arXiv subjects

Naokazu Shibata

Publications and source records attributed to Naokazu Shibata.

At least 37 records · Page 2Linked to original sources

arXiv2013

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 grand canonical density matrix renormalization group, an unbiased state-of-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.

cond-mat.str-el↗

arXiv2013

"Grand Canonical" Finite Size Numerical Approaches : a Route to Measuring Bulk Properties under Applied Field

We exploit a prescription to observe directly the physical properties of the thermodynamic limit under continuously applied field in one-dimensional quantum finite lattice systems. By systematically scaling down the energy of the Hamiltonian of the open system from center toward both ends, one could adopt the edge sites with negligibly small energy scale as a "grand canonical" small particle bath, and an equilibrium states with non-integer arbitrary conserved numbers, e.g., electron numbers or sz, are realized in the main part of the system. This will enable the evaluation of the response functions under continuously varying external field in a small lattice without any fine tuning or scaling of parameters while keeping the standard numerical accuracy. Demonstrations are given on quantum spin systems as well as on a Hubbard model by the density matrix renormalization group calculation.

cond-mat.str-el↗

arXiv2012

Quasi-Particle Tunneling in Anti-Pfaffian Quantum Hall State

We study tunneling phenomena at the edge of the anti-Pfaffian quantum Hall state at the filling factor $ν=5/2$. The edge current in a single point-contact is considered. We focus on nonlinear behavior of two-terminal conductance with the increase in negative split-gate voltage. Expecting the appearance of the intermediate conductance plateau we calculate the value of its conductance by using the renormalization group (RG) analysis. Further, we show that non-perturbative quasi-particle tunneling is effectively described as perturbative electron tunneling by the instanton method. The two-terminals conductance is written as a function of the gate voltage. The obtained results enable us to distinguish the anti-Pfaffian state from the Pfaffian state experimentally.

cond-mat.mes-hall↗

arXiv2011

Boundary effects in the density-matrix renormalization group calculation

We investigate the boundary effect of the density matrix renormalization group calculation (DMRG), which is an artifactual induction of symmetry-breaking pseudo-long-range order and takes place when the long-range quantum fluctuation cannot be properly included in the variational wave function due to numerical limitation. The open boundary condition often used in DMRG suffers from the boundary effect the most severely, which is directly reflected in the distinct spatial modulations of the local physical quantity. By contrast, the other boundary conditions such as the periodic one or the sin^2-deformed interaction [A. Gendiar, R. Krcmar, and T. Nishino, Prog. Theor. Phys. {\bf 122}, 953 (2009)] keep spatial homogeneity, and are relatively free from the boundary effect. By comparing the numerical results of those various boundary conditions, we show that the open boundary condition sometimes gives unreliable results even after the finite-size scaling. We conclude that the examination of the boundary condition dependence is required besides the usual treatment based on the system size or accuracy dependence in cases where the long-range quantum fluctuation is important.

cond-mat.str-el↗

arXiv2010

Modulation Induced Phase Transition from Fractional Quantum Hall to Stripe State at nu=5/3

We have investigated the effect of unidirectional periodic potential modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3 and 4/3. For large enough modulation amplitude, we find that the resistivity minimum at nu=5/3 gives way to a peak that grows with decreasing temperature. Density matrix renormalization group calculation reveals that phase transition from FQH state to unidirectional striped state having a period sim 4 l (with l the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the particle-hole symmetry) with the increase of the modulation amplitude, suggesting that the observed peak is the manifestation of the stripe phase.

cond-mat.mes-hall↗

arXiv2010

Stability of the Excitonic Phase in Bilayer Quantum Hall Systems at Total Filling One -- Effects of Finite Well Width and Pseudopotentials --

The ground state of a bilayer quantum Hall system at $ν_{\rm T}=1$ with model pseudopotential is investigated by the DMRG method. Firstly, pseudopotential parameters appropriate for the system with finite layer thickness are derived, and it is found that the finite thickness makes the excitonic phase more stable. Secondly, a model, where only a few pseudopotentials with small relative angular momentum have finite values, is studied, and it is clarified how the excitonic phase is destroyed as intra-layer pseudopotential becomes larger. The importance of the intra-layer repulsive interaction at distance twice of the magnetic length for the destruction of the excitonic phase is found.

cond-mat.str-el↗

arXiv2009

Quantum Hall Systems Studied by the Density Matrix Renormalization Group Method

The ground-state and low-energy excitations of quantum Hall systems are studied by the density matrix renormalization group (DMRG) method. From the ground-state pair correlation functions and low-energy excitions, the ground-state phase diagram is determined, which consists of incompressible liquid states, Fermi liquid type compressible liquid states, and many kinds of CDW states called stripe, bubble and Wigner crystal. The spin transition and the domain formation are studied at v=2/3. The evolution from composite fermion liquid state to an excitonic state in bilayer systems is investigated at total filling factor v=1.

cond-mat.str-el↗

arXiv2009

Fractional Quantum Hall Effects in Graphene and Its Bilayer

Single-layer and Bilayer of graphene are new classes of two-dimensional electron systems with unconventional band structures and valley degrees of freedom. The ground states and excitations in the integer and fractional quantum Hall regimes are investigated on torus and spherical geometries with the use of the density matrix renormalization group (DMRG) method. At nonzero Landau level indices, the ground states at effective filling factors 1, 1/3, 2/3 and 2/5 are valley polarized both in single-layer and bilayer graphenes. We examine the elementary charge excitations which could couple with the valley degrees of freedom (so called valley skyrmions). The excitation gaps are calculated and extrapolated to the thermodynamic limit. The largest excitation gap at effective filling 1/3 is obtained in bilayer graphene, which is a good candidate for experimental observation of fractional quantum Hall effect.

cond-mat.str-el↗

arXiv2008

Coupled charge and valley excitations in graphene quantum Hall ferromagnets

Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-type low-energy spectrum. In a strong magnetic field, where Coulomb interactions dominate against disorder broadening, quantum Hall ferromagnetic states realize at integer fillings. Extending the quantum Hall ferromagnetism to the fractional filling case of massless Dirac fermions, we study the elementally charge excitations which couple with the valley degrees of freedom (so-called valley skyrmions). With the use of the density matrix renomalization group (DMRG) method, the excitation gaps are calculated and extrapolated to the thermodynamic limit. These results exhibit numerical evidences and criterions of the skyrmion excitations in graphene.

cond-mat.mes-hall↗

arXiv2007

Domain Formation in v=2/3 Fractional Quantum Hall Systems

We study the domain formation in the v=2/3 fractional quantum Hall systems basing on the density matrix renormalization group (DMRG) analysis. The ground-state energy and the pair correlation functions are calculated for various spin polarizations. The results confirm the domain formation in partially spin polarized states, but the presence of the domain wall increases the energy of partially spin polarized states and the ground state is either spin unpolarized state or fully spin polarized state depending on the Zeeman energy. We expect coupling with external degrees of freedom such as nuclear spins is important to reduce the energy of partially spin polarized state.

cond-mat.mes-hall↗

arXiv2006

Least Action Principle for the Real-Time Density Matrix Renormalization Group

A kind of least action principle is introduced for the discrete time evolution of one-dimensional quantum lattice models. Based on this principle, we obtain an optimal condition for the matrix product states on succeeding time slices generated by the real-time density matrix renormalization group method. This optimization can also be applied to classical simulations of quantum circuits. We discuss the time reversal symmetry in the fully optimized MPS.

cond-mat.stat-mech↗

arXiv2006

Ground State of v=1 Bilayer Quantum Hall Systems

The ground-state wave function and the energy gap are calculated for various layer separations d and for up to 24 electrons by the density matrix renormalization group (DMRG) method. Two-particle distribution function and excitonic correlation function are calculated from the ground-state wave function, and the evolution of the ground state with increasing d is analyzed. The results indicate that the transition is continuous. A smooth crossover of the ground state is found at around d/l ~ 1.6 from the excitonic character at small d/l to the independent Fermi-liquid character at large d/l, where l is the magnetic length.

cond-mat.str-el↗

arXiv2005

Comments on "Dimerization Induced by the RKKY Interaction"

Recently, Xavier et al. claimed the existence of an insulating spin dimer state in the one-dimensional Kondo lattice model at quarter-filling amidst the paramagnetic metallic phase. In this comment we show that the dimer-dimer correlation decays algebraically at long distances and the charge gap vanishes in the bulk limit.

cond-mat.str-el↗

arXiv2004

Theory of Fano-Kondo effect of transport properties through quantum dots

The Fano-Kondo effect in zero-bias conductance is investigated based on a theoretical model for the T-shaped quantum dot. The conductance as a function of the gate voltage is generally characterized by a Fano asymmetric parameter q. With varying temperature the conductance shows a crossover between the high and low temperature regions compared with the Kondo temperature T_K: two Fano asymmetric peaks at high temperatures and the Fano-Kondo plateau inside a Fano peak at low temperatures. Temperature dependence of conductance is calculated numerically by the Finite temperature density matrix renormalization group method (FT-DMRG).

cond-mat.mes-hall↗

arXiv2004

Real Space Effective Interaction and Phase Transition in the Lowest Landau Level

The transition between the stripe state and the liquid state in a high magnetic field is studied by the density-matrix renormalization-group (DMRG) method. Systematic analysis on the ground state of two-dimensional electrons in the lowest Landau level shows that the transition from the stripe state to the liquid state at v=3/8 is caused by a reduction of repulsive interaction around r=3. The same reduction of the interaction also stabilizes the incompressible liquid states at v=1/3 and 2/5, which shows a similarity between the two liquid states at v=3/8 and 1/3. It is also shown that the strong short-range interaction around r=1 in the lowest Landau level makes qualitatively different stripe correlations compared with that in higher Landau levels.

cond-mat.mes-hall↗

arXiv2004

Kondo hole in one-dimensional Kondo insulators

Properties of a nonmagnetic impurity in Kondo insulators are investigated by considering a one-dimensional Kondo lattice model with depletion of a localized spin. The ground-state phase diagram determined by the Lanczos method shows that the magnetic moment is more stable than in ordinary metals. Temperature dependence of impurity susceptibilities is also studied by using the finite-temperature density-matrix renormalization group.

cond-mat.str-el↗

arXiv2004

Conductance through Quantum Dots Studied by Finite Temperature DMRG

With the Finite temperature Density Matrix Renormalization Group method (FT-DMRG), we depeloped a method to calculate thermo-dynamical quantities and the conductance of a quantum dot system. Conductance is written by the local density of states on the dot. The density of states is calculated with the numerical analytic continuation from the thermal Green's function which is obtained directly from the FT-DMRG. Typical Kondo behaviors in the quantum dot system are observed conveniently by comparing the conductance with the magnetic and charge susceptibilities: Coulomb oscillation peaks and the unitarity limit. We discuss advantage of this method compared with others.

cond-mat.mes-hall↗

arXiv2004

Ground state phase diagram of 2D electrons in high magnetic field

The ground state phase diagram of two-dimensional electrons in high magnetic field is studied by the density matrix renormalization group (DMRG) method. The low energy excitations and pair correlation functions in Landau levels of N=0,1,2 are calculated for wide range of fillings. The obtained results for systems with up to 25 electrons confirm the existence of various electronic states in quantum Hall systems. The ground state phase diagram for N=0,1,2 consisting of incompressible liquids, compressible liquids, charge density waves called stripe, bubble and Wigner crystal is determined.

cond-mat.mes-hall↗