Naokazu Shibata

Thermodynamics of the Anisotropic Heisenberg Chain Calculated by the Density Matrix Renormalization Group Method

The density matrix renormalization group (DMRG) method is applied to the anisotropic Heisenberg chain at finite temperatures. The free energy of the system is obtained using the quantum transfer matrix which is iteratively enlarged in the imaginary time direction. The magnetic susceptibility and the specific heat are calculated down to T =0.01 J and compared with the Bethe ansatz results. The agreement including the logarithmic correction in the magnetic susceptibility at the isotropic point is fairly good.

Ground-state phase diagram of 2D electrons in a high Landau level: a density-matrix renormalization group study.

The ground state phase diagram of 2D electrons in a high Landau level (index N=2) is studied by the density matrix renormalization group method. Pair correlation functions are systematically calculated for various filling factors from v=1/8 to 1/2. It is shown that the ground state phase diagram consists of three different CDW states called stripe-phase, bubble-phase, and Wigner crystal. The boundary between the stripe and the bubble phases is determined to be v_c = 0.38, and that for the bubble phase and Wigner crystal is v_c = 0.24. Each transition is of first order.

SU(4) SPIN-ORBIT CRITICAL STATE IN ONE DIMENSION

Effect of quantum fluctuations concerned with the orbital degrees of freedom is discussed for the model with SU(4) symmetry in one dimension. An effective Hamiltonian is derived from the orbitally degenerate Hubbard model at quarter filling. This model is equivalent to the Bethe soluble SU(4) exchange model. Quantum numbers of the ground state and the lowest branch of excitations are determined. The spin-spin correlation functions are obtained numerically by the density matrix renormalization group method. It shows a power-law decay with oscillations of the period of four sites. The period originates from the interference between the spin and orbital degrees of freedom. The exponent of the power-law decay estimated from the finite size data is consistent with the prediction by the conformal field theory.

Controlling frustrated liquids and solids with an applied field in a kagome Heisenberg antiferromagnet

Quantum spin-1/2 kagome Heisenberg antiferromagnet is the representative frustrated system possibly hosting a spin liquid. Clarifying the nature of this elusive topological phase is a key challenge in condensed matter; however, even identifying it still remains unsettled. Here we apply a magnetic field and discover a series of spin-gapped phases appearing at five different fractions of magnetization by means of a grand canonical density matrix renormalization group, an unbiased state-of-the-art numerical technique. The magnetic field dopes magnons and first gives rise to a possible Z₃ spin liquid plateau at 1/9 magnetization. Higher field induces a self-organized super-lattice unit, a six-membered ring of quantum spins, resembling an atomic orbital structure. Putting magnons into this unit one by one yields three quantum solid plateaus. We thus find that the magnetic field could control the transition between various emergent phases by continuously releasing the frustration.

Greenish-white electroluminescence from p-type CuGaS2 heterojunction diodes using n-type ZnO as an electron injector

Greenish-white electroluminescence (EL) was observed from p-type (001) CuGaS2 chalcopyrite semiconductor epilayers grown on a (001) GaP substrate by metalorganic vapor phase epitaxy, due to the electron injection from preferentially (0001)-oriented polycrystalline n-type ZnO thin films deposited by the surface-damage-free helicon-wave-excited-plasma sputtering method. The structure was designed to enable the electron injection from n-type wide band gap partner forp‐CuGaS2 forming the ZnO∕CuGaS2 TYPE-I heterojunction. The EL spectra exhibited emission peaks and bands between 1.6 and 2.5eV, although the higher energy portion was absorbed by the GaP substrate. Since the spectral line shape resembled that of the photoluminescence from the identical CuGaS2 epilayers, the EL was assigned to originate from p‐CuGaS2.

Dielectric SiO2∕ZrO2 distributed Bragg reflectors for ZnO microcavities prepared by the reactive helicon-wave-excited-plasma sputtering method

Reactive helicon-wave-excited-plasma sputtering method is shown to be a suitable technique for the fabrication of high reflectivity (R) distributed Bragg reflectors (DBRs), in particular, operating at the resonance wavelength of B excitons in ZnO (366.5nm), utilizing quarter-wavelength multilayers of SiO2 and ZrO2 dielectric films. According to the surface-damage-free nature and proper stoichiometry controllability of the method, dense dielectric films exhibiting ideal refractive indices (1.46 for SiO2 and 2.10 for ZrO2 at 633nm) and small root-mean-square values for the surface roughness (0.20nm for SiO2 and 0.53nm for ZrO2) were deposited using Si and Zr targets and O2 gas at room temperature. Optical reflectance spectra of the SiO2∕ZrO2 DBRs agreed with those calculated using the optical multilayer film theory, and eight-pair DBR exhibited R higher than 99.5% at 366.5nm and 82nm stop bandwidth (R⩾95%). The results indicate that the DBR can be used for the realization of polariton lasers using ZnO micro...

Spin and charge gaps in the one-dimensional Kondo-lattice model with Coulomb interaction between conduction electrons.

The density-matrix renormalization-group method is applied to the one-dimensional Kondo-lattice model with the Coulomb interaction between the conduction electrons. The spin and charge gaps are calculated as a function of the exchange constant

Fractional Quantum Hall Effects in Graphene and Its Bilayer

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Thermodynamic Properties of the S=1/2 Heisenberg Chain with Staggered Dzyaloshinsky–Moriya Interaction

and the Coulomb interaction

Numerical Study of the One-Dimensional Spin-Orbit Coupled System with SU(2)⊗SU(2) Symmetry

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