arXiv subjects

Takayuki Ishitobi

Publications and source records attributed to Takayuki Ishitobi.

10 recordsLinked to original sources

arXiv2026

Purely Electronic Chirality without Structural Chirality

We introduce the concept of purely electronic chirality (PEC), which arises in the absence of structural chirality. In condensed matter physics and chemistry, chirality has conventionally been understood as a mirror-image asymmetry in crystal or molecular structures. We demonstrate that certain electronic orders exhibit chirality-related properties without atomic displacement. Specifically, we investigate quadrupole orders to realize such purely electronic chirality with handedness that can be tuned by magnetic fields. As a representative example, we analyze a model featuring $120^circ$ antiferro quadrupole orders on a distorted kagomé lattice, predicting various chirality-related responses in the nonmagnetic ordered phase of URhSn. Furthermore, as a phonon analog, chiral phonons can emerge in achiral crystals through coupling with the PEC order. Our results provide a distinct origin of chirality and a fundamental basis for exploring the interplay between electronic and structural chirality.

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arXiv2025

Three-stage Phase Transitions and Field-induced Phases in CeCoSi:A Landau Theory

We investigate both the nonmagnetic and magnetic ordered phases of CeCoSi using Landau theory. Our analysis predicts three successive phase transitions at zero magnetic field. A quadrupole order parameter that emerges below $T_0=12$ K acts as a weak symmetry-breaking field on the antiferromagnetic ordering below $T_{\rm N}=9.4$ K, leading to two-stage magnetic transitions. In the higher-temperature antiferromagnetic phase within the range $T_{\rm s2}=8~{\rm K}<T<T_{\rm N}$, an out-of-plane component of the antiferromagnetic moment may or may not be present. If present, magnetic fields applied along the [100] and [110] directions induce additional magnetic phases.

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arXiv2025

Superzone gap formation induced by ferroic orders

We demonstrate that a superzone gap, typically associated with antiferroic ordering, can also emerge from ferroic orders in systems with sublattice degrees of freedom. By analyzing a $p$-orbital tight-binding model on a zigzag chain, we show that a Su--Schrieffer--Heeger-type gap is induced by ferroquadrupolar or ferromagnetic order or by applying an external magnetic field.

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arXiv2025

Inverse Single-sided Magnet

A single-sided magnet generates a magnetic field on only one side while canceling it on the opposite side, a feature that has enabled diverse applications in both fundamental science and engineering. Here, we propose the {\it inverse} single-sided magnet: a non-ferromagnetic system that selectively attracts either the north or south pole of a ferromagnet while remaining unresponsive to the opposite pole. We demonstrate that such behavior can arise in microscopic octupolar magnets. To illustrate this, we analyze two minimal models: a coplanar magnetic structure with 120-degree ordering and a collinear magnetic structure with site-dependent anisotropy. In both cases, we find that the magnetization response is nonreciprocal with respect to the sign of the applied magnetic field. Notably, in the latter model, the system exhibits strong magnetization in one field direction and negligible response in the opposite direction. This diode-like behavior for magnetic fields suggests that inverse single-sided magnets could play a pivotal role in controlling magnetic interference, with potential impact comparable to conventional single-sided magnets.

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arXiv2023

Quadrupole partial orders and triple-$q$ states on the face-centered cubic lattice

We study $Γ_3$ quadrupole orders in a face-centered cubic lattice. The $Γ_3$ quadrupole moments under cubic symmetry possess a unique cubic invariant in their free energy in the uniform ($q=0$) sector and the triple-q sector for the X points $q=(2π,0,0),(0,2π,0)$, and $(0,0,2π)$. Competition between this cubic anisotropy and anisotropic quadrupole-quadrupole interactions causes a drastic impact on the phase diagram both in the ground state and at finite temperatures. We show details about the model construction and its properties, the phase diagram, and the mechanism of the various triple-$q$ quadrupole orders reported in our preceding letter [J. Phys. Soc. Jpn. 90, 43701 (2021), arXiv:2102.06346]. By using a mean-field approach, we analyze a quadrupole exchange model that consists of a crystalline-electric field scheme with the ground-state $Γ_3$ non-Kramers doublet and the excited singlet $Γ_1$ state. We find various triple-$q$ orders in the four-sublattice mean-field approximation. A few partial orders of quadrupoles are stabilized in a wide range of parameter space at a higher transition temperature than single-$q$ orders. With lowering the temperature, these partial orders undergo phase transitions into further symmetry broken phases in which nonvanishing quadrupole moments emerge at previously disordered sites. The obtained phases in the mean-field approximation are investigated by a phenomenological Landau theory, which clearly shows that the cubic invariant plays an important role for stabilizing the triple-$q$ states. We also discuss its implications for recent experiments in a few f- and d-electron compounds.

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arXiv2023

Electronic origin of ferroic quadrupole moment under antiferroic quadrupole orders and finite magnetic moment in $J_{\rm eff}=3/2$ systems

We study the electronic origin of parasitic ferroic quadrupole moments in antiferroic quadrupole orders by extending a model studied in G. Chen et al., Phys. Rev. B 82, 174440 (2010) with the effective angular momentum $J_{\rm eff}=3/2$ quartet ground states. Taking into account the first crystalline-electric-field (CEF) excited doublet, cubic anisotropy in the quadrupole moments emerges, which leads to the induced ferroic quadrupole moments in the antiferro quadrupolar phases. The hybridization with the CEF excited quartet states also causes finite magnetic moments compatible to the observed size of the effective moment in typical $J_{\rm eff}=3/2$ systems, as opposed to the naive expectation of vanishing moments in the $J_{\rm eff}=3/2$ systems. These results suggest the importance of the corrections arising from the high-energy CEF excited states in the $J_{\rm eff}=3/2$ systems.

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arXiv2023

Triple-$\mathcal{Q}$ partial magnetic orders induced by quadrupolar interactions: Triforce order scenario for UNi$_4$B

We theoretically investigate possible symmetry-broken states in ${\rm UNi_4B}$, constructing a localized pseudo triplet crystalline-electric field model. For a long time, its low-temperature symmetry-broken phase in ${\rm UNi_4B}$ has been considered to be a magnetic toroidal order forming atomic-scale vortices lattice with disordered sites at each center of the vortices. However, recent observation of current-induced magnetizations offers a reinvestigation about the validity of this order parameter because of the contradiction in their anisotropy. Our model takes into account the quadrupole degrees of freedom, whose importance is recently evidenced by the sound-velocity softening. We find that the quadrupole moments play an important role in determining the magnetic structure in the ordered states. For a wide range of parameter space, we obtain two triple-$\mathcal{Q}$ magnetic orders in our 36-site mean-field analysis: toroidal order and another one with the same number of disordered sites as in the toroidal order. We name the latter ``triforce'' order after its magnetic structure. Importantly, the triforce order possesses exactly the same spin structure factor as the toroidal order does, while the phase factors in the superposition of the triple-$\mathcal{Q}$ structure are different. We show that the triforce order is consistent with the observed current-induced magnetization when the realistic crystal structure of ${\rm UNi_4B}$ is taken into account. We compare the predictions of the triforce order with the experimental data available at present in detail and also discuss possible applications of the present mechanism of triple-$\mathcal{Q}$ orders to anisotropic correlated systems.

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arXiv2021

Triple-$q$ quadrupole-octupole order scenario for PrV$_2$Al$_{20}$

We propose novel triple-${\bm q}$ multipole orders as possible candidates for the two distinct low-temperature symmetry broken phases in quadrupolar system PrV$_2$Al$_{20}$. An analysis of the experiment under [111] magnetic fields indicates that the {\it ferro} octupole moments in the lower temperature phase arise from the {\it antiferro} octupole interactions. We demonstrate that the triple-${\bm q}$ multipole orders can solve this seemingly inconsistent issue. Anisotropies of quadrupole moments stabilize a triple-${\bm q}$ order, which further leads to the second transition to a coexisting phase with triple-${\bm q}$ octupole moments. The cubic invariant of quadrupole moments formed by the triple-${\bm q}$ components and the characteristic couplings with the octupole moments in their free energy play important roles. We analyze a multipolar exchange model by mean-field approximation and discuss the temperature and magnetic field phase diagrams. Many of the microscopic results, such as the number of phases and the magnitudes of critical fields in the phase diagrams, are qualitatively consistent with the experiments in PrV$_2$Al$_{20}$.

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arXiv2021

Quadrupole Orders on the fcc Lattice

We theoretically study electric quadrupole orders in f-electron systems on the fcc lattice. Qudrupole degrees of freedom $O_{20}$ and $O_{22}$ originate in the non-Kramers doublet ground state $Γ_3$ of ion with $f^2$ electron configuration. For discussing quadrupole orders, we use a minimal model with isotropic ($J$) and anisotropic ($K$) nearest-neighbor interactions, and determine the phase diagram using a four-site mean-field approximation at zero and finite temperatures. Quadrupoles couple the $Γ_3$ doublet to the singlet excited state $Γ_1$, and its effects on canting antiferro orders are examined in detail. We found that this coupling leads to a rich phase diagram including two- and four-sublattice antiferro phases, and that two phases show a partial order of quadrupoles at finite temperatures.

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arXiv2019

Magnetoelectric Effects and Charge-Imbalanced Solenoids: Antiferro Quadrupole Orders in a Diamond Structure

We study magnetoelectric (magneto-current) effects in a diamond structure under antiferro quadrupole (AFQ) orders. The AFQ orders break the spacial inversion symmetry and cause the current-induced magnetization. The current-induced magnetization strongly depends on the types of the order parameters and the direction of the current. This gives a way to the experimental identification of AFQ order parameters. We also discuss the current-induced magnetization under the AFQ orders in the diamond structure can be intuitively understood in terms of charge-imbalanced solenoids.

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