arXiv subjects

Yukitoshi Motome

Publications and source records attributed to Yukitoshi Motome.

At least 235 records · Page 13Linked to original sources

arXiv2010

Phase competition in the double-exchange model on the frustrated pyrochlore lattice

Competition between the ferromagnetic double-exchange interaction and the super-exchange antiferromagnetic interaction is theoretically studied in the presence of geometrical frustration. As increasing the super-exchange interaction, the ferromagnetic metal becomes unstable, and is taken over by a cooperative paramagnetic metal, in sharp contrast with a discontinuous transition to the antiferromagnetic insulator in the absence of frustration. In the critical region, the system exhibits a peculiar temperature-independent behavior with highly incoherent transport, suggesting a large residual entropy at low temperatures. We discuss the relevance of the results to the pressure-induced behaviors in Mo pyrochlore oxides [S. Iguchi et al., Phys. Rev. Lett. 102, 136407 (2009)].

cond-mat.str-el↗

arXiv2010

Nematic, vector-multipole, and plateau-liquid states in the classical O(3) pyrochlore antiferromagnet with biquadratic interactions in applied magnetic field

The classical bilinear-biquadratic nearest-neighbor Heisenberg antiferromagnet on the pyrochlore lattice does not exhibit conventional Neel-type magnetic order at any temperature or magnetic field. Instead spin correlations decay algebraically over length scales r ~ \sqrt{T}, behavior characteristic of a Coulomb phase arising from a strong local constraint. Despite this, its thermodynamic properties remain largely unchanged if Neel order is restored by the addition of a degeneracy-lifting perturbation, e.g., further neighbor interactions. Here we show how these apparent contradictions can be resolved by a proper understanding of way in which long-range Neel order emerges out of well-formed local correlations, and identify nematic and vector-multipole orders hidden in the different Coulomb phases of the model. So far as experiment is concerned, our results suggest that where long range interactions are unimportant, the magnetic properties of Cr spinels which exhibit half-magnetization plateaux may be largely independent of the type of magnetic order present.

cond-mat.str-el↗

arXiv2009

Spiral charge frustration in molecular conductor (DI-DCNQI)2Ag

We theoretically study the effect of spiral-type charge frustration in a quasi-one-dimensional molecular conductor (DI-DCNQI)2Ag. We clarify how the spiral frustration in the interchain Coulomb repulsion is relieved and leads to a self-organization of complex charge-lattice ordered chains, in agreement with the recent synchrotron X-ray study [T. Kakiuchi et al., Phys. Rev. Lett. 98, 066402 (2007)]. In addition, we find that a keen competition between charge and lattice degrees of freedom under the frustration gives rise to a characteristic temperature within the ordered phase, below which a drastic growth of molecular displacements occurs. Our results enlighten the relevance of the spiral frustration and provide a possible reconciliation among puzzling experimental data.

cond-mat.str-el↗

arXiv2009

Phase competitions and coexistences in quasi-one-dimensional molecular conductors: exact diagonalization study

We investigate ground state properties of a quasi-one-dimensional electron-lattice coupled model for quarter-filled molecular conductors. The effective one-dimensional extended Hubbard model coupled to adiabatic lattice degree of freedom is derived by the inter-chain mean-field approximation and solved by Lanczos exact diagonalization method. We find that the critical behavior among lattice tetramerized states with different charge-lattice ordered patterns is sensitively affected by the inter-chain Coulomb interaction, lattice anharmonicity, and intrinsic dimerization. This indicates a subtle balance between these states originating from strong correlation and reduced dimensionality.

cond-mat.str-el↗

arXiv2009

Reply to Comment on "Evolution of electronic structure of doped Mott insulator - reconstruction of poles and zeros of Green's function" [arXiv:0904.0454] by P. Phillips

In a recent Comment [arXiv:0904.0454] on our Letter [Phys. Rev. Lett. 102, 056404 (2009); arXiv:0809:0950], Phillips criticizes one specific issue discussed in our paper, concerning the spectral weight transfer, among our wide-ranging clarifications of the filling-control Mott transition. Here we show that all his criticisms have nothing to do with our contributions. Our nontrivial finding on this particular aspect is a very quick, non-linear increase in the spectral weight just above the chemical potential caused by tiny doping to the Mott insulator, which is indeed unprecedented in the literature. We call attention to its novelty in comparison with the perturbation theory, and reemphasize our picture for the quick weight transfer, that is, an avalanchine screening of doublon-holon binding energy.

cond-mat.str-el↗

arXiv2008

Finite-Temperature Phase Diagram of Quasi-One-Dimensional Molecular Conductors: Quantum Monte Carlo Study

Finite-temperature phase transitions in quasi-one-dimensional quarter-filled systems are investigated by the extended Hubbard model with electron-lattice coupling. Using a quantum Monte Carlo method combined with the inter-chain mean-field approximation, we clarify competing and coexisting behaviors among charge ordering, dimer Mott, and spin-Peierls states. It is pointed out that an anharmonicity of lattice distortions plays an important role in multi-critical behaviors. The results are compared with experimental data for quasi-one-dimensional molecular conductors such as DCNQI and TMTTF compounds.

cond-mat.str-el↗

arXiv2008

Evolution of electronic structure of doped Mott insulators - reconstruction of poles and zeros of Green's function

We study evolution of metals from Mott insulators in the carrier-doped 2D Hubbard model using a cluster extension of the dynamical mean-field theory. While the conventional metal is simply characterized by the Fermi surface (pole of the Green function G), interference of the zero surfaces of G with the pole surfaces becomes crucial in the doped Mott insulators. Mutually interfering pole and zero surfaces are dramatically transferred over the Mott gap, when lightly doped holes synergetically loosen the doublon-holon binding. The heart of the Mott physics such as the pseudogap, hole pockets, Fermi arcs, in-gap states, and Lifshitz transitions appears as natural consequences of this global interference in the frequency space.

cond-mat.str-el↗

arXiv2007

Exact diagonalization study of Mott transition in the Hubbard model on an anisotropic triangular lattice

We study Mott transition in the two-dimensional Hubbard model on an anisotropic triangular lattice. We use the Lanczos exact diagonalization of finite-size clusters up to eighteen sites, and calculate Drude weight, charge gap, double occupancy and spin structure factor. We average these physical quantities over twisted boundary conditions in order to reduce finite-size effects. We find a signature of the Mott transition in the dependence of the Drude weight and/or charge gap on the system size. We also examine the possibility of antiferromagnetic order from the spin structure factor. Combining these information, we propose a ground-state phase diagram which has a nonmagnetic insulating phase between a metallic phase and an insulating phase with antiferromagnetic order. Finally, we compare our results with those reported in the previous theoretical studies, and discuss the possibility of an unconventional insulating state.

cond-mat.str-el↗

arXiv2007

Charge ordering and coexistence of charge fluctuations in quasi-two-dimensional organic conductors $θ$-(BEDT-TTF)$_2X$

We present a scenario for the peculiar coexistence of charge fluctuations observed in quasi-2D 1/4-filled organic conductors $θ$-(BEDT-TTF)$_2X$ in the quantum critical regime where the charge ordering is suppressed down to zero temperature. The scenario is explored in the extended Hubbard model including electron-phonon couplings on an anisotropic triangular lattice. We find that the coexisting fluctuations emerge from two different instabilities, the ``Wigner crystallization on lattice'' driven by the off-site Coulomb repulsion and the charge-density-wave formation due to the nesting of the Fermi surface, not from phase competition or real-space inhomogeneity. This mechanism explains the contrastive temperature dependence of two fluctuations in experiments.

cond-mat.str-el↗

arXiv2006

Finite-temperature phase transitions in quasi-one-dimensional molecular conductors

Phase transitions in 1/4-filled quasi-one-dimensional molecular conductors are studied theoretically on the basis of extended Hubbard chains including electron-lattice interactions coupled by interchain Coulomb repulsion. We apply the numerical quantum transfer-matrix method to an effective one-dimensional model, treating the interchain term within mean-field approximation. Finite-temperature properties are investigated for the charge ordering, the "dimer Mott" transition (bond dimerization), and the spin-Peierls transition (bond tetramerization). A coexistent state of charge order and bond dimerization exhibiting dielectricity is predicted in a certain parameter range, even when intrinsic dimerization is absent.

cond-mat.str-el↗

arXiv2006

Antiferromagnetic Ising Model on Inverse Perovskite Lattice

We study thermodynamic properties of an antiferromagnetic Ising model on the inverse perovskite lattice by using Monte Carlo simulations. The lattice structure is composed of corner-sharing octahedra and contains three-dimensional geometrical frustration in terms of magnetic interactions. The system with the nearest-neighbor interactions alone does not exhibit any phase transition, leading to a degenerate ground state with large residual entropy. The degeneracy is lifted by an external magnetic field or by an anisotropy in the interactions. Depending on the anisotropy, they stabilize either a 3D ferrimagnetic state or a partially-disordered antiferromagnetic (PDAF) state with a dimensionality reduction to 2D. By the degeneracy-lifting perturbations, all the transition temperatures of these different ordered states continuously grow from zero, leaving an unusual zero-temperature critical point at the unperturbed point. Such a zero-temperature multicriticality is not observed in other frustrated structures such as face-centered cubic and pyrochlore. The transition to the PDAF state is represented by either the first- or second-order boundaries separated by tricritical lines, whereas the PDAF phase shows 1/3 magnetization plateaus.

cond-mat.stat-mech↗

arXiv2006

Spin Singlet State in Heptamers Emerging in Spinel Oxide AlV$_2$O$_4$

We present our theoretical results on the electronic state in vanadium spinel oxide AlV$_2$O$_4$. The material is a mixed-valent system with the average valence V$^{2.5+}$, and V cations constitute a frustrated pyrochlore structure. It shows a structural transition at $\sim 700$ K, leading to the formation of seven V-sites clusters -- heptamers. We study the electronic state of the heptamer by explicitly taking account of orbital degree of freedom as well as electron correlations. We show that the ground state of the heptamer for realistic parameters becomes spin-singlet because of strong $σ$-type bonding states of $t_{2g}$ orbitals. The temperature dependence of the magnetic susceptibility in experiments is naturally understood by this singlet formation in heptamers. Our results indicate that in AlV$_2$O$_4$ orbital physics is relevant to stabilize a cluster-type singlet state instead of a previously proposed charge-ordered state with valence skipping.

cond-mat.str-el↗

arXiv2006

Symmetry considerations on the magnetization process of the Heisenberg model on the pyrochlore lattice

We present a detailed symmetry analysis of the degeneracy lifting due to higher order spin exchanges in the pyrochlore lattice in applied magnetic field. Under the assumption of the 4 sublattice ordering, the criteria for a stable half-magnetization plateau are deduced. The higher order exchange terms may originate from spin-lattice coupling, or can describe quantum and thermal fluctuations.

cond-mat.str-el↗

arXiv2006

Half-magnetization plateaux in Cr spinels

Magnetization plateaux, visible as anomalies in magnetic susceptibility at low temperatures, are one of the hallmarks of frustrated magnetism. An extremely robust half-magnetization plateau is observed in the spinel oxides CdCr2O4 and HgCr2O4, where it is accompanied by a substantial lattice distortion. We give an overview of the present state experiment for CdCr2O4 and HgCr2O4, and show how such a half-magnetization plateau arises quite naturally in a simple model of these systems, once coupling to the lattice is taken into account.

cond-mat.str-el↗

arXiv2005

A metamagnetic critical point in a three dimensional frustrated antiferromagnet

The competition between different forms of order is central to the problem of strong correlation. This is particularly true of frustrated systems, which frequently exist at or near to a zero-temperature critical point. Here we show that a state with a half-magnetization plateau but no long range order can arise when a three dimensional frustrated antiferromagnet is tuned to a critical point bordering a metamagnetic state. We use classical Monte Carlo simulation and low-temperature expansion techniques to accurately characterize this `spin pseudogap' state, and show how its properties relate to those of the critical point. Our results provide an example of three dimensional spin model which can be used to study the relationship between gap and `pseudogap', - i.e. long range and preformed local order - near a metamagnetic critical point.

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↗

arXiv2005

Monte Carlo study of half-magnetization plateau and magnetic phase diagram in pyrochlore antiferromagnetic Heisenberg model

The antiferromagnetic Heisenberg model on a pyrochlore lattice under external magnetic field is studied by classical Monte Carlo simulation. The model includes bilinear and biquadratic interactions; the latter effectively describes the coupling to lattice distortions. The magnetization process shows a half-magnetization plateau at low temperatures, accompanied with strong suppression of the magnetic susceptibility. Temperature dependence of the plateau behavior is clarified. Finite-temperature phase diagram under the magnetic field is determined. The results are compared with recent experimental results in chromium spinel oxides.

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