arXiv subjects

Hirokazu Tsunetsugu

Publications and source records attributed to Hirokazu Tsunetsugu.

At least 37 records · Page 2Linked to original sources

arXiv2011

Transport criticality in triangular lattice Hubbard model

We study electric transport near the Mott metal-insulator transition. Optical conductivity of the half-filled Hubbard model on a triangular lattice is calculated based on a cellular dynamical mean field theory including vertex corrections inside the cluster. By investigating the spectrum at low frequencies, we find that a Drude peak on the metallic side smoothly connects to an "ingap" peak on the insulating side. The optical weight of these peaks exhibits a critical behavior with power-law near the Mott critical end point, $|D-D^*|\propto|U-U^*|^{1/δ}$. We find that the critical exponent $1/δ$ differs from the exponents in the thermodynamics.

cond-mat.str-el↗

arXiv2011

Theory of Impurity Effects on the Spin Nematic State

The effect of magnetic bond disorder in otherwise antiferro nematic ordered system is investigated. We introduced triangular-shaped ferromagnetic bond disorder in the S=1 bilinear-biquadratic model on a triangular lattice. It is shown that the coupling between the impurity magnetic moment and nonmagnetic excitation in the bulk yields single-moment anisotropy and long-range anisotropic interaction between impurity magnetic moments. This interaction can induce unconventional spin-freezing phenomena observed in triangular magnet, NiGa2S4.

cond-mat.str-el↗

arXiv2011

Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6

We investigate with a microscopic model anharmonic K-cation oscillation observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which also shows a mysterious first-order structural transition at Tp=7.5 K. We have identified a set of microscopic model parameters that successfully reproduce the observed temperature dependence and the superconducting transition temperature. Considering changes in the parameters at Tp, we can explain puzzling experimental results about electron-phonon coupling and neutron data. Our analysis demonstrates that the first-order transition is multipolar transition driven by the octupolar component of K-cation oscillations. The octupole moment does not change the symmetry and is characteristic to noncentrosymmetric K-cation potential.

cond-mat.supr-con↗

arXiv2010

Magnon pairing in quantum spin nematic

Competing ferro- and antiferromagnetic exchange interactions may lead to the formation of bound magnon pairs in the high-field phase of a frustrated quantum magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior to the onset of a conventional one-magnon instability. We develop an analytical approach to study the zero-temperature properties of the magnon-pair condensate, which is a bosonic analog of the BCS superconductors. Representation of the condensate wave-function in terms of the coherent bosonic states reveals the spin-nematic symmetry of the ground-state and allows one to calculate various static properties. Sharp quasiparticle excitations are found in the nematic state with a small finite gap. We also predict the existence of a long-range ordered spin-nematic phase in the frustrated chain material LiCuVO4 at high fields.

cond-mat.str-el↗

arXiv2010

Strong coupling superconductivity mediated by three-dimensional anharmonic phonons

We investigate three dimensional anharmonic phonons in tetrahedral symmetry and superconductivity mediated by these phonons. Three dimensional anharmonic phonon spectra are calculated directly by solving Schrödinger equation and the superconducting transition temperature is determined by using the theory of strong coupling superconductivity assuming an isotropic gap function. With increasing the third order anharmonicity $b$ of the tetrahedral ion potential, we find a crossover in the energy spectrum to a quantum tunneling regime. We obtain strongly enhanced transition temperatures around the crossover point. We also investigate the anharmonic effects on the Debye-Waller factor, the phonon spectral functions and the density profile, as a function of the anharmonicity $b$ and temperature. The isomorphic first order transition observed in KOs_2O_6 is discussed in terms of the first excited state energy $Δ$, and the coupling constant $λ$ in the strong coupling theory of superconductivity. Our results suggest the decrease of $λ$ and increase of $Δ$ below the first order transition temperature. We point out that the change in the oscillation amplitude and characterizes this isomorphic transition. The chemical trends of the superconducting transition temperature, $λ$, and $Δ$ in the $β$-pyrochlore compounds are also discussed.

cond-mat.supr-con↗

arXiv2009

Mobile holes in frustrated quantum magnets and itinerant fermions on frustrated geometries

As discussed in several chapters of this volume, frustration leads to unconventional (insulating) ground states. On the other hand, doped holes are known to have profound effects in Mott insulators. Therefore doped frustrated systems offer the prospect of novel phases with some of the most fascinating, challenging and exotic behavior. In addition, at commensurate electron fillings and in the presence of strong (screened) Coulomb repulsion, geometrical frustration can also manifests itself as an extensive degeneracy of the classical ground-state manifold providing profound similarities with the field of quantum frustrated magnetism.

cond-mat.str-el↗

arXiv2008

Finite-Temperature Mott Transition in Two-Dimensional Frustrated Hubbard Models

We investigate the Hubbard model on two typical frustrated lattices in two dimensions, the kagome lattice and the anisotropic triangular lattice, by means of the cellular dynamical mean field theory. We show that the metallic phase is stabilized up to fairly large Hubbard interactions under strong geometrical frustration in both cases, which results in heavy fermion behavior and several anomalous properties around the Mott transition point. In particular, for the anisotropic triangular lattice, we find novel reentrant behavior in the Mott transition in the moderately frustrated parameter regime, which is caused by the competition between Fermi-liquid formation and magnetic correlations. It is demonstrated that the reentrant behavior is a generic feature inherent in the Mott transition with intermediate geometrical frustration, and indeed in accordance with recent experimental findings for organic materials.

cond-mat.str-el↗

arXiv2008

A Possible Phase Transition in beta-pyrochlore Compounds

We investigate a lattice of interacting anharmonic oscillators by using a mean field theory and exact diagonalization. We construct an effective five-state hopping model with intersite repulsions as a model for beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase transition line from large to small oscillation amplitude phases as temperature decreases. We also discuss the possibility of a phase with local electric polarizations. Our theory can explain the origin of the mysterious first order transition in KOs_2O_6.

cond-mat.str-el↗

arXiv2008

Effective Hamiltonian of Three-orbital Hubbard Model on Pyrochlore Lattice: Application to LiV_2O_4

We investigate heavy fermion behaviors in the vanadium spinel LiV_2O_4. We start from a three-orbital Hubbard model on the pyrochlore lattice and derive its low-energy effective Hamiltonian by an approach of real-space renormalization group type. One important tetrahedron configuration in the rochlore lattice has a three-fold orbital degeneracy and spin S=1, and correspondingly, the effective Hamiltonian has spin and orbital exchange interactions of Kugel-Khomskii type as well as correlated electron hoppings. Analyzing the effective Hamiltonian, we find that ferromagnetic double exchange processes compete with antiferromagnetic superexchange processes and various spin and orbital exchange processes are competing to each other. These results suggest the absence of phase transition in spin and orbital spaces down to very low temperatures and their large fluctuations in the low-energy sector, which are key issues for understanding the heavy fermion behavior in LiV_2O_4.

cond-mat.str-el↗

arXiv2008

Spontaneous trimerization in a bilinear-biquadratic S=1 zig-zag chain

Recent theoretical studies raised the possibility of a realization of spin nematic states in the S=1 triangular lattice compound NiGa_2S_4. We study the bilinear-biquadratic spin 1 chain in a zig-zag geometry by means of the density matrix renormalization group method and exact diagonalization. We present the phase diagram focusing on antiferromagnetic interactions. Adjacent to the known Haldane-double Haldane and the extended critical phase with dominant spin nematic correlations we find a trimerized phase with a nonvanishing energy gap. We discuss results for different order parameters, energy gaps, correlation functions, and the central charge, and make connection to field theoretical predictions for the phase diagram.

cond-mat.str-el↗

arXiv2007

Finite temperature Mott transition in Hubbard model on anisotropic triangular lattice

We investigate the Hubbard model on the anisotropic triangular lattice by means of the cellular dynamical mean field theory. The phase diagram determined in the Hubbard interaction versus temperature plane shows novel reentrant behavior in the Mott transition due to the competition between Fermi-liquid formation and magnetic correlations under geometrical frustration. We demonstrate that the reentrant behavior is characteristic of the Mott transition with intermediate geometrical frustration and indeed consistent with recent experimental results of organic materials.

cond-mat.str-el↗

arXiv2006

Anomalous magnetic properties near Mott transition in Kagomé lattice Hubbard model

We investigate the characteristics of the metallic phase near the Mott transition in the Kagomé lattice Hubbard model using the cellular dynamical mean field theory. By calculating the specific heat and spin correlation functions, we demonstrate that the quasiparticles show anomalous properties in the metallic phase close to the Mott transition. We find clear evidence for the multi-band heavy quasiparticles in the specific heat, which gives rise to unusual temperature dependence of the spin correlation functions.

cond-mat.str-el↗

arXiv2006

Magnetic correlations around the Mott transition in the Kagomé lattice Hubbard model

We study the magnetic properties around the Mott transition in the Kagomé lattice Hubbard model by the cellular dynamical mean field theory combined with quantum Monte Carlo simulations. By investigating the q-dependence of the susceptibility, we find a dramatic change of the dominant spin fluctuations around the Mott transition. The spin fluctuations in the insulating phase favor down to the lowest temperature a spatial spin configuration in which antiferromagnetic correlations are strong only in one chain direction but almost vanishing in the others.

cond-mat.str-el↗

arXiv2006

Mott transition in Kagomé lattice Hubbard model

We investigate the Mott transition in the Kagomé lattice Hubbard model using a cluster extension of dynamical mean field theory. The calculation of the double occupancy, the density of states, the static and dynamical spin correlation functions demonstrates that the system undergoes the first-order Mott transition at the Hubbard interaction $U/W \sim 1.4$ ($W$:bandwidth). In the metallic phase close to the Mott transition, we find the strong renormalization of three distinct bands, giving rise to the formation of heavy quasiparticles with strong frustration. It is elucidated that the quasiparticle states exhibit anomalous behavior in the temperature-dependent spin correlation functions.

cond-mat.str-el↗

arXiv2005

Spin Nematic Phase in S=1 Triangular Antiferromagnets

Spin nematic order is investigated for a S=1 spin model on triangular lattice with bilinear-biquadratic interactions. We particularly studied an antiferro nematic order phase with three-sublattice structure, and magnetic properties are calculated at zero temperature by means of bosonization. Two types of bosonic excitations are found. One is a gapless excitation with linear energy dispersion around $k \sim 0$, and this leads to a finite spin susceptibility at T=0 and would have a specific heat $C(T) \sim T^2$ at low temperatures. These behaviors can explain many of characteristic features of recently discovered spin liquid state in the triangular magnet, NiGa2S4.

cond-mat.str-el↗

arXiv2005

A Simple Model for the Checkerboard Pattern of Modulated Hole Densities in Underdoped Cuprates

A simple model is proposed as a possible explanation for the checkerboard pattern of modulations in the hole density observed in recent tunneling experiments on underdoped cuprates. Two assumptions are made; first, an enhanced hole density near the acceptor dopants and secondly short range correlations in the positions of these dopants caused by their electrostatic and anisotropic elastic interactions. Together these can lead to a structure factor in qualitative agreement with experiment.

cond-mat.str-el↗

arXiv2005

Theory of successive transitions in vanadium spinels and order of orbitals and spins

We have theoretically studied successive transitions in vanadium spinel oxides with (t_2g)^2 electron configuration. These compounds show a structural transition at ~ 50K and an antiferromagnetic transition at ~ 40K. Since threefold t_2g orbitals of vanadium cations are occupied partially and vanadiums constitute a geometrically-frustrated pyrochlore lattice, the system provides a particular example to investigate the interplay among spin, orbital and lattice degrees of freedom on frustrated lattice. We examine the models with the Jahn-Teller coupling and/or the spin-orbital superexchange interaction, and conclude that keen competition between these two contributions explains the thermodynamics of vanadium spinels. Effects of quantum fluctuations as well as relativistic spin-orbit coupling are also discussed.

cond-mat.str-el↗

arXiv2005

Interplay among Spin, Orbital and Lattice Degrees of Freedom in $t_{2g}$ Electron Systems with Edge-Sharing Network of Octahedra

Transition metal oxides whose lattice structure has edge-sharing network of octahedra constitute a diverse group of intriguing materials besides compounds with corner-sharing octahedra such as perovskites. We present a theoretical investigation of the interplay among spin, orbital and lattice degrees of freedom in these materials. We focus on $t_{2g}$ electron systems where a keen competition among those degrees of freedom is expected to emerge under a relatively weak Jahn-Teller coupling. We study the interplay between spin and orbital degrees of freedom in vanadium spinels and titanium pyroxenes. We clarify the important role of the strong anisotropy in the orbital interactions due to the edge-sharing geometry. We also discuss the interplay between spin and lattice in chromium spinels focusing on the magnetization process under the external magnetic field.

cond-mat.str-el↗