arXiv subjects

Masaaki Nakamura

Publications and source records attributed to Masaaki Nakamura.

At least 37 records · Page 2Linked to original sources

arXiv2017

Rotational state-selection and alignment of chiral molecules by electrostatic hexapoles

Electrostatic hexapoles are revealed as a powerful tool in the rotational state-selection and alignment of molecules to be utilized in beam experiments on collisional and photoinitiated processes. In the paper, we report results on the application of the hexapolar technique on the recently studied chiral molecules propylene oxide, 2-butanol and 2bromobutane, to be investigated in selective photodissociation and enantiomeric discrimination.

physics.chem-ph↗

arXiv2016

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in dimerized quantum magnet Ba$_2$CuSi$_2$O$_6$Cl$_2$

We synthesized single crystals of composition Ba$_2$CuSi$_2$O$_6$Cl$_2$ and investigated its quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi$_2$O$_6$ also known as Han purple. Ba$_2$CuSi$_2$O$_6$Cl$_2$ has a singlet ground state with an excitation gap of $Δ/k_{\rm B}\,{=}\,20.8$ K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the $g$-factor except for a small jump anomaly for a magnetic field perpendicular to the $c$ axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in Ba$_2$CuSi$_2$O$_6$Cl$_2$ is described as the quasi-2D Bose-Einstein condensation of triplets.

cond-mat.str-el↗

arXiv2016

Detection of topological states in two-dimensional Dirac systems by the dynamic spin susceptibility

We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac electrons with spin-orbit interactions to characterize topological insulators. The imaginary part of the DSS appears as an absorption rate in response to a transverse ac magnetic field, just as in an electron spin resonance experiment for localized spin systems. We found that when the system is in a static magnetic field, the topological state can be identified by an anomalous resonant peak of the imaginary part of the DSS as a function of the frequency of the transverse magnetic field $ω$. In the absence of a static magnetic field, the imaginary part of the DSS becomes a continuous function of $ω$ with a threshold frequency $ω_{\rm c}$. In this case, the topological and the trivial phases can also be distinguished by the values of $ω_{\rm c}$ and by the line shapes. Thus the DSS is an experimentally observable physical quantity to characterize a topological insulator directly from bulk properties, without observing a topological transition.

cond-mat.str-el↗

arXiv2014

Non-Symmetry-Breaking Ground State of the S=1 Heisenberg Model on the Kagome lattice

The $S=1$ antiferromagnetic Heisenberg model on a Kagome lattice is studied using the density-matrix renormalization group method. To identify the ground state, we take four kinds of clusters into account; periodic, cylindrical, and two open ones. The hexagonal singlet solid (HSS) and triangular valence bond solid (TVBS) states are artificially generated by modulating edge shapes of the open clusters. We find that the energy par sites of the HSS state is $e_0=E_0/N =-1.41095$, which is readily lower than that of the TVBS state ($e_0=-1.3912 \pm 0.0025$). This agrees well with those of the cylindrical ($e_0=-1.40988$) and periodic ($e_0=-1.409 \pm 0.005$) clusters, where no assumption as to the ordering is posed. Thus, we conclude that the HSS picture is consistent to describe the ground state of the $S=1$ Kagome Heisenberg model. This is further confirmed by finding non-symmetry-breaking state in the calculations of the dimer-dimer correlation functions as well as the entanglement entropy of cylindrical clusters. Finally, we estimate the single-triplet energy gap: The HSS ground state has $Δ=0.1919$, while the TVBS excited state has a larger one $Δ=0.2797$.

cond-mat.str-el↗

arXiv2014

Dimerizations in spin-$S$ antiferromagnetic chains with three-spin interaction

We discuss spin-$S$ antiferromagnetic Heisenberg chains with three-spin interactions, next-nearest-neighbor interactions, and bond alternation. First, we prove rigorously that there exist parameter regions of the exact dimerized ground state in this system. This is a generalization of the Majumdar-Ghosh model to arbitrary $S$. Next, we discuss the ground-state phase diagram of the models by introducing several effective field theories and the universality classes of the transitions are described by the level-$2S$ $\mathrm{SU}(2)$ Wess-Zumino-Witten model and the Gaussian model. Finally, we determine the phase diagrams of $S=1$ and $S=3/2$ systems by using exact diagonalization and level spectroscopy.

cond-mat.str-el↗

arXiv2013

One-dimensional lattice model with an exact matrix-product ground state describing the Laughlin wave function

We introduce one-dimensional lattice models with exact matrix-product ground states describing the fractional quantum Hall (FQH) states in Laughlin series (given by filling factors $ν=1/q$) on torus geometry. Surprisingly, the exactly solvable Hamiltonian has the same mathematical structure as that of the pseudopotential for the Laughlin wave function, and naturally derives the general properties of the Laughlin wave function such as the $Z_2$ properties of the FQH states and the fermion-boson relation. The obtained exact ground states have high overlaps with the Laughlin states and well describe their properties. Using the matrix product method, density functions and correlation functions are calculated analytically. Especially, obtained entanglement spectra reflects gapless edge states as was discussed by Li and Haldane.

cond-mat.str-el↗

arXiv2013

Matrix-Product Ansatz for Excited States of Fractional Quantum Hall Systems

Recently, it was discussed that the $ν=1/3$ fractional quantum Hall state can be expressed by a one-dimensional lattice model with an exact matrix-prduct ground state, when toroidal boundary conditions are assumed for a narrow strip [Phys. Rev. Lett. {\bf 109} (2012) 016401]. In this article, we discuss how the excitation spectra of this system are calculated analytically based on the matrix-product formalism. We introduce variational wave functions for various momenta and optimize them. The obtained spectra of the charge neutral excitations show the magneto-roton behavior.

cond-mat.str-el↗

arXiv2012

Spin-chain description of fractional quantum Hall states in the Jain series

We discuss the relationship between fractional quantum Hall (FQH) states at filling factor $ν=p/(2p+1)$ and quantum spin chains. This series corresponds to the Jain series $ν=p/(2mp+1)$ with $m=1$ where the composite fermion picture is realized. We show that the FQH states with toroidal boundary conditions beyond the thin-torus limit can be mapped to effective quantum spin S=1 chains with $p$ spins in each unit cell. We calculate energy gaps and the correlation functions for both the FQH systems and the corresponding effective spin chains, using exact diagonalization and the infinite time-evolving block decimation (iTEBD) algorithm. We confirm that the mass gaps of these effective spin chains are decreased as $p$ is increased which is similar to $S=p$ integer Heisenberg chains. These results shed new light on a link between the hierarchy of FQH states and the Haldane conjecture for quantum spin chains.

cond-mat.str-el↗

arXiv2012

Exactly Solvable Fermion Chain Describing a $ν=1/3$ Fractional Quantum Hall State

We introduce an exactly solvable fermion chain that describes a $ν=1/3$ fractional quantum Hall (FQH) state beyond the thin-torus limit. The ground state of our model is shown to be unique for each center of mass sector, and it has a matrix product representation that enables us to exactly calculate order parameters, correlation functions, and entanglement spectra. The ground state of our model shows striking similarities with the BCS wave functions and quantum spin-1 chains. Using the variational method with matrix product ansatz, we analytically calculate excitation gaps and vanishing of the compressibility expected in the FQH state. We also show that the above results can be related to a $ν=1/2$ bosonic FQH state.

cond-mat.str-el↗

arXiv2011

Layer-resolved conductivities in multilayer graphenes

We study interlayer transport of multilayer graphenes in magnetic field with various stacking structures (AB, ABC, and AA types) by calculating the Hall and longitudinal conductivities as functions of Fermi energy. Their behavior depends strongly on the stacking structures and selection of the layers. The Hall conductivity between different layers is no longer quantized. Moreover, for AB stacking, the interlayer conductivity vanishes around zero energy with increasing layer separation, and shows negative values in particular cases. The fact that longitudinal interlayer conductivity suppressed by the magnetic field indicates that this system can be applied as a switching device.

cond-mat.mes-hall↗

arXiv2011

Beyond the Tao-Thouless limit of the fractional quantum Hall effect: spin chains and Fermi surface deformation

We discuss the relationship between the fractional quantum Hall effect in the vicinity of the thin-torus, a.k.a. Tao-Thouless (TT), limit and quantum spin chains. We argue that the energetics of fractional quantum Hall states in Jain sequence at filling fraction $ν=p/(2p+1)$ (and $ν=1-p/(2p+1)$) in the lowest Landau level is captured by S=1 spin chains with $p$ spins in the unit cell. These spin chains naturally arise at sub-leading order in $\e^{-2π^2/L_1^2}$ which serves as an expansion parameter away from the TT limit ($L_1\rightarrow 0$). We also corroborate earlier results on the smooth Fermi surface deformation of the gapless state at $ν=1/2$, interpolating between a state described by a critical $S=1/2$ chain and the bulk.

cond-mat.str-el↗

arXiv2010

Effective spin chains for fractional quantum Hall states

Fractional quantum Hall (FQH) states are topologically ordered which indicates that their essential properties are insensitive to smooth deformations of the manifold on which they are studied. Their microscopic Hamiltonian description, however, strongly depends on geometrical details. Recent work has shown how this dependence can be exploited to generate effective models that are both interesting in their own right and also provide further insight into the quantum Hall system. We review and expand on recent efforts to understand the FQH system close to the solvable thin-torus limit in terms of effective spin chains. In particular, we clarify how the difference between the bosonic and fermionic FQH states, which is not apparent in the thin-torus limit, can be seen at this level. Additionally, we discuss the relation of the Haldane-Shastry chain to the so-called QH circle limit and comment on its significance to recent entanglement studies.

cond-mat.str-el↗

arXiv2010

Link between the hierarchy of fractional quantum Hall states and Haldane's conjecture for quantum spin chains

We study a strong coupling expansion of the $ν=1/3$ fractional quantum Hall state away from the Tao-Thouless limit and show that the leading quantum fluctuations lead to an effective spin-1 Hamiltonian that lacks parity symmetry. By analyzing the energetics, discrete symmetries of low-lying excitations, and string order parameters, we demonstrate that the $ν=1/3$ fractional quantum Hall state is adiabatically connected to both Haldane and large-$D$ phases, and is characterized by a string order parameter which is dual to the ordinary one. This result indicates a close relation between (a generalized form of) the Haldane conjecture for spin chains and the fractional quantum Hall effect.

cond-mat.str-el↗

arXiv2010

Metal-insulator transition of the Kagome lattice fermions at 1/3 filling

We discuss the metal-insulator transition of the spinless fermion model on the Kagomé lattice at 1/3-filling. The system is analyzed using exact diagonalization, the density-matrix renormalization group methods and the random phase approximation. In the strong-coupling region, the charge-ordered ground state is consistent with the predictions of the effective model with a plaquette order. We find that the qualitative properties of the metal-insulator transition are totally different depending on the sign of the hopping integrals, reflecting the difference of band structure at the Fermi level.

cond-mat.str-el↗

arXiv2009

Valley symmetry breaking in bilayer graphene: a test to the minimal model

Physical properties reflecting valley asymmetry of Landau levels in a biased bilayer graphene under magnetic field are discussed. Within the $4-$band continuum model with Hartree-corrected self-consistent gap and finite damping factor we predict the appearance of anomalous steps in quantized Hall conductivity due to the degeneracy lifting of Landau levels. Moreover, the valley symmetry breaking effect appears as a non-semiclassical de Haas-van Alphen effect where the reduction of the oscillation period to half cannot be accounted for through quasi-classical quantization of the orbits in reciprocal space, still valley degenerate.

cond-mat.mes-hall↗

arXiv2008

Quantum Hall effect in bilayer and multilayer graphenes with finite gate voltage

We discuss the quantum Hall effect of bilayer graphene with finite gate voltage where the Fermi energy exceeds the interlayer hopping energy. We calculated magnetic susceptibility, diagonal and off-diagonal conductivities in finite-magnetic-field formalism, and observed crossover of integer quantum Hall effect from two independent monolayer type system to strongly coupled bilayer systems by changing the ratio of interlayer hopping energy and the gate voltage. We also discuss the case of multilayer systems with Bernal stacking

cond-mat.mes-hall↗

arXiv2008

Electric transport and magnetic properties in multilayer graphene

We discuss electric transport and orbital magnetism of multilayer graphenes in a weak-magnetic field using the matrix decomposition technique. At zero temperature, the minimum conductivity is given by that of the monolayer system multiplied by the layer number $N$, independent of the interlayer hopping $t$. When the interlayer hopping satisfies the condition $t\gg \hbar/τ$ with $τ$ being collision time of impurity scattering, $[N/2]$ kinks and $[N/2]+1$ plateaux appear in the Fermi-energy (gate voltage) dependence of the conductivity and the Hall conductivity, respectively. These behaviors are interpreted as multiband effects. We also found that the Hall conductivity and the magnetic susceptibility take minimum value as a function of temperature, for certain value of the gate voltage. This behavior is explained by Fermi-energy dependence of these functions at zero temperature.

cond-mat.mes-hall↗

arXiv2007

Orbital Magnetism and Transport Phenomena in Two Dimensional Dirac Fermions in Weak Magnetic Field

We discuss the orbital magnetism and the Hall effect in the weak magnetic field in two dimensional Dirac fermion systems with energy gap. This model is related to the graphene sheet, organic conductors, and $d$-density wave superconductors. We found the strong diamagnetism and finite Hall conductivity even in gapped systems. We also discuss the relation between the weak-magnetic field formalism and the Landau quantization with the Euler-Maclaurin expansion.

cond-mat.str-el↗