arXiv subjects

D. Yoshioka

Publications and source records attributed to D. Yoshioka.

18 recordsLinked to original sources

arXiv2006

Real-Space Imaging of Alternate Localization and Extension of Quasi Two-Dimensional Electronic States at Graphite Surfaces in Magnetic Fields

We measured the local density of states (LDOS) of a quasi two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.

cond-mat.mes-hall↗

arXiv2003

Numerical Investigation on Asymmetric Bilayer System at Integer Filling Factor

Deformation of the easy-axis ferromagnetic state in asymmetric bilayer systems are investigated numerically. Using the exact diagonalization the easy-axis to easy-plane ferromagnetic transition at total filling factor 3 or 4 is investigated. At still higher filling, novel stripe state in which stripes are aligned in the vertical direction occurs. The Hartree-Fock energies of relevant ordered states are calculated and compared.

cond-mat.mes-hall↗

arXiv2002

Ground State Phase Diagram of 2D Electrons in High Magnetic Field

The ground state of 2D electrons in high magnetic field is studied by the density matrix renormalization group method. The ground state energy, excitation gap, and pair correlation functions are systematically calculated at various fillings in the lowest and the second lowest Landau levels. The ground state phase diagram, which consists of incompressible liquid state, compressible liquid state, stripe state, pairing state, and Wigner crystal is determined.

cond-mat.mes-hall↗

arXiv2001

Ground state phase diagram of 2D electrons in a high Landau level: - DMRG 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.

cond-mat.mes-hall↗

arXiv1997

Susceptibilities of Sr(Cu_(1-x)Zn_x)_2O_3 Studied by Quantum Monte Carlo Simulation

The effects of non-magnetic impurities randomly doped into a two-leg Heisenberg spin ladder are investigated. Using the continuous time quantum Monte Carlo loop algorithm we calculate the uniform and staggered susceptibilities of such a system. The obtained uniform susceptibility is well described in terms of an effective model of weakly interacting local moments induced by non-magnetic impurities for a 1% doping case, but not for higher concentrations. The staggered susceptibility however is significantly enhanced over that in the effective model already at 1% doping. Using a mean field approximation for the interladder coupling, we explain qualitatively the phase diagram of Sr(Cu_{1-x}Zn_x)_2O_3.

cond-mat.str-el↗

arXiv1997

Fine structure in the off-resonance conductance of small Coulomb blockade systems

We show how a fine, multiple-peak structure can arise in the off-resonance, zero-bias conductance of Coulomb blockade systems. In order to understand how this effect comes about one must abandon the orthodox, mean-field understanding of the Coulomb blockade phenomenon and consider quantum fluctuations in the occupation of the single-particle electronic levels. We illustrate such an effect with a spinless Anderson-like model for multi-level systems and an equation-of-motion method for calculating Green's functions that combines two simple decoupling schemes.

cond-mat.mes-hall↗

arXiv1996

Inter-layer Edge Tunneling and Transport Properties in Double-Layer Quantum Hall Systems

A theory of transport in the quantum Hall regime is developed for separately contacted double-layer electron systems. Inter-layer tunneling provides a channel for equilibration of the distribution functions in the two layers and influences transport properties through the resulting influence on steady-state distribution functions. Resistences for various configurations of the electrodes are calculated as a function of the inter-layer tunneling amplitude. The effect of misalignment of the edges of the two layers and the effect of tilting the magnetic field away from the normal to the layers on the inter- layer tunneling amplitude near the sample edges are investigated. The results obtained in this work is consistent with recent experiments.

cond-mat↗

arXiv1996

Spontaneous Interlayer Coherence in Double-Layer Quantum Hall Systems: Symmetry Breaking Interactions, In-Plane Fields and Phase Solitons

At strong magnetic fields double-layer two-dimensional-electron-gas systems can form an unusual broken symmetry state with spontaneous inter-layer phase coherence. The system can be mapped to an equivalent system of pseudospin $1/2$ particles with pseudospin-dependent interactions and easy-plane magnetic order. In this paper we discuss how the presence of a weak interlayer tunneling term alters the properties of double-layer systems when the broken symmetry is present. We use the energy functional and equations of motion derived earlier to evaluate the zero-temperature response functions of the double-layer system and use our results to discuss analogies between this system and Josephson-coupled superconducting films. We also present a qualitative picture of the low-energy charged excitations of this system. We show that parallel fields induce a highly collective phase transition to an incommensurate state with broken translational symmetry.

cond-mat↗

arXiv1996

Edge State Transport in Separately Contacted Double-Layer Quantum Hall Systems

We develop a theory of edge state transport in separately contacted double-layer quantum Hall systems which are tuned close to the resonance condition for tunneling between the layers. When applied to the case where contact is made to only one layer, the theory gives a quantized Hall resistance and zero longitudinal resistance in both weak and strong inter-layer coupling limits. For weak coupling, the leading correction to the Hall resistance occurs at second order, while the longitudinal resistance appears at first order. Recent experiments which show almost quantized Hall resistance and a substantial longitudinal resistance are explained and predictions are made for other contact configurations.

cond-mat↗

arXiv1996

Long-lived charged multiple-exciton complexes in strong magnetic fields

We consider the charged exciton complexes of an ideal two-dimensional electron-hole system in the limit of strong magnetic fields. A series of charged multiple-exciton states is identified and variational and finite-size exact diagonalization calculations are used to estimate their binding energies. We find that, because of a hidden symmetry, bound states of excitons and an additional electron cannot be created by direct optical absorption and, once created, have an infinite optical recombination lifetime. We also estimate the optical recombination rates when electron and hole layers are displaced and the hidden symmetry is violated.

cond-mat↗

arXiv1996

Resonating-Valence-Bond Ground-state of CaV$_4$O$_9$ by the Gutzwiller-projected Schwinger-boson method

An antiferromagnetic Heisenberg model on a 1/5-depleted two-dimensional square-lattice, a model of CaV$_4$O$_9$, is investigated by variational Monte Carlo simulation. A prototype of a trial wave function is made by projecting out the doubly occupied states from the Schwinger-boson mean-field solution. Then variational Monte Carlo simulation is performed up to $40 \times 40 $ sites(including $320$ vacant sites). The optimized state has the lowest energy, $-0.5510J$, ever reported. For this state energies of a dimer bond and a plaquette bond, staggered magnetization, static structure factor, and excitation spectrum are calculated. It is shown that the Néel order survives and there is no gap at isotropic coupling.

cond-mat↗

arXiv1995

Bi-layer Heisenberg model studied by the Schwinger-boson Gutzwiller-projection method

A two-dimensional bi-layer, square lattice Heisenberg model with different intraplane($J_{\parallel}$) and interplane($J_{\perp}$) couplings is investigated. The model is first solved in the Schwinger boson mean-field approximation. %It is shown that order-disorder transition occurs as the interplane Coupling %is increased. The critical ratio is $J_{\perp/\p=4.48J$ Then the solution is Gutzwiller projected to satisfy the local constraint that there should be only one boson at each site. For these wave functions, we perform variational Monte Carlo simulation up to $24 \times 24 \times 2$ sites. It is shown that the Néel order is destroyed as the interplane coupling is increased. The obtained critical value, $J_{\perp}/J_{\parallel}=3.51$, is smaller than that by the mean-field theory. Excitation spectrum is calculated by a single mode approximation. It is shown that energy gap develops once the Néel order is destroyed.

cond-mat↗

arXiv1995

The effect of the Coulomb interaction on the mesoscopic persistent current

The persistent current in three-dimensional mesoscopic rings is investigated numerically. The model is tight-binding one with random site-energies and interaction between electrons. The self-consistent Hartree-Fock approximation is adopted for the interaction between electrons, and models with up to $6\times 6\times 20$ sites are investigated. It is shown that the long-range Coulomb interaction enhances the persistent current for rings with finite width. The origin of the enhancement is discussed.

cond-mat↗

arXiv1994

Enhancement of Persistent Currents by the Coulomb Interaction

The persistent current in three-dimensional mesoscopic rings is investigated numerically. The model is tight-binding one with random site-energies and interaction between electrons. The Hartree-Fock approximation is adopted for the interaction between electrons, and models with up to $3\times 4\times 40$ sites are investigated. It is shown that the long-range Coulomb interaction enhances the persistent current considerably for rings with finite width. It is also shown that long-rangedness of the interaction is essential for the enhancement. Screening of the random site-energy is attributed to this enhancement.

cond-mat↗

arXiv1994

Anisotropic two-dimensional Heisenberg model by Schwinger-boson Gutzwiller projected method

Two-dimensional Heisenberg model with anisotropic couplings in the $x$ and $y$ directions ($J_x \neq J_y$) is considered. The model is first solved in the Schwinger-boson mean-field approximation. Then the solution is Gutzwiller projected to satisfy the local constraint that there is only one boson at each site. The energy and spin-spin correlation of the obtained wavefunction are calculated for systems with up to $20 \times 20$ sites by means of the variational Monte Carlo simulation. It is shown that the antiferromagnetic long-range order remains down to the one-dimensional limit.

cond-mat↗

arXiv1994

Spontaneous Inter-layer Coherence in Double-Layer Quantum-Hall Systems I: Charged Vortices and Kosterlitz-Thouless Phase Transitions

At strong magnetic fields double-layer two-dimensional-electron-gas systems can form an unusual broken symmetry state with spontaneous inter-layer phase coherence. In this paper we explore the rich variety of quantum and finite-temperature phase transitions associated with this broken symmetry. We describe the system using a pseudospin language in which the layer degree-of-freedom is mapped to a fictional spin 1/2 degree-of-freedom. With this mapping the spontaneous symmetry breaking is equivalent to that of a spin 1/2 easy-plane ferromagnet. In this language spin-textures can carry a charge. In particular, vortices carry e/2 electrical charge and vortex-antivortex pairs can be neutral or carry charge e. We derive an effective low-energy action and use it to discuss the charged and collective neutral excitations of the system. We have obtained the parameters of the Landau-Ginzburg functional from first-principles estimates and from finite-size exact diagonalization studies. We use these results to estimate the dependence of the critical temperature for the Kosterlitz-Thouless phase transition on layer separation.

cond-mat↗

arXiv1994

Suppression of Persistent Currents in 1-D Disordered Rings by Coulomb Interaction

Effects of Coulomb interaction on persistent currents in disordered one-dimensional rings are numerically investigated. First of all effectiveness of the Hartree-Fock approximation is established on small systems. Then the calculations are done for systems with 40 electrons in 100 sites. It is found that the amplitude of the average persistent current in the diffusive regime is suppressed as the strength of the Coulomb interaction increases. The suppression of the current is stronger in larger rings than in smaller ones. The enhancement of the current by the electron-electron interaction was not observed in the diffusive regime.

cond-mat↗

arXiv1993

Quantum Ferromagnetism and Phase Transitions in Double-Layer Quantum Hall Systems

Double layer quantum Hall systems have interesting properties associated with interlayer correlations. At $ν=1/m$ where $m$ is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin $1/2$ easy-plane ferromagnets, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite temperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order.

cond-mat↗