Searcharxiv⌕ Search

arXiv subjects

Takuma Ohashi

Publications and source records attributed to Takuma Ohashi.

17 recordsLinked to original sources
arXiv2026

Study on the validity of IPT+parquet method as an impurity solver in DMFT focusing on orbital fluctuations

A breakdown of calculations with exact impurity solvers in the dynamical mean field theory in multiband systems easily occurs due to the expensive numerical cost. To overcome this practical difficulty, three of the present authors developed an inexpensive and reliable impurity solver by combining the iterative perturbation theory (IPT) and parquet equation, and named it IPT+parquet [R. Mizuno, et al., Phys. Rev. B 104, 035160 (2021).]. In this study, we validate IPT+parquet focusing on the orbital fluctuation by comparing the numerically exact impurity solvers. We confirm that IPT+parquet can capture competition between orbital fluctuation channels, which the conventional IPT cannot capture.

cond-mat.str-el↗
arXiv2010

Mott transition in the Hubbard model on the anisotropic kagomé lattice

We investigate the Mott transition in the anisotropic kagomé lattice Hubbard model using the cellular dynamical mean field theory combined with continuous-time quantum Monte Carlo simulations. By calculating the double occupancy and the density of states, we determine the interaction strength of the first-order Mott transition and show that it becomes small as the anisotropy increases. We also calculate the spin correlation functions and the single-particle spectrum, and reveal that the quasiparticle and magnetic properties change dramatically around the Mott transition; the spin correlations are strongly enhanced and the quasiparticle bands are deformed. We conclude that such dramatic changes are due to the enhancement of anisotropy associated with the relaxation of frustration around the Mott transition.

cond-mat.str-el↗
arXiv2009

Spatial fluctuations of spin and orbital in two-orbital Hubbard model

We investigate the quasiparticle dynamics in the two-orbital Hubbard model on the square lattice at quarter filling by means of the cellular dynamical mean field theory. We show that the Fermi-liquid state is stabilized up to the large Hubbard interactions in the symmetric case without the Hund's coupling, and find the heavy quasiparticles around the metal-insulator boundary. It is elucidated that the Hund's coupling enhances the antiferro-orbital correlations, which give rise to the pseudo gap behavior in the single-particle excitations. We also find the nonmonotonic temperature dependence in the quasiparticle dynamics for intermediate strength of the Hund's coupling, and clarify that it is caused by the competition between the Fermi-liquid formation and the antiferro-orbital fluctuations.

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

Three-orbital Kondo effect in single quantum dot system with plural electrons

We study the Kondo effect and related transport properties in orbitally degenerate vertical quantum dot systems with plural electrons. Applying the non-crossing approximation to the three-orbital Anderson impurity model with the finite Coulomb interaction and Hund-coupling, we investigate the magnetic-field dependence of the conductance and thermopower. We also introduce an additional orbital splitting to take account of the realistic many-body effect in the vertical quantum dot system. It is clarified how the three-orbital Kondo effect influences the transport properties via the modulation of the Kondo temperature and unitary limit of transport quantities due to the change of the symmetry in the system.

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

Non-Crossing Approximation Study of Multi-Orbital Kondo Effect in Quantum Dot Systems

We study the three-orbital Kondo effect in quantum dot (QD) systems by applying the non-crossing approximation to the three-orbital Anderson impurity model. By investigating the tunneling conductance through a QD, we show that the competition between the Hund-coupling and the orbital level-splitting gives rise to characteristic behavior in transport properties. It is found that the Hund-coupling becomes more important in the three-orbital case than in the two-orbital case. We also show that the enhancement of Kondo temperature due to the singlet-triplet mechanism suggested for the two-orbital model tends to be suppressed by the existence of the third orbital.

cond-mat.mes-hall↗
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

Magnetic properties of a Kondo insulator with RKKY interaction: Extended dynamical mean field study

We study the Kondo lattice model with the Heisenberg-type RKKY-exchange coupling among localized f-spins in the presence of a magnetic field. By means of an extended dynamical mean field theory combined with the non-crossing approximation, we investigate the one-particle spectral function and the dynamical spin correlation function in the Kondo insulating phase. It is shown that the magnetic field and the RKKY exchange interaction both cause the instability to the antiferromagnetic order with enhanced transverse spin fluctuations, which give rise to a strong renormalization of quasi-particles as the system approaches the quantum critical point. This leads to a tendency to retain the Kondo insulating gap up to rather large fields.

cond-mat.str-el↗
arXiv2004

Field-induced phase transition in the periodic Anderson model

We investigate the effect of magnetic fields on a Kondo insulator by using the periodic Anderson model. The analysis by dynamical mean field theory combined with quantum Monte Carlo simulations reveals that the magnetic field drives the Kondo insulator to a transverse antiferromagnetic insulator at low temperatures. We calculate the staggered spin susceptibility and find its divergence signaling the antiferromagnetic instability. Further investigation of the spin correlation functions and the magnetization process clarifies how the magnetic field suppresses the Kondo singlet formation and induces the transverse antiferromagnetic ordering.

cond-mat.str-el↗
arXiv2004

Field-induced phase transitions in a Kondo insulator

We study the magnetic-field effect on a Kondo insulator by exploiting the periodic Anderson model with the Zeeman term. The analysis using dynamical mean field theory combined with quantum Monte Carlo simulations determines the detailed phase diagram at finite temperatures. At low temperatures, the magnetic field drives the Kondo insulator to a transverse antiferromagnetic phase, which further enters a polarized metallic phase at higher fields. The antiferromagnetic transition temperature $T_c$ takes a maximum when the Zeeman energy is nearly equal to the quasi-particle gap. In the paramagnetic phase above $T_c$, we find that the electron mass gets largest around the field where the quasi-particle gap is closed. It is also shown that the induced moment of conduction electrons changes its direction from antiparallel to parallel to the field.

cond-mat.str-el↗
arXiv2004

Periodic Anderson model with degenerate orbitals: linearized dynamical mean field theory approach

We investigate a multi-orbital extension of the periodic Anderson model with particular emphasis on electron correlations including orbital fluctuations. By means of a linearized version of the dynamical mean-field theory, we compute the renormalization factor, the density of states, the spectral gap and the local correlation functions for a given set of the intra- and inter-orbital Coulomb interactions as well as the Hund coupling. It is found that when a certain condition is met for the intra- and inter-orbital interactions for $f$ electrons, orbital fluctuations are enhanced, thereby enlarging the Kondo insulating gap. This effect is suppressed in the presence of the Hund coupling. We also clarify how the Kondo insulator is continuously changed to the Mott insulator when electron correlations among conduction electrons are increased.

cond-mat.str-el↗
arXiv2003

Effects of degenerate orbitals on the Hubbard model

Stability of a metallic state in the two-orbital Hubbard model at half-filling is investigated. We clarify how spin and orbital fluctuations are enhanced to stabilize the formation of quasi-particles by combining dynamical mean field theory with the quantum Monte Carlo simulations. These analyses shed some light on the reason why the metallic phase is particularly stable when the intra- and inter-band Coulomb interactions are nearly equal.

cond-mat.str-el↗
arXiv2002

Susceptibility of a Magnetic Impurity in Two-Dimensional Disordered Electron Systems

We investigate the Kondo effect in two-dimensional disordered electron systems using a finite-temperature quantum Monte Carlo method. Depending on the position of a magnetic impurity, the local moment is screened or unscreened by the spin of the conduction electron. The results suggest that the Kondo temperature takes different values depending on the position of a magnetic impurity. We show that the distribution of the Kondo temperature becomes wide and the weight at $T_K=0$ becomes large as randomness increases. The average susceptibility shows a weak power-law divergence at low temperature in strongly disordered systems, indicating a non-Fermi-liquid behavior. We calculate the correlation function between the local moment and the spin of the conduction electron. The results are discussed in connection with Kondo screening.

cond-mat.str-el↗