arXiv subjects

Tomonari Mizoguchi

Publications and source records attributed to Tomonari Mizoguchi.

At least 19 recordsLinked to original sources

arXiv2026

Orbital-Zeeman cross correlation in $p$- and $d$-wave altermagnets

Altermagnets are a novel class of magnets that exhibit a large spin splitting but the total magnetic moment is vanishing. This unconventional spin splitting gives rise to various characteristic phenomena, such as spin current generation. In this paper, we study the orbital-Zeeman (OZ) cross term in altermagnets. Specifically, we consider the Rashba metal and the surface Dirac cones of three-dimensional topological insulators (TIs) in the presence of the altermagnetic order parameters. For the Rashba metals, the $p$-wave order parameter exerts only a limited influence on the OZ term, whereas the $d$-wave one causes the sign change of it when the order parameter becomes sufficiently large. For the TI surface, the $p$-wave order parameter retains the step-function-type dependence of the OZ term as a function of the chemical potential ($μ$) associated with the jump at $μ=0$, observed in the TI surface without magnetism, but its magnitude is reduced. For the $d$-wave case, the magnitude of jump at $μ=0$ is preserved but the OZ term decreases as increasing $|μ|$.

cond-mat.mes-hall↗

arXiv2025

Topological two-body interaction obstructing trivial ground states: an indicator of fractional Chern insulators

The search for candidate materials for fractional Chern insulators (FCIs) has mainly focused on the topological and geometrical structures of single-particle Chern bands. However, there are inherent limitations in approaches that neglect interaction effects, highlighting the need for complementary methods. In this work, we discuss how the Chern number defined for the effective interaction projected onto a Chern band is related to the stabilization of FCIs. Specifically, by formulating both the effective interaction and the two-particle problem using a common matrix, we establish a connection between the two-particle band structure and the effective interaction. This formulation allows us to characterize the effective interaction through the topology of the two-particle band. To investigate the relationship between topological effective interactions and FCIs, we perform numerical calculations primarily based on exact diagonalization. We find a notable correlation between the fact that the dominant two-particle bands carry a unit Chern number and the realization of a robust FCI at the filling fraction $ν= 1/3$. This result is consistent with the presumed correspondence between pseudopotentials in the fractional quantum Hall effect and the two-particle band structure. From another perspective, our findings suggest that the topology inherent in the interaction itself can obstruct trivial ground states. We also discuss this in the context of scattering channels. Extending such topological two-body interactions could pave the way for realizing exotic states beyond FCIs.

cond-mat.str-el↗

arXiv2025

Emergent Shastry-Sutherland network from square-kagome Heisenberg antiferromagnet with trimerization

We study the $S=1/2$ square-kagome lattice Heisenberg antiferromagnet with the trimarized modulation. In the trimerized limit, each trimer hosts the four-fold degenearte ground states characterized by the spin and chirality degrees of freedom. We find that, within the first-order perturbation theory with respect to the inter-trimer coupling, the effective Hamiltonian is the Kugel-Khomskii-type model on a Shastry-Sutherland lattice. Based on a mean-field decoupling, we propose a dimer-covering ansatz for the effective Hamiltonian; however, the validity of these states in the low-energy sector remains an open question.

cond-mat.str-el↗

arXiv2025

Quasi-periodic flat-band model constructed by molecular-orbital representation

We construct a tight-binding model that hosts both a quasi-periodic nature and marcoscopically-dengenerate zero-energy modes. The model can be regarded as a counterpart of the Aubry-André-Harper (AAH) model, which is a paradigmatic example of the quasi-periodic tight-binding model. Our main focus is on the many-body state where the flat-band-like degenerate zero-energy modes are fully occupied. We find a characteristic sublattice dependence of the particle density distribution. Further, by analyzing the hyperuniformity of the particle density distribution, we find that it belongs to the class-I hyperuniform distribution, regardless of the model parameter. We also show that, upon changing the parameter, the finite-energy modes exhibit the same extended-to-localized transition as that for the original AAH model.

cond-mat.mes-hall↗

arXiv2025

Topological pump and its plateau transitions of $N$-leg spin ladder

A topological pump on an $N\textrm{-}$leg spin ladder is discussed by introducing spatial clusterization whose adiabatic limit is a set of $2N\textrm{-}$site staircase clusters. We set a pump path in the parameter space that connects two different symmetry protected topological phases. By introducing a symmetry breaking staggered magnetic field, the system is always gapped during the pump. In the topological pump {thus obtained}, the bulk Chern number is given by the number of the critical points enclosed by the pump path. Plateau transitions characterized by the Chern number are demonstrated associated with deformation of the pump path. We find that there are $N$ critical points enclosed by the pump path for the $N\textrm{-}$leg ladder. The ground state phase diagram without symmetry breaking terms is numerically investigated by using the quantized Berry phase. We also discuss the physical picture of edge states in the diagonal boundary, and numerically demonstrate the bulk-edge correspondence for $N=2,3$ cases.

cond-mat.stat-mech↗

arXiv2025

Orbital Magnetism in Honeycomb Ladder

We investigate the orbital magnetic susceptibility of the tight-binding model for the honeycomb ladder with the additional vertical hopping. Despite being one-dimensional, the magnetic flux penetrating the hexagonal rings affects the energetics of electrons, resulting in the finite orbital magnetic response. We find that the orbital magnetic susceptibility is sensitive to the parameters as well as the chemical potential. At half-filling, the response is diamagnetic when the system is close to the pure honeycomb ladder, whereas it turns to paramagnetic when it is close to the two-leg ladder. We also find several characteristic properties away from half-filling, such as the diamagnetic response at the band top and bottom, and the large paramagnetic response at the band gap sandwiched by the divergent density of states.

cond-mat.mes-hall↗

arXiv2024

Orbital magnetic susceptibility of type-I, II, and III massless Dirac fermions in two dimensions

We study the orbital magnetic susceptibility of tilted massless Dirac fermions in two dimensions. It is well-known that the type-I massless Dirac fermions exhibit divergingly-large diamagnetic susceptibility, whereas less is known about the types II and III cases. We first clarify that the orbital magnetic susceptibility is vanishing for the types II and III in the continuum model. We then compare the three types of Dirac fermions for the lattice models. We employ three tight-binding models with different numbers of Dirac points, all of which are two-band models defined on a square lattice. For all three models, we find that the type-I Dirac fermions show the divergingly-large orbital diamagnetic susceptibility, whereas the type-II Dirac fermions exhibit non-diverging paramagnetic susceptibility. The type-III Dirac fermions exhibit diamagnetism but its susceptibility is small compared with the type-I case.

cond-mat.mes-hall↗

arXiv2024

Evolution of flat bands in two-dimensional fused pentagon network

Theoretical quest of flat-band tight-binding models usually relies on lattice structures on which electrons reside. Typical examples of candidate lattice structures include the Lieb-type lattices and the line graphs. Meanwhile, there can be accidental flat-band systems that belong to neither of such typical classes and deriving flat-band energies and wave functions for such systems is not straightforward. In this work, we investigate the characteristic band structure for the tight-binding model on a network composed of pentagonal rings, which is inspired by the theoretically-predicted carbon-based material. Although the lattice does not belong to conventional classes of flat band models, the exact flat bands appear only for fine-tuned parameters. We analytically derive the exact eigenenergies and eigenstates of the flat bands. By using the analytic form of the Bloch wave function, we construct the corresponding Wannier function and reveal its characteristic real-space profile. We also find that, even away from the exact flat-band limits, the nearly flat band exists near the Fermi level for the half-filled systems, which indicates that the present system will be a suitable platform for questing flat-band-induced correlated electron physics if it is realized in the real material.

cond-mat.mtrl-sci↗

arXiv2024

Hidden chiral symmetry for kagome lattice and its analogs

Chiral symmetry plays an essential role in condensed matter physics. In tight-binding models, it is often attributed to bipartite lattice structures, and its typical consequence is the ``particle-hole symmetric" band structures, that is, the positive and negative eigenenergies appear in a pairwise manner. In this work, we address the chiral symmetry for non-bipartite lattice models with flat bands. Our argument relies on what we call the molecular-orbital representation, by which we can guarantee the existence of the flat band. We show that the chiral symmetry is preserved for the non-flat bands when the molecular orbitals are normalized and divided into two sets within which they are non-overlapping. The chiral operator is constructed by the same molecular orbitals as the Hamiltonian. This results in the characteristic relation between the chiral operator and the Hamiltonian, which we call the Pythagoras relation. We present the examples of such models, i.e., the kagome lattice and its one-dimensional analog, and address the characteristics originating from the chiral symmetry, such as the emergence of the topological edge modes. We also show an example where the numbers of molecular orbitals in each set are different from each other, resulting in multiple flat bands with different energies.

cond-mat.mes-hall↗

arXiv2024

On equivalence of two formulas of orbital magnetic susceptibility for tight-binding models

We prove that two formulas of the orbital magnetic susceptibility, namely, Koshino-Ando's formula and Gómez-Santos and Stauber's formula, are equivalent for the tight-binding models. The difference between these two formulas can be written in a form containing the surface terms arising from the momentum-space integration, and we show that the surface terms are exactly vanishing although the primitive functions are seemingly non-periodic with respect to the translation by the reciprocal lattice vector.

cond-mat.mes-hall↗

arXiv2023

Blocking particle dynamics in diamond chain with spatially increasing flux

Spatial non-uniformity in tight-binding models serves as a source of rich phenomena. In this paper, we study a diamond-chain tight-binding model with a spatially-modulated magnetic flux at each plaquette. In the numerical studies with various combinations of the minimum and maximum flux values, we find the characteristic dynamics of a particle, namely, a particle slows down when approaching the plaquette with $π$-flux. This originates from the fact that the sharply localized eigenstates exist around the $π$-flux plaquette. These localized modes can be understood from a squared model of the original one. This characteristic blocked dynamics will be observed in photonic waveguides or cold atoms.

cond-mat.mes-hall↗

arXiv2023

Unconventional gapless semiconductor in an extended martini lattice in covalent honeycomb materials

We study characteristic electronic structures in an extended martini lattice model and propose its materialization in $π$-electron networks constructed by designated chemisorption on graphene and silicene. By investigating the minimal tight-binding model, we reveal rich electronic structures tuned by the ratio of hopping parameters, ranging from the band insulator to the unconventional gapless semiconductor. Remarkably, the unconventional gapless semiconductor is characterized by a flat band at the Fermi level. Further, the density functional theory calculations for candidate materials reveal that the characteristic electronic structures can be realized by designated chemisorption or chemical substitution on graphene and silicene, and that the electronic structure near the Fermi level is tunable by the choice of the atomic species of adsorbed atoms. Our results open the way to search exotic electronic structures and their functionalities induced by an extended martini lattice.

cond-mat.mtrl-sci↗

arXiv2023

Molecular-orbital representation with random U(1) variables

We propose random tight-binding models that host macroscopically degenerate zero energy modes and belong to the unitary class. Specifically, we employ the molecular-orbital representation, where a Hamiltonian is constructed by a set of non-orthogonal orbitals composed of linear combinations of atomic orbitals. By setting the coefficients appearing in molecular orbitals to be random U(1) variables, we can make the models belong to the unitary class. We find two characteristic behaviors that are distinct from the random-real-valued molecular-orbital model. Firstly, a finite energy gap opens on top of the degenerate zero energy modes. Secondly, besides the zero energy modes, we also argue that the band center of the finite energy modes is critical, which is inherited from the dual counterpart, namely, the random-phase model on a bipartite lattice. Furthermore, as a by-product of this model-construction scheme, we also construct the random tight-binding model on a composite lattice, where we also find a realization of critical states.

cond-mat.mes-hall↗

arXiv2023

Higher-Order Topological Insulator on a Martini Lattice and Its Square Root Descendant

Notion of square-root topological insulators have been recently generalized to higher-order topological insulators. In two-dimensional square-root higher-order topological insulators, emergence of in-gap corner states are inherited from the squared Hamiltonian which hosts higher-order topology. In this paper, we propose that the martini lattice model serves as a concrete example of higher-order topological insulators. Furthermore, we also propose a suquare-root higher-order topological insulator based on the martini model. Specifically, we propose that the honeycomb lattice model with two-site decoration, whose squared Hamiltonian consists of two martini lattice models, realizes square-root higher-order topological insulators. We show, for both of these two models, that in-gap corner states appear at finite energies and that they are portected by non-trivial bulk $\mathbb{Z}_3$ topological invariant.

cond-mat.mes-hall↗

arXiv2022

Relationship between two-particle topology and fractional Chern insulator

Lattice generalizations of fractional quantum Hall (FQH) systems, called fractional Chern insulators (FCIs), have been extensively investigated in strongly correlated systems. Despite many efforts, previous studies have not revealed all of the guiding principles for the FCI search. In this paper, we investigate a relationship between the topological band structure in the two-particle problem and the FCI ground states in the many-body problem. We first formulate the two-particle problem of a bosonic on-site interaction projected onto the lowest band of a given tight-binding Hamiltonian. We introduce a reduced Hamiltonian whose eigenvalues correspond to the two-particle bound-state energies. By using the reduced Hamiltonian, we define the two-particle Chern number and numerically check the bulk-boundary correspondence that is predicted by the two-particle Chern number. We then propose that a nontrivial two-particle Chern number of dominant bands roughly indicates the presence of bosonic FCI ground states at filling factor $ν=1/2$. We numerically investigate this relationship in several tight-binding models with Chern bands and find that it holds well in most of the cases, albeit two-band models being exceptions. Although the two-particle topology is neither a necessary nor a sufficient condition for the FCI state as other indicators in previous studies, our numerical results indicate that the two-particle topology characterizes the degree of similarity to the FQH systems.

cond-mat.str-el↗

arXiv2022

Thermoelectric transport of type-I, II, and III massless Dirac fermions in two-dimensional lattice model

We study longitudinal electric and thermoelectric transport coefficients of Dirac fermions on a simple lattice model where tuning of a single parameter enables us to change the type of Dirac cones from type-I to type-II. We pay particular attention to the behavior of the critical situation, i.e., the type-III Dirac cone. We find that the transport coefficients of the type-III Dirac fermions behave as the limiting case of neither the type-I nor type-II. On the one hand, the qualitative behaviors of the type-III case are similar to those of the type-I case. On the other hand, the transport coefficients do not change monotonically upon increasing the tilting; namely, the largest thermoelectric response is obtained not for the type-III case but for the optimally tilted type-I case. For the optimal case, the sizable transport coefficients are obtained; for example, the dimensionless figure of merit is 0.18.

cond-mat.mtrl-sci↗

arXiv2022

Machine Learning Study on the Flat-Band States Constructed by Molecular-Orbital Representation with Randomness

We study the characteristic probability density distribution of random flat band models by machine learning. The models considered here are constructed on the basis of the molecular-orbital representation, which guarantees the existence of the macroscopically degenerate zero-energy modes even in the presence of randomness. We find that flat band states are successfully distinguished from conventional extended and localized states, indicating the characteristic feature of the flat band states. We also find that the flat band states can be detected when the target data are defined in the different lattice from the training data, which implies the universal feature of the flat band states constructed by the molecular-orbital representation.

cond-mat.mes-hall↗

arXiv2022

Construction of interacting flat-band models by molecular-orbital representation: Correlation functions, energy gap, and entanglement

We calculate correlation functions of exactly-solvable one-dimensional flat-band models by utilizing the "molecular-orbital" representation. The models considered in this paper have a gapped ground state with flat-band being fully occupied, even in the presence of the interaction. In this class of models, the space spanned by the "molecular-orbitals" is the co-space of that spanned by the flat bands. Thanks to this property, the correlation functions are calculated by using the information of the molecular-orbitals rather than the explicit forms of the flat-band wave functions, which simplifies the calculations. As a demonstration, several one-dimensional models and their correlation functions are presented. We also calculate the entanglement entropy by using the correlation function.

cond-mat.str-el↗