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Sep 23, 2013
09/13

by
Akihisa Koga

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We investigate the nonequilibrium phenomena through the quantum dot coupled to the normal and superconducting leads using a weak-coupling continuous-time Monte Carlo method. Calculating the time evolution of particle number, double occupancy, and pairing correlation at the quantum dot, we discuss how the system approaches the steady state. We also deduce the steady current through the quantum dot beyond the linear response region. It is clarified that the interaction decreases the current in...

Source: http://arxiv.org/abs/1303.0288v2

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89

Jul 20, 2013
07/13

by
Akihisa Koga

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We investigate quantum phase transitions in the three-dimensional orthogonal-dimer system of the compound $\rm SrCu_2(BO_3)_2$ by means of the series expansion technique. We then discuss the phase diagram where the dimer phase, the Haldane phase, the frustration-induced disordered phase and the magnetically ordered phase compete with each other. It is found that the compound $\rm SrCu_2(BO_3)_2$ is located in the dimer phase close to the phase boundary.

Source: http://arxiv.org/abs/cond-mat/0009266v1

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58

Sep 21, 2013
09/13

by
Akihisa Koga; Philipp Werner

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We investigate the attractive Hubbard model in infinite spatial dimensions by combining dynamical mean-field theory with a strong-coupling continuous-time quantum Monte Carlo method. By calculating the superfluid order parameter and the density of states, we discuss the stability of the superfluid state. In the intermediate coupling region above the critical temperature, the density of states exhibits a heavy fermion behavior with a quasi-particle peak in the dense system, while a dip structure...

Source: http://arxiv.org/abs/1106.4559v1

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68

Sep 22, 2013
09/13

by
Akihisa Koga; Norio Kawakami

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We investigate the quantum phase transitions in the frustrated antiferromagnetic Heisenberg model for $\rm SrCu_2(BO_3)_2$ by using the series expansion method. It is found that a novel spin-gap phase, which is adiabatically connected to the plaquette-singlet phase, exists between the dimer and the magnetically ordered phases known so far. When the ratio of the competing exchange couplings $\alpha(=J'/J)$ is varied, this spin-gap phase exhibits the first- (second-) order quantum phase...

Source: http://arxiv.org/abs/cond-mat/0003435v1

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118

Jul 20, 2013
07/13

by
Akihisa Koga; Philipp Werner

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We investigate the attractive Hubbard model in infinite spatial dimensions at quarter filling. By combining dynamical mean-field theory with continuous-time quantum Monte Carlo simulations in the Nambu formalism, we directly deal with the superfluid phase in the population imbalanced system. We discuss the low energy properties in the polarized superfluid state and the pseudogap behavior in the vicinity of the critical temperature.

Source: http://arxiv.org/abs/1007.3307v1

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58

Sep 22, 2013
09/13

by
Kensuke Inaba Akihisa Koga

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We investigate the two-orbital Hubbard model in infinite dimensions by means of the self-energy functional method. By calculating the entropy, susceptibility, and quasi-particle weight at zero temperature, we determine the phase diagram for the system with same and different bandwidths, which is compared with that obtained recently. It is clarified that orbital fluctuations play a key role in controlling the nature of the Mott transitions in the system.

Source: http://arxiv.org/abs/cond-mat/0603470v1

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61

Sep 21, 2013
09/13

by
Kensuke Inaba; Akihisa Koga

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We investigate the infinite-dimensional two-orbital Hubbard model at arbitrary band fillings. By means of the self-energy functional approach, we discuss the stability of the metallic state in the systems with same and different bandwidths. It is found that the Mott insulating phases are realized at commensurate band fillings. Furthermore, it is clarified that the orbital selective Mott phase with one orbital localized and the other itinerant is stabilized even at fractional band fillings in...

Source: http://arxiv.org/abs/0706.3948v1

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85

Sep 21, 2013
09/13

by
Yasuharu Okawauchi; Akihisa Koga

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We investigate the stability of the superfluid state in a bilayer fermionic optical lattice system with a confining potential, using the Bogoliubov de-Gennes equations. It is clarified that in the imbalanced case, the introduction of the interlayer hopping stabilizes the radial Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while makes the angular FFLO state unstable. We also discuss the system size dependence of the superfluid ground state. It is clarified that in a certain ring region the...

Source: http://arxiv.org/abs/1204.4187v1

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64

Sep 19, 2013
09/13

by
Akihisa Koga; Norio Kawakami

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We investigate quantum phase transitions for the $s=1/2$ antiferromagnetic Heisenberg model on a pyrochlore lattice. By means of a series expansion starting from isolated tetrahedra, the ground-state phase diagram is determined. When the ratio of the two competing exchange couplings is varied, the first-order (second-order) quantum phase transition occurs between the two spin gap phases (the spin-gap and the antiferromagnetic phases). We also discuss some properties expected for the s=1...

Source: http://arxiv.org/abs/cond-mat/0010138v1

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64

Sep 23, 2013
09/13

by
Akihisa Koga; Norio Kawakami

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By using the series expansion techniques, we study the excitation spectrum for the two-dimensional quantum spin systems with ladder, plaquette and mixed-spin structures. We calculate the spin excitation gap and thus determine the phase boundary between the spin-gap phase and the magnetically ordered phase rather precisely. It is found that the phase diagram obtained improves fairly well the one previously obtained via the ground-state susceptibility.

Source: http://arxiv.org/abs/cond-mat/9908458v1

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6.0

Jun 30, 2018
06/18

by
Akihisa Koga; Philipp Werner

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We study the two-band Hubbard model in infinite dimensions by solving the dynamical mean-field equations with a strong coupling continuous-time quantum Monte Carlo method and show that an $s$-wave superconducting state can be stabilized in the repulsively interacting case. We discuss how this superconducting state competes with the metallic and paired Mott states. The effects of the Hund coupling and crystalline electric field are also addressed.

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1412.1883

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7.0

Jun 29, 2018
06/18

by
Hiromasa Yanatori; Akihisa Koga

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We investigate the SU($N$) Hubbard model for the multi-component fermionic optical lattice system, combining dynamical mean-field theory with the continuous-time quantum Monte Carlo method. We obtain the finite temperature phase diagrams with $N\le 6$ and find that low temperature properties depends on the parity of the components. The magnetically ordered state competes with the correlated metallic state in the system with the even number of components $(N\ge 4)$, yielding the first-order...

Topics: Condensed Matter, Quantum Gases, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1603.02647

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4.0

Jun 30, 2018
06/18

by
Nayuta Takemori; Akihisa Koga

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We study electron correlations in the half-filled Hubbard model on two-dimensional Penrose lattice. Applying the real-space dynamical mean-field theory to large clusters, we discuss how low-temperature properties are affected by the quasiperiodic structure. By calculating the double occupancy and renormalization factor at each site, we clarify the existence of the Mott transition. The spatially-dependent renormalization characteristic of geometrical structure is also addressed.

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1411.7067

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4.0

Jun 28, 2018
06/18

by
Hiromasa Yanatori; Akihisa Koga

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We investigate finite temperature properties in the half-filled three-component (colors) fermion systems. It is clarified that a color density-wave (CDW) state is more stable than a color-selective "antiferromagnetic" (CSAF) state against thermal fluctuations. The reentrant behavior in the phase boundary for the CSAF state is found. We also address the maximum critical temperature of the translational symmetry breaking states in the multicomponent fermionic systems.

Topics: Quantum Gases, Condensed Matter

Source: http://arxiv.org/abs/1510.01471

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5.0

Jun 29, 2018
06/18

by
Akihisa Koga; Hiromasa Yanatori

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We investigate spontaneously symmetry breaking states in the attractive SU($N$) Hubbard model at half filling. Combining dynamical mean-field theory with the continuous-time quantum Monte Carlo method, we obtain the finite temperature phase diagrams for the superfluid state. When $N>2$, the second-order phase transition occurs in the weak coupling region, while the first-order phase transition with the hysteresis appears in the strong coupling region. We also discuss the stability of the...

Topics: Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1612.06384

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59

Sep 21, 2013
09/13

by
Akihisa Koga; Philipp Werner

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We investigate the periodic Anderson model with attractive interactions by means of dynamical mean-field theory (DMFT). Using a continuous-time quantum Monte Carlo impurity solver, we study the competition between the superfluid state and the paramagnetic Kondo insulating state, and determine the phase diagram. At the chemical potential-induced phase transition from the Kondo insulating state to the superfluid state, a low-energy peak characteristic of the superfluid state appears inside the...

Source: http://arxiv.org/abs/1008.0036v1

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39

Sep 20, 2013
09/13

by
Akihisa Koga; Philipp Werner

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We investigate the attractive Hubbard model in infinite spatial dimensions by means of dynamical mean-field theory. Using a continuous-time Monte Carlo algorithm in the Nambu formalism as an impurity solver, we directly deal with the superfluid phase in the population imbalanced system. By calculating the superfluid order parameter, the magnetization, and the density of states, we discuss how the polarized superfluid state is realized in the attractive Hubbard model at quarter filling. We find...

Source: http://arxiv.org/abs/1002.2958v1

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55

Sep 22, 2013
09/13

by
Akihisa Koga; Norio Kawakami

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We investigate quantum phase transitions in the frustrated orthogonal-dimer chain with an arbitrary spin $S \geq 1/2$. When the ratio of the competing exchange couplings is varied, first-order phase transitions occur 2S times among distinct spin-gap phases. The introduction of single-ion anisotropy further enriches the phase diagram. The phase transitions described by the present model possess most of the essential properties inherent in frustrated quantum spin systems.

Source: http://arxiv.org/abs/cond-mat/0201518v1

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44

Sep 23, 2013
09/13

by
Akihisa Koga; Norio Kawakami

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We present a novel mixed-spin cluster expansion method for a quasi-one-dimensional Haldane system with bond alternation. By mapping the s=1 antiferromagnetic spin model on square and cubic lattices to the equivalent mixed-spin model, we study the competition among the Haldane, the dimer and the magnetically ordered phases. The mixed-spin cluster expansion proposed here allows us to directly deal with the Haldane phase, which may not be reached by standard series expansion methods. The phase...

Source: http://arxiv.org/abs/cond-mat/9908289v1

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40

Sep 21, 2013
09/13

by
Nayuta Takemori; Akihisa Koga

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We study the attractive Hubbard model with mass imbalance to clarify low temperature properties of the fermionic mixtures in the optical lattice. By combining dynamical mean-field theory with the continuous-time quantum Monte Carlo simulation, we discuss the competition between the superfluid and density wave states at half filling. By calculating the energy and the order parameter for each state, we clarify that the coexisting (supersolid) state, where the density wave and superfluid states...

Source: http://arxiv.org/abs/1204.1783v1

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117

Sep 22, 2013
09/13

by
Takuya Yoshioka; Akihisa Koga; Norio Kawakami

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We investigate the quantum phase transitions in the half-filled Hubbard model on the triangular lattice by means of the path-integral renormalization group (PIRG) method with a new iteration scheme proposed recently. It is found that as the Hubbard interaction $U$ increases, the paramagnetic metallic state undergoes a first-order phase transition to a nonmagnetic insulating (NMI) state at $U_{c1}\sim 7.7t$, which is followed by another first-order transition to a $120^\circ$ N\'eel ordered...

Source: http://arxiv.org/abs/0811.1575v1

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55

Sep 21, 2013
09/13

by
Takuya Yoshioka; Akihisa Koga; Norio Kawakami

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We study the ground state properties of the geometrically frustrated Hubbard model on the anisotropic checkerboard lattice with nearest-neighbor hopping $t$ and next nearest-neighbor hopping $t'$. By using the path-integral renormalization group method, we study the phase diagram in the parameter space of the Hubbard interaction $U$ and the frustration-control parameter $t'/t$. Close examinations of the effective hopping, the double occupancy, the momentum distribution and the spin/charge...

Source: http://arxiv.org/abs/0808.1573v1

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71

Sep 21, 2013
09/13

by
Takuya Yoshioka; Akihisa Koga; Norio Kawakami

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We investigate a frustrated Ising spin system on the garnet lattice composed of a specific network of corner-sharing triangles. By means of Monte Carlo simulations with the heat bath algorithm, we discuss the magnetic properties at finite temperatures. It is shown that the garnet spin system with the nearest-neighbor couplings does not exhibit any magnetic transitions, yielding the large residual entropy at zero temperature. We also investigate the effect of the long-range dipolar interaction...

Source: http://arxiv.org/abs/cond-mat/0402604v1

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7.0

Jun 28, 2018
06/18

by
Nayuta Takemori; Akihisa Koga; Hartmut Hafermann

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We study intersite electron correlations in the half-filled Hubbard model on square lattices with periodic and open boundary conditions by means of a real-space dual fermion approach. By calculating renormalization factors, we clarify that nearest-neighbor intersite correlations already significantly reduce the critical interaction. The Mott transition occurs at U/t~6.4, where $U$ is the interaction strength and t is the hopping integral. This value is consistent with the quantum Monte Carlo...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1510.04749

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53

Sep 21, 2013
09/13

by
Takamitsu Saitou; Akihisa Koga; Atsushi Yamamoto

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We investigate the half-filled Hubbard model with spatially alternating interactions by means of the two-site dynamical mean-field theory. It is found that a single Mott transition occurs when two kinds of interactions are increased. This implies that the different interactions are essentially irrelevant at the critical point. The nature of the Mott states is also addressed.

Source: http://arxiv.org/abs/1204.4188v1

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58

Sep 22, 2013
09/13

by
Akihisa Koga; Kouichi Okunishi; Norio Kawakami

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We investigate the low-energy properties of the orthogonal-dimer spin chain characterized by a frustrated dimer-plaquette structure. When the competing antiferromagnetic couplings are varied, the first-order quantum phase transition occurs between the dimer and the plaquette phases, which is accompanied by nontrivial features due to frustration: besides the discontinuity in the lowest excitation gap at the transition point, a sharp level-crossing occurs for the spectrum in the plaquette phase....

Source: http://arxiv.org/abs/cond-mat/0003437v1

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51

Sep 19, 2013
09/13

by
Shoji Yamamoto; Toru Sakai; Akihisa Koga

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In response to recent chemical attempts to construct higher-spin ladder materials from organic polyradicals, we study the ground-state properties of a wide class of antiferromagnetic spin-1 ladders. Employing various numerical tools, we reveal the rich phase diagram and correct a preceding nonlinear-sigma-model prediction. A variational analysis well interprets the phase competition with particular emphasis on the {\it re-entrant phase boundary} as a function of the rung interaction.

Source: http://arxiv.org/abs/cond-mat/0206553v1

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67

Sep 20, 2013
09/13

by
Yusuke Fujihara; Akihisa Koga; Norio Kawakami

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We investigate the one-dimensional Hubbard model with a confining potential, which may describe cold fermionic atoms trapped in an optical lattice. Combining the variational Monte Carlo simulations with the new stochastic reconfiguration scheme proposed by Sorella, we present an efficient method to systematically treat the ground state properties of the confined system with a site-dependent potential. By taking into account intersite correlations as well as site-dependent on-site correlations,...

Source: http://arxiv.org/abs/cond-mat/0612171v2

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8.0

Jun 28, 2018
06/18

by
Ryu Shinzaki; Joji Nasu; Akihisa Koga

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We study valence fluctuations at finite temperatures in the extended periodic Anderson model, where the Coulomb interaction between conduction and localized $f$-electrons is taken into account, using dynamical mean-field theory combined with the continuous-time quantum Monte Carlo (CT-QMC) method. The valence transition with the hysteresis is clearly found, indicating the first-order phase transition between the Kondo and mixed-valence states. We demonstrate that spin correlation rapidly...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1510.04788

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8.0

Jun 30, 2018
06/18

by
Yuki Okanami; Nayuta Takemori; Akihisa Koga

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Three-component fermionic optical lattice systems are investigated in dynamical mean-field theory for the Hubbard model. Solving the effective impurity model by means of continuous-time quantum Monte Carlo simulations in the Nambu formalism, we find that the $s$-wave superfluid state proposed recently is indeed stabilized in the repulsively interacting case and appears along the first-order phase boundary between the metallic and paired Mott states in the paramagnetic system. The BCS-BEC...

Topics: Quantum Gases, Condensed Matter

Source: http://arxiv.org/abs/1401.5610

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7.0

Jun 29, 2018
06/18

by
Shoya Kojima; Joji Nasu; Akihisa Koga

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We study low temperature properties of the Hubbard model for the bismuth nickelate, where degenerate orbitals in the nickel ions and a single orbital in the bismuth ions are taken into account, combining dynamical mean-field theory with the continuous-time quantum Monte Carlo method. We discuss the effect of the attractive interactions to mimic the valence skipping phenomenon in the bismuth ions. We demonstrate how the charge and magnetically ordered states are stable against thermal...

Topics: Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1604.05916

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48

Sep 24, 2013
09/13

by
Yoshihiro Takushima; Akihisa Koga; Norio Kawakami

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We investigate quantum phase transitions among the spin-gap phases and the magnetically ordered phases in a two-dimensional frustrated antiferromagnetic spin system, which interpolates several important models such as the orthogonal-dimer model as well as the model on the 1/5-depleted square lattice. By computing the ground state energy, the staggered susceptibility and the spin gap by means of the series expansion method, we determine the ground-state phase diagram and discuss the role of...

Source: http://arxiv.org/abs/cond-mat/0103264v1

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10.0

Jun 26, 2018
06/18

by
Shinichi Takemura; Nayuta Takemori; Akihisa Koga

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We study the extended Anderson model on the two-dimensional Penrose lattice, combining the real-space dynamical mean-field theory with the non-crossing approximation. It is found that the Coulomb repulsion between localized and conduction electrons does not induce a valence transition, but the crossover between the Kondo and mixed valence states in contrast to the conventional periodic system. In the mixed-valence region close to the crossover, nontrivial valence distributions appear...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1502.03907

5
5.0

Jun 29, 2018
06/18

by
Ryu Shinzaki; Joji Nasu; Akihisa Koga

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We study valence fluctuations in the extended Anderson model on two-dimensional Penrose lattice, using the real-space dynamical mean-field theory combined with the continuous-time Monte Carlo method. Calculating $f$-electron number, $c$-$f$ spin correlations, and magnetic susceptibility at each site, we find site-dependent formations of the singlet state and valence distribution at low temperatures, which are reflected by the quasiperiodic lattice structure. The bulk magnetic susceptibility is...

Topics: Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1606.06503

46
46

Sep 20, 2013
09/13

by
Akihisa Koga; Takamitsu Saitou; Atsushi Yamamoto

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We study two-component fermions in optical lattices with spatially alternating on-site interactions using dynamical mean-field theory. Calculating the quasi-particle weight, double occupancy, and order parameters for each sublattice, we discuss the low-temperature properties of the system. When both interactions are repulsive, the magnetically ordered state is realized at half-filling. In the attractive case, the superfluid state is, in general, realized with a particle number imbalance. On the...

Source: http://arxiv.org/abs/1207.2150v2

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52

Sep 18, 2013
09/13

by
Yusuke Fujihara; Akihisa Koga; Norio Kawakami

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We investigate two-component ultracold fermionic atoms with repulsive interactions trapped in an optical lattice with a ladder structure. By applying the Bogoliubov-de Gennes equations to an effective t-J model in the strong correlation limit, we discuss how the spatially-modulated spin-singlet pairs with d-wave like symmetry are formed in the systems with trapping potentials. Furthermore, a close examination of the condensation energy as well as the local average of potential, kinetic and...

Source: http://arxiv.org/abs/0809.4114v1

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54

Sep 19, 2013
09/13

by
Akihisa Koga; Yoshiki Imai; Norio Kawakami

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Electron correlations in the two-orbital Hubbard model at half-filling are investigated by combining dynamical mean field theory with the exact diagonalization method. We systematically study how the interplay of the intra- and inter-band Coulomb interactions, together with the Hund coupling, affects the metal-insulator transition. It is found that if the intra- and inter-band Coulomb interactions are nearly equal, the Fermi-liquid state is stabilized due to orbital fluctuations up to fairly...

Source: http://arxiv.org/abs/cond-mat/0206064v1

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68

Sep 21, 2013
09/13

by
Takuya Yoshioka; Akihisa Koga; Norio Kawakami

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We investigate the bandwidth-controlled Mott transition in the Hubbard model on the checkerboard lattice at half filling using the path-integral renormalization group (PIRG) method. It is demonstrated that the system undergoes a first-order phase transition to the plaquette-singlet insulating phase at a finite Hubbard interaction. This conclusion is drawn via a detailed analysis of the spin and charge correlations around the phase transition point by means of the PIRG method aided with a new...

Source: http://arxiv.org/abs/0808.1116v2

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50

Sep 22, 2013
09/13

by
Kazuto Noda; Akihisa Koga; Norio Kawakami; Thomas Pruschke

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We investigate two-component ultracold fermions loaded into a decorated square lattice, which are described by the Hubbard model with repulsive interactions and nearest neighbor hoppings. By combining the real-space dynamical mean-field theory with the numerical renormalization group method, we discuss how a ferromagnetically ordered ground state in the weak coupling regime, which originates from the existence of a dispersionless band, is adiabatically connected to a Heisenberg ferrimagnetic...

Source: http://arxiv.org/abs/1004.4650v1

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52

Sep 20, 2013
09/13

by
Akihisa Koga; Akira Kawaguchi; Kouichi Okunishi; Norio Kawakami

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We give a short review of our recent works on the first-order quantum phase transitions in frustrated spin chains with orthogonal-dimer structure. When the ratio of the competing antiferromagnetic exchange couplings is varied, a first-order transition occurs between the dimer phase and the plaquette phase, which is accompanied by the discontinuity in the spin excitation gap. We further show that strong frustration triggers the phase transitions in a magnetic field, which exhibit plateaus and...

Source: http://arxiv.org/abs/cond-mat/0204505v1

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83

Sep 19, 2013
09/13

by
Akihisa Koga; Norio Kawakami; Robert Peters; Thomas Pruschke

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We investigate quantum phase transitions in the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. We calculate zero and finite temperature properties of the system using the combination of dynamical mean-field theory and the numerical renormalization group. At half filling, a phase transition between a Mott insulating state and a Kondo insulating state occurs in the strong coupling regime. We furthermore find that a metallic...

Source: http://arxiv.org/abs/0708.1765v1

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44

Sep 20, 2013
09/13

by
Ryota Sato; Takuma Ohashi; Akihisa Koga; Norio Kawakami

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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...

Source: http://arxiv.org/abs/cond-mat/0404553v1

69
69

Sep 23, 2013
09/13

by
Akihisa Koga; Norio Kawakami; Robert Peters; Thomas Pruschke

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We study magnetic properties of the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. By combining dynamical mean-field theory with the numerical renormalization group we calculate the sublattice magnetization and the staggered susceptibility to determine the phase diagram in the particle-hole symmetric case. We find that two kinds of magnetically ordered states compete with the Kondo insulating state at zero temperature,...

Source: http://arxiv.org/abs/0801.3668v1

6
6.0

Jun 29, 2018
06/18

by
Shiro Sakai; Nayuta Takemori; Akihisa Koga; Ryotaro Arita

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We study a possible superconductivity in quasiperiodic systems, by portraying the issue within the attractive Hubbard model on a Penrose lattice. Applying a real-space dynamical mean-field theory to the model consisting of 4181 sites, we find a superconducting phase at low temperatures. Reflecting the nonperiodicity of the Penrose lattice, the superconducting state exhibits an inhomogeneity. According to the type of the inhomogeneity, the superconducting phase is categorized into three...

Topics: Superconductivity, Condensed Matter, Quantum Gases, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1608.07210

55
55

Sep 22, 2013
09/13

by
Akira Kawaguchi; Akihisa Koga; Kouichi Okunishi; Norio Kawakami

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Hole-doping effects are investigated on the {\it t-J} ladder model with the linked-tetrahedra structure. We discuss how a metal-insulator transition occurs upon hole doping with particular emphasis on the effects of geometrical frustration. By computing the electron density and the spin correlation function by the density matrix renormalization group, we show that strong frustration triggers a first-order transition to a metallic phase, when holes are doped into the plaquette-singlet phase. By...

Source: http://arxiv.org/abs/cond-mat/0205635v1

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Sep 18, 2013
09/13

by
Akira Kawaguchi; Akihisa Koga; Kouichi Okunishi; Norio Kawakami

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We investigate the magnetization process for a quasi-one-dimensional S=1 antiferromagnet with bond alternation. By combining the density matrix renormalization group method with the interchain mean-field theory, we discuss how the interchain coupling affects the magnetization curve. It is found that the width of the magnetization plateau is considerably reduced upon introducing the interchain coupling. We obtain the phase diagram in a magnetic field. The effect of single-ion anisotropy is also...

Source: http://arxiv.org/abs/cond-mat/0202080v1

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70

Sep 21, 2013
09/13

by
Takuma Ohashi; Akihisa Koga; Sei-ichiro Suga; Norio Kawakami

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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...

Source: http://arxiv.org/abs/cond-mat/0407518v1

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50

Sep 18, 2013
09/13

by
Akihisa Koga; Norio Kawakami; T. M. Rice; Manfred Sigrist

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We investigate the Mott transitions in two-band Hubbard models with different bandwidths. Applying dynamical mean field theory, we discuss the stability of itinerant quasi-particle states in each band. We demonstrate that separate Mott transitions occur at different Coulomb interaction strengths in general, which merge to a single transition only under special conditions. This kind of behavior may be relevant for the physics of the single-layer ruthenates, Ca$_{2-x}$Sr$_x$RuO$_4$.

Source: http://arxiv.org/abs/cond-mat/0401223v1

62
62

Jul 20, 2013
07/13

by
Akihisa Koga; Norio Kawakami; T. M. Rice; Manfred Sigrist

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We investigate the Mott transitions in the two-orbital Hubbard model with different bandwidths. By combining dynamical mean field theory with the exact diagonalization, we discuss the stability of itinerant quasi-particle states in each band. We demonstrate that separate Mott transitions occur at different Coulomb interaction strengths in general, which merge to a single transition only under special conditions. In particular, it is clarified that the $xy$ and pair-hopping components of the...

Source: http://arxiv.org/abs/cond-mat/0406457v1

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50

Sep 21, 2013
09/13

by
Takuma Ohashi; Akihisa Koga; Sei-ichiro Suga; Norio Kawakami

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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 $T_c$...

Source: http://arxiv.org/abs/cond-mat/0407142v1