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ABSTRACT: 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 geometrical frustration. In particular, it is found that a RVB-type spin-gap phase proposed recently for the orthogonal-dimer system is adiabatically connected to the plaquette phase known for the 1/5-depleted square-lattice model. Comment: 6 pages, to appear in JPSJ 70 (2001)
03/2001;
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ABSTRACT: The NMR relaxation rate $1/T_{1}$ is studied for one-dimensional multicomponent spin-orbital systems. By combining the bosonization techniques and the exact solution of the SU($n$) model, we evaluate the power-law exponent and the enhancement factor for $1/T_1$ at low temperatures. We discuss how the band splitting affects the relaxation rate, and find that $1/T_{1}$ may be enhanced around the critical value of the band splitting. The crossover behavior in $1/T_1$ around the critical point is discussed in terms of the low-frequency dynamical spin susceptibility. The effect of the hole-doping is also addressed.
03/2001;
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ABSTRACT: Magnetic properties for the one-dimensional multicomponent quantum spin system with the excitation gap are studied based on the integrable spin model introduced by Bariev {\it et al}. By exactly computing the magnetization, we show how the characteristic structure with plateaus and cusps appears in the magnetization process. To study low-energy dynamics of the system, we apply the finite-size scaling analysis to the excitation spectrum, and thereby evaluate the power-law exponent as well as the enhancement factor for the low-temperature NMR relaxation rate $1/T_1$. We discuss the critical properties of $1/T_1$ around plateaus and cusps in the magnetization curve. Comment: 7 pages, 8 eps figures, to appear in PRB
01/2001;
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ABSTRACT: We study how the multichannel Kondo effect is dynamically induced to affect the photoemission and the inverse photoemission spectrum when an electron is emitted from (or added to) the completely screened Kondo impurity with spin S>1/2. The spectrum thereby shows a power-law edge singularity characteristic of the multichannel Kondo model. We discuss this anomalous behavior by using the exact solution of the multichannel Kondo model and boundary conformal field theory. The idea is further applied to the photoemission for quantum spin systems, in which the edge singularity is controlled by the dynamically induced overscreening effect with a mobile Kondo impurity.
Phys. Rev. B. 01/2001; 63(6).
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ABSTRACT: The two-orbital Hubbard model with the Hund coupling is investigated in a metallic phase close to the Mott insulator. We calculate the one-particle spectral function and the optical conductivity within dynamical mean field theory, for which the effective impurity problem is solved by using the non-crossing approximation. For a metallic system close to quarter filling, a heavy quasi-particle band is formed by the Hubbard interaction, the effective mass of which is not so sensitive to the orbital splitting and the Hund coupling. In contrast, a heavy quasi-particle band near half filling disappears in the presence of the orbital splitting, but is induced again by the introduction of the Hund coupling, resulting in a different type of heavy quasi-particles.
Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/2001; 70(8):2365-2370.
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ABSTRACT: 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 pyrochlore spin system. Comment: 4 pages, 4 figures
10/2000;
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ABSTRACT: The effect of the Hubbard interaction among conduction electrons on the double exchange model is investigated in a ferromagnetic metallic phase. Applying iterative perturbation theory to the Hubbard interaction within dynamical mean field theory, we calculate the one-particle spectral function and the optical conductivity, in which coherent-potential approximation is further used to treat the ferromagnetic Hund coupling between conduction electrons and localized spins. Identifying the decrease of the magnetization for the localized spin with the increase of the temperature, we discuss the temperature dependence of the one-particle spectrum and the optical conductivity. It is found that the interplay between the Hund coupling and the Hubbard interaction dramatically changes the spectral function, while it is somehow obscured in the optical conductivity. Comment: 5 pages, 4 eps figures, to appear in J. Phys. Soc. Jpn
07/2000;
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ABSTRACT: 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 transition to the dimer (the magnetically ordered) phase at the critical point $\alpha_{c1}=0.677(2)$ ($\alpha_{c2}=0.86(1)$). Our results shed light on some controversial arguments about the nature of the quantum phase transitions in this model. Comment: 4 pages, accepted for publication in Phys. Rev. Lett
03/2000;
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ABSTRACT: 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. We further reveal that the plateau in the magnetization curve at 1/4 of the full moment dramatically changes its character in the vicinity of the critical point. It is argued that the first-order phase transition in this system captures some essential properties found in the two-dimensional orthogonal-dimer model proposed for $\rm SrCu_2(BO_3)_2$. Comment: 7 pages, submitted to Phys. Rev. B
03/2000;
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ABSTRACT: We present a mixed-spin cluster expansion method to discuss the quantum phase transitions for the Haldane system in two and three dimensions. By mapping the s51 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 realizes the notion of the valence bond solid in a perturbation theory. This method allows us to directly deal with the Haldane phase, which may not be reached by standard series expansion methods. The zero-temperature phase diagram is thus determined rather precisely for the two- and three-dimensional Haldane systems.
Physical Review B 01/2000; 61(9):6133. · 3.69 Impact Factor
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ABSTRACT: We study the effects of the Hubbard interaction among conduction electrons on the double exchange model with the orbital degeneracy. Employing a slave-boson approach to treat the correlation for conduction electrons and a coherent-potential approximation for the Hund coupling, we calculate the density of states and the optical conductivity. The Hund coupling splits the density of states into two parts, which are modified by the band narrowing factor arising from the Hubbard interaction. Identifying the decrease of the magnetization for the localized spin with the increase of the temperature, we discuss the temperature dependence of the optical conductivity.
Physica B Condensed Matter 01/2000; 281:536-537. · 1.06 Impact Factor
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ABSTRACT: 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.
09/1999;
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ABSTRACT: 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 diagram is determined rather precisely by making use of an additional symmetry property in the effective mixed-spin model introduced.
09/1999;
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ABSTRACT: We investigate low-energy properties of two-dimensional quantum spin systems with the ladder and plaquette structures,which are described by a generalized antiferromagnetic Heisenberg model with both of the bond and spin alternations.By exploiting a non-linear sigma model technique and a modified spin wave approach, we evaluate the spin gap and the spontaneous magnetization todiscuss the quantum phase transition between the ordered and disordered states. We argue how the spin-gapped phase is driven to the antiferromagnetic phase in the phase diagram.
Journal of The Physical Society of Japan - J PHYS SOC JPN. 01/1999; 68(2):642-649.
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ABSTRACT: The critical properties of Tomonaga-Luttinger liquids including a mobile impurity are studied by means of the Bethe ansatz method, conformal field theory (CFT), and bosonization. We calculate conformal dimensions by performing the CFT analysis of the energy spectrum for an integrable impurity model, and show that this system is classified as c=1 shifted Gaussian CFT. The exact critical exponents of various correlation functions are obtained as functions of the impurity mass and its momentum. Passing to the continuum limit, we then construct an effective field theory in terms of bosonization, and directly calculate some correlation functions. A possible application to the Fermi-edge singularity in quantum wires is also discussed.
Phys. Rev. B. 08/1998; 58(7).
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ABSTRACT: We investigate the Mott transition in the anisotropic kagome 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.
Phys. Rev. B. 82(16).
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ABSTRACT: We investigate the singlet-pairing superfluid state of two-component correlated fermions with repulsive interactions in a one-dimensional harmonic potential. By introducing a BCS-type wave function with spatially modulated order parameter, we show how the spin-singlet pairs are formed in the inhomogeneous potential. We then perform variational calculations with particular emphasis on the effect of particle correlations in order to examine the stability of the superfluid state.
Journal of Physics and Chemistry of Solids.
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ABSTRACT: We study the ground-state properties of the two-dimensional Hubbard model with a confining potential. To discuss how the attractive (repulsive) interactions induce a spatially modulated superfluid (magnetically ordered) state at zero temperature, we use the variational Monte Carlo method, which can incorporate not only onsite but also intersite correlations. We then elucidate how the intersite correlations affect the ground-state properties of the system.
Physica B: Condensed Matter.
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ABSTRACT: We investigate electron correlations in the two-dimensional Hubbard model on the planar pyrochlore lattice. By means of variational Monte Carlo simulations, we discuss how stable the paramagnetic semi-metallic state is at half-filling. It is found that the increase of the Coulomb interaction induces the first-order transition around U/t∼9, where the magnetically ordered state is more stable than the Mott insulating phase without symmetry breaking.
Journal of Magnetism and Magnetic Materials 310(2):867-869. · 1.78 Impact Factor
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ABSTRACT: We study a geometrically frustrated Hubbard model on the checkerboard lattice with nearest neighbor (t) and diagonal (t′) hoppings. By using the path-integral renormalization group method, we calculate the double occupancy and the plaquette singlet correlation function to discuss the instability of the plaquette valence-bond crystal (P-VBC) phase at half filling. It is found that the increase of Coulomb interaction induces a first-order Mott transition to the plaquette singlet insulating (PSI) phase for t′/t=1.0, which naturally leads to the P-VBC phase in the Heisenberg limit. For t′/t=0.8, the double quantum phase transitions occur, and the PSI phase is stabilized between paramagnetic metallic and antiferromagnetic insulating phase.
Physica B Condensed Matter 403:1248-1250. · 1.06 Impact Factor
