arXiv subjects

Masafumi Udagawa

Publications and source records attributed to Masafumi Udagawa.

At least 55 records · Page 3Linked to original sources

arXiv2013

Invariant Energy Levels and Flat Band Engineering in a Kondo Lattice Model on Geometrically Frustrated Lattices

We show the existence of invariant energy levels in a Kondo lattice model on an isolated complete graph, such as a triangle and a tetrahedron. These energy levels always have fixed eigenenergies $t \pm J/2$, irrespective of the configuration of localized moments ($t$ is the transfer integral of conduction electrons and $J$ is the spin-charge coupling constant). We also extend the analysis to geometrically frustrated lattices by using the complete graphs as basic building blocks. We show that the construction rule for the invariant energy levels leads to the existence condition of localized states, if the model is defined on the triangle-based line graphs, such as a kagome lattice. We further propose a procedure of engineering isolated flat bands with broken time-reversal symmetry, which are separated from other dispersive bands with finite energy gaps.

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arXiv2013

Linear Spin Wave Analysis for General Magnetic Orders in the Kondo Lattice Model

We extend the formulation of the spin wave theory for the Kondo lattice model, which was mainly used for the ferromagnetic metallic state, to general magnetic orders including complex noncollinear and noncoplanar orders. The 1/S expansion is reformulated in the matrix form depending on the size of the magnetic unit cell. The noncollinearity and noncoplanarity of the localized moments are properly taken into account by the matrix elements of the para-unitary matrix used in the diagonalization of the Bogoliubov-de Gennes type Hamiltonian for magnons. We apply the formulation within the linear spin wave approximation to a typical noncollinear case, the $120^{\circ}$ N{é}el order on a triangular lattice at half filling. We calculate the magnon excitation spectrum and the quantum correction to the magnitude of ordered moments as functions of the strength of the Hund's-rule coupling, $J_{\rm H}/t$. We find that the magnon excitation shows softening at $J_{\rm H}/t \simeq 2.9$, which indicates that the $120^{\circ}$ order is destabilized for smaller $J_{\rm H}/t$. On the other hand, we show that the $120^{\circ}$ order is stable in the entire range of $J_{\rm H}/t \gtrsim 2.9$, and, in the limit of $J_{\rm H}/t \to \infty$, the form of the spin wave spectrum approaches that for the antiferromagnetic Heisenberg model, while the bandwidth is proportional to $t^2/J_{\rm H}$. The reduction of the ordered moment is smaller than that for the spin-only model, except in the vicinity of the softening.

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arXiv2013

Monte Carlo Study of an Effective Ising Model for the Spin-ice type Kondo Lattice Model

An effective Ising model for the spin-ice type Kondo lattice model is investigated by the classical Monte Carlo simulation. We clarify the magnetic phase diagram with four phases: ice-ferro, ice-(0,0,2π), 32-sublattice, and all-in/all-out ordered states. The result well reproduces the phase diagram of the Kondo lattice model studied previously [H. Ishizuka et al.: J. Phys. Soc. Jpn. 81 (2012) 113706], which suggests that the RKKY interactions up to third neighbors are sufficient to describe the magnetic properties of the itinerant electron model. We discuss the peculiar nature of phase transitions: the suppression of the critical temperatures down to zero temperature between two ice phases and the presence of the tricritical points on the phase boundary of the 32-sublattice ordered state.

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arXiv2013

Polynomial Expansion Monte Carlo Study of Frustrated Itinerant Electron Systems: Application to a Spin-ice type Kondo Lattice Model on a Pyrochlore Lattice

We present the benchmark of the polynomial expansion Monte Carlo method to a Kondo lattice model with classical localized spins on a geometrically frustrated lattice. The method enables to reduce the calculation amount by using the Chebyshev polynomial expansion of the density of states compared to a conventional Monte Carlo technique based on the exact diagonalization of the fermion Hamiltonian matrix. Further reduction is brought by a real-space truncation of the vector-matrix operations. We apply the method to the model with spin-ice type Ising spins on a three-dimensional pyrochlore lattice, and carefully examine the convergence in terms of the order of polynomials and the truncation distance. We find that, in a wide range of electron density at a relatively weak Kondo coupling compared to the noninteracting bandwidth, the results by the polynomial expansion method show good convergence to those by the conventional method within reasonable numbers of polynomials. This enables us to study the systems up to 4x8^3 = 2048 sites, while the previous study by the conventional method was limited to 4x4^3 = 256 sites. On the other hand, the real-space truncation is not helpful in reducing the calculation amount for the system sizes that we reached, as the sufficient convergence is obtained when most of the sites are involved within the truncation distance. The necessary truncation distance, however, appears not to show significant system size dependence, suggesting that the truncation method becomes efficient for larger system sizes.

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arXiv2012

Anomalous Hall effect from frustration-tuned scalar chirality distribution in Pr2Ir2O7

We analyse the Ising Kondo lattice model on a pyrochlore structure in order to study the anomalous Hall effect due to non-coplanar magnetism. We focus on the frustration-induced spatial inhomogeneity of different magnetic low-temperature regimes, between which one can efficiently tune using an external magnetic field. We incorporate non-magnetic scattering on a phenomenological level so that we can distinguish between the effects of short-range correlations and short-range coherence. We obtain a Hall conductivity (σ_H) as function of field strength and direction which compares well to the experimental data of Pr2Ir2O7. In particular, we show that the observed peak in σ_H for H||[111] signals the crossover from zero-field spin ice to Kagome ice.

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arXiv2012

Magnetic Order and Charge Disproportionation in a Spin-ice type Kondo Lattice Model: Large Scale Monte Carlo Study

Phase diagram of a spin-ice type Kondo lattice model, potentially relevant to metallic pyrochlore oxides, is obtained by the Monte Carlo simulation implementing the polynomial expansion technique up to the system size with 2048 sites. We identified a new 32-sublattice magnetic phase with concomitant charge disproportionation, along with other phases such as two-in two-out and all-in/all-out orders. The spin and charge pattern can be switched by external magnetic field to a different one accompanied by a half magnetization plateau.

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arXiv2012

Partial Disorder and Metal-Insulator Transition in the Periodic Anderson Model on a Triangular Lattice

Ground state of the periodic Anderson model on a triangular lattice is systematically investigated by the mean-field approximation. We found that the model exhibits two different types of partially disordered states: one is at half filling and the other is at other commensurate fillings. In the latter case, the kinetic energy is lowered by forming an extensive network involving both magnetic and nonmagnetic sites, in sharp contrast to the former case in which the nonmagnetic sites are rather isolated. This spatially extended nature of nonmagnetic sites yields a metallic partially-disordered state by hole doping. We discuss the mechanism of the metal-insulator transition by the change of electronic structure.

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arXiv2012

Hidden Multiple-spin Interactions as an Origin of Spin Scalar Chiral Order in Frustrated Kondo Lattice Models

We reveal the significance of kinetic-driven multiple-spin interactions hidden in geometrically-frustrated Kondo lattice models. Carefully examining the perturbation in terms of the spin-charge coupling up to the fourth order, we find that a positive biquadratic interaction is critically enhanced and plays a crucial role on stabilizing a spin scalar chiral order near 1/4 filling in a triangular lattice case. This is a generalized Kohn anomaly, appearing only when the second-order perturbation is inefficient because of the degeneracy under frustration. The mechanism is potentially common to frustrated spin-charge coupled systems, leading to emergence of unusual magnetic orders.

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arXiv2012

Non-Kondo mechanism for resistivity minimum in spin ice conduction systems

We present a mechanism of resistivity minimum in conduction electron systems coupled with localized moments, which is distinguished from the Kondo effect. Instead of the spin-flip process in the Kondo effect, electrons are elastically scattered by local spin correlations which evolve in a particular way under geometrical frustration as decreasing temperature. This is demonstrated by the cellular dynamical mean-field theory for a spin-ice type Kondo lattice model on a pyrochlore lattice. Peculiar temperature dependences of the resistivity, specific heat, and magnetic susceptibility in the non-Kondo mechanism are compared with the experimental data in metallic Ir pyrochlore oxides.

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arXiv2011

Carrier doping to a partially disordered state in the periodic Anderson model on a triangular lattice

We investigate the effect of hole and electron doping to half-filling in the periodic Anderson model on a triangular lattice by the Hartree-Fock approximation at zero temperature. At half-filling, the system exhibits a partially disordered insulating state, in which a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork coexists with nonmagnetic state at the remaining sites. We find that the carrier doping destabilizes the partially disordered state, resulting in a phase separation to a doped metallic state with different magnetic order. The partially disordered state is restricted to the half-filled insulating case, while its metallic counterpart is obtained as a metastable state in a narrow electron doped region.

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arXiv2011

Application of polynomial-expansion Monte Carlo method to a spin-ice Kondo lattice model

We present the results of Monte Carlo simulation for a Kondo lattice model in which itinerant electrons interact with Ising spins with spin-ice type easy-axis anisotropy on a pyrochlore lattice. We demonstrate the efficiency of the truncated polynomial expansion algorithm, which enables a large scale simulation, in comparison with a conventional algorithm using the exact diagonalization. Computing the sublattice magnetization, we show the convergence of the data with increasing the number of polynomials and truncation distance.

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arXiv2011

Variational Monte Carlo Study of the Kondo Necklace Model with Geometrical Frustration

We investigate the ground state of the Kondo necklace model on geometrically-frustrated lattices by the variational Monte Carlo simulation. To explore the possibility of a partially-ordered phase, we employ an extension of the Yosida-type wave function as a variational state, which can describe a coexistence of spin-singlet formation due to the Kondo coupling and magnetic ordering by the Ruderman-Kittel-Kasuya-Yosida interaction. We show the benchmark of the numerical simulation to demonstrate the high precision brought by the optimization of a large number of variational parameters. We discuss the ground-state phase diagram for the model on the kagome lattice in comparison with that for the triangular-lattice case.

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arXiv2011

Partial Disorder in the Periodic Anderson Model on a Triangular Lattice

We report our theoretical results on the emergence of a partially-disordered state at zero temperature and its detailed nature in the periodic Anderson model on a triangular lattice at half filling. The partially-disordered state is characterized by coexistence of a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites. This state appears with opening a charge gap between a noncollinear antiferromagnetic metal and Kondo insulator while changing the hybridization and Coulomb repulsion. We also find a characteristic crossover in the low-energy excitation spectrum as a result of coexistence of magnetic order and nonmagnetic sites. The result demonstrates that the partially-disordered state is observed distinctly even in the absence of spin anisotropy, in marked contrast to the partial Kondo screening state found in the previous study for the Kondo lattice model.

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arXiv2011

Metal-insulator transition caused by the coupling to localized charge-frustrated systems under ice-rule local constraint

We report the results of our theoretical and numerical study on electronic and transport properties of fermion systems with charge frustration. We consider an extended Falicov-Kimball model in which itinerant spinless fermions interact repulsively by U with localized particles whose distribution satisfies a local constraint under geometrical frustration, the so-called ice rule. We numerically calculate the density of states, optical conductivity, and inverse participation ratio for the models on the pyrochlore, checkerboard, and kagome lattices, and discuss the nature of metal-insulator transitions at commensurate fillings. As a result, we show that the ice-rule local constraint leads to several universal features in the electronic structure; a charge gap opens at a considerably small U compared to the bandwidth, and the energy spectrum approaches a characteristic form in the large U limit, that is, the noninteracting tight-binding form in one dimension or the $δ$-functional peak. In the large U region, the itinerant fermions are confined in the macroscopically-degenerate ice-rule configurations, which consist of a bunch of one-dimensional loops: We call this insulating state the charge ice. On the other hand, transport properties are much affected by the geometry and dimensionality of lattices; e.g., the pyrochlore lattice model exhibits a transition from a metallic to the charge-ice insulating state by increasing U, while the checkerboard lattice model appears to show Anderson localization before opening a gap. Meanwhile, in the kagome lattice case, we do not obtain clear evidence of Anderson localization. Our results elucidate the universality and diversity of phase transitions to the charge-ice insulator in fully frustrated lattices.

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arXiv2011

Mean-field Study of Charge, Spin, and Orbital Orderings in Triangular-lattice Compounds ANiO2 (A=Na, Li, Ag)

We present our theoretical results on the ground states in layered triangular-lattice compounds ANiO2 (A=Na, Li, Ag). To describe the interplay between charge, spin, orbital, and lattice degrees of freedom in these materials, we study a doubly-degenerate Hubbard model with electron-phonon couplings by the Hartree-Fock approximation combined with the adiabatic approximation. In a weakly-correlated region, we find a metallic state accompanied by \sqroot3x\sqroot3 charge ordering. On the other hand, we obtain an insulating phase with spin-ferro and orbital-ferro ordering in a wide range from intermediate to strong correlation. These phases share many characteristics with the low-temperature states of AgNiO2 and NaNiO2, respectively. The charge-ordered metallic phase is stabilized by a compromise between Coulomb repulsions and effective attractive interactions originating from the breathing-type electronphonon coupling as well as the Hund's-rule coupling. The spin-orbital-ordered insulating phase is stabilized by the cooperative effect of electron correlations and the Jahn-Teller coupling, while the Hund's-rule coupling also plays a role in the competition with other orbital-ordered phases. The results suggest a unified way of understanding a variety of low-temperature phases in ANiO2. We also discuss a keen competition among different spin-orbital-ordered phases in relation to a puzzling behavior observed in LiNiO2.

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arXiv2010

Interplay of Spin-Orbit Interaction and Electron Correlation on the Van Vleck Susceptibility in Transition Metal Compounds

We have studied the effects of electron correlation on Van Vleck susceptibility ($χ_{\rm{VV}}$) in transition metal compounds. A typical crossover behavior is found for the correlation effect on $χ_{\rm{VV}}$ as sweeping spin-orbit interaction, $λ$. For a small $λ$, orbital fluctuation plays a dominant role in the correlation enhancement of $χ_{\rm{VV}}$; however, the enhancement rate is rather small. In contrast, for an intermediate $λ$, $χ_{\rm{VV}}$ shows a substantial increase, accompanied by the development of spin fluctuation. We will discuss the behavior of $χ_{\rm{VV}}$ in association with the results of Knight-shift experiments on Sr$_2$RuO$_4$ and an anomalously large magnetic susceptibility observed for $5d$ Ir compounds.

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arXiv2010

Partial Kondo screening in frustrated Kondo lattice systems

We investigate the effect of geometrical frustration on the competition between the Kondo coupling and the Ruderman-Kittel-Kasuya-Yosida interaction in Kondo lattice systems. By variational Monte Carlo simulations, we reveal an emergent quantum phase with partial ordering in which the frustration is relieved by forming a magnetic order on a sublattice and leaving the rest in the Kondo screening with spin-singlet formation. The role of quantum fluctuations and spin-charge interplay is elucidated.

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arXiv2010

Quantum melting of charge ice and non-Fermi-liquid behavior: An exact solution for the extended Falicov-Kimball model in the ice-rule limit

An exact solution is obtained for a model of itinerant electrons coupled to ice-rule variables on the tetrahedron Husimi cactus, an analogue of the Bethe lattice of corner-sharing tetrahedra. It reveals a quantum critical point with the emergence of non-Fermi-liquid behavior in melting of the "charge ice" insulator. The electronic structure is compared with the numerical results for the pyrochlore-lattice model to elucidate the physics of electron systems interacting with the tetrahedron ice rule.

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