized ones for the SU($K$,1) supersymmetric t-J model are derived via solution of the Sutherland model in the strong coupling limit.

cond-mat.str-el↗

arXiv1999

Dynamical Properties of the One-Dimensional Supersymmetric t-J Model: A View from Elementary Excitations

Dynamical properties, such as dynamical spin and charge structure factors and single-particle spectral functions, are studied for the one-dimensional supersymmetric t-J model with inverse-square interaction. Exact diagonalization and the recursion method are used for finite systems up to 16 sites. A simple rule is proposed to understand the supermultiplet structure in the excitation spectrum. Numerical calculations show that the dynamical spin structure factor is independent of the electron density in the region where only two spinons contribute. In the electron removal spectrum from half-filling, close correspondence is found between the dominant contribution from three-spinon plus one-holon states in the t-J model and the two-spinon excitations in the Haldane-Shastry model. Comparison is made with dynamics of the nearest-neighbor supersymmetric t-J model.

cond-mat.str-el↗

arXiv1999

Charge Kondo effect toward a non-Fermi-liquid fixed point in the orbitally degenerate exchange model

We show that a Kondo-type model with an orbital degeneracy has a new non-Fermi-liquid fixed point. Near the fixed point the spin degrees of freedom are completely quenched, and the residual charge degrees of freedom lead to the multi-channel Kondo effect. Anomalous behavior appears in electric and thermal properties, but the magnetic susceptibility should show the local Fermi-liquid behavior. The non-Fermi-liquid fixed point becomes unstable against perturbations breaking the particle-hole symmetry. We derive these results using the third-order scaling for a spherically symmetric model with a fictitious spin. In contrast to the Coqblin-Schrieffer model, the present model respects different time-reversal properties of multipole operators.

cond-mat.str-el↗

arXiv1998

Perturbative renormalization of multi-channel Kondo-type models

The poor man's scaling is extended to higher order by the use of the open-shell Rayleigh-Schroedinger perturbation theory. A generalized Kondo-type model with the SU(n) times SU(m) symmetry is proposed and renormalized to the third order. It is shown that the model has both local Fermi-liquid and non-Fermi-liquid fixed points, and that the latter becomes unstable in the special case of n=m=2. Possible relevance of the model to the newly found phase IV in Ce_{x}La_{1-x}B_6 is discussed.

cond-mat↗

arXiv1998

Fluctuation Effects on the Quadrupolar Ordering in Magnetic Field

Effects of magnetic field on the quadrupolar ordering are investigated with inclusion of fluctuation of order parameters. For the simplest model with the nearest-neighbor quadrupolar interaction, the transition temperature and the specific heat are derived by the use of the recently proposed effective medium theory. It is shown that magnetic field H has two competing effects on the quadrupolar ordering; one is to encourage the ordering by suppressing the fluctuation among different components of order parameters, and the other is to block the ordering as in antiferromagnets. The former is found to be of order H^2 and the latter of order H^4. Hence the fluctuation is suppressed for weak fields, and the transition temperature increases with magnetic field. The fluctuation effect is so strong that the entropy released at the quadrupolar ordering is only about half of the full value ln 4 even without the Kondo effect.

cond-mat.str-el↗

arXiv1998

Dynamical Structure Factors of the Spin-1/2 XXZ Chain with Inverse-Square Exchange and Ising Anisotropy

The dynamical properties of the S=1/2 antiferromagnetic XXZ chain are studied by the exact diagonalization and the recursion method of finite systems up to 24 sites. Two types of the exchange interaction are considered: one is the nearest-neighbor type, and the other is the inverse-square one. As the Ising anisotropy becomes larger, there appears a noticeable difference in the transverse component S^{xx}(q,ω) between the two types of the exchange. For the nearest-neighbor type, the peak frequency of S^{xx}(q,ω) for each q approaches the center of the continuum spectrum. On the contrary, the peak frequency for the inverse-square type moves to the upper edge of the continuum, and separates from the continuum for the anisotropy larger than the threshold value. Whether the interaction between domain walls (solitons) is absent or repulsive in the Ising limit leads to this difference in the behavior of S^{xx}(q,ω). In the longitudinal component S^{zz}(q,ω), on the other hand, the feature of the dynamics is scarcely different between the two types. The energy gap and the static properties are also discussed.

cond-mat.str-el↗

arXiv1997

Dynamical Effective Medium Theory for Quantum Spins and Multipoles

A dynamical effective medium theory is presented for quantum spins and higher multipoles such as quadrupole moments. The theory is a generalization of the spherical model approximation for the Ising model, and is accurate up to O(1/z_n) where z_n is the number of interacting neighbors. The polarization function is optimized under the condition that it be diagonal in site indices. With use of auxiliary fields and path integrals, the theory is flexibly applied to quantum spins and higher multipoles with many interacting neighbors. A Kondo-type screening of each spin is proposed for systems with extreme quantum fluctuations but without conduction electrons.

cond-mat↗

arXiv1997

Competition between crystalline electric field singlet and itinerant states of f electrons

A new kind of phase transition is proposed for lattice fermion systems with simplified f^2 configurations at each site. The free energy of the model is computed in the mean-field approximation for both the itinerant state with the Kondo screening, and a localized state with the crystalline electric field (CEF) singlet at each site. The presence of a first-order phase transition is demonstrated in which the itinerant state changes into the localized state toward lower temperatures. In the half-filled case, the insulating state at high temperatures changes into a metallic state, in marked contrast with the Mott transition in the Hubbard model. For comparison, corresponding states are discussed for the two-impurity Kondo system with f^1 configuration at each site.

cond-mat.str-el↗

arXiv1996

Origin of Magic Angular Momentum in a Quantum Dot under Strong Magnetic Field

This paper investigates origin of the extra stability associated with particular values (magic numbers) of the total angular momentum of electrons in a quantum dot under strong magnetic field. The ground-state energy, distribution functions of density and angular momentum, and pair correlation function are calculated in the strong field limit by numerical diagonalization of the system containing up to seven electrons. It is shown that the composite fermion picture explains the small magic numbers well, while a simple geometrical picture does better as the magic number increases. Combination of these two pictures leads to identification of all the magic numbers. Relation of the magic-number states to the Wigner crystal and the fractional quantum Hall state is discussed.

cond-mat↗

arXiv1996

Partially Solvable Anisotropic t-J Model with Long-Range Interactions

A new anisotropic t-J model in one dimension is proposed which has long-range hopping and exchange. This t-J model is only partially solvable in contrast to known integrable models with long-range interaction. In the high-density limit the model reduces to the XXZ chain with the long-range exchange. Some exact eigenfunctions are shown to be of Jastrow-type if certain conditions for an anisotropy parameter are satisfied. The ground state as well as the excitation spectrum for various cases of the anisotropy parameter and filling are derived numerically. It is found that the Jastrow-type wave function is an excellent trial function for any value of the anisotropy parameter.

cond-mat↗

arXiv1996

Fractional Exclusion Statistics for the t-J Model

We construct thermodynamics of the one-dimensional supersymmetric {\it t-J} model with the $ 1/\sin^2$ interaction and hopping. The thermodynamics is described exactly in terms of free spinons and holons obeying Haldane's fractional exclusion statistics at all temperatures. Moreover, at low temperatures the semionic spinons and holons decouple resulting in the spin-charge separation in thermodynamic properties. We obtain explicit results for the spin and charge susceptibilities and specific heat, and interpret them in terms of the fractional exclusion statistics. Extension to the multi-component {\it t-J} model shows that the excitations obey either fractional statistics for g-ons with partial polarization of components, or the parafermionic one without polarization.

cond-mat↗