arXiv subjects

Shin Miyahara

Publications and source records attributed to Shin Miyahara.

At least 19 recordsLinked to original sources

arXiv2025

Generalization of the Affleck-Kennedy-Lieb-Tasaki Model for Quantum Ferromagnetism

We study a spin-$S$ ferromagnetic model with exactly-written ground states, known as the partially-magnetized valence bond solid (VBS) states with magnetization $m=(S-1)/S$, which is a ferromagnetic generalization of the Affleck-Kennedy-Lieb-Tasaki model. We find that the VBS state and an antiferromagnetic ground state with magnetization $m=0$ are degenerate for $S=3/2$ and $S=2$ by using the Lanczos method and the density matrix renormalization group method (DMRG). However, increasing $S$, the magnetization of the ground states is uniquely determined as the fraction $m=(S-1)/S$. This is not just a ferromagnet, but a quantum ferromagnet due to quantum entanglement inherent in VBS states. In the low-energy excitation spectrum, we find the coexistence of the Haldane gap and Goldstone-like ferromagnetic magnon excitation. This ``magnetic chimera'' clearly appears under a finite magnetic field. Finally, we discuss an application to the measurement-based quantum computation and an extension of the Haldane's conjecture.

cond-mat.str-el↗

arXiv2024

Ferromagnetic Haldane state and dimer multiplet state of quantum ferromagnets

We present a theory of the realization of a ferromagnetic Haldane state in a spin-2 bilinear-biquadratic spin system on an orthogonal-dimer chain. The coexistence of a ferromagnetic state and a Haldane state is due to the rigorous correspondence between the eigenstates of a spin-2 model and a spin-1/2 Heisenberg model; i.e., "eigensystem embedding." Numerical exact-diagonalization calculations indicate that the ground state in the model is a fractionally magnetized M = 3/4 Haldane state. Moreover, a ferromagnetic-dimer multiplet state is an exact ground state on a lattice, where the direct product of dimer singlet states is the ground state in a spin-1/2 Heisenberg model that includes one-, two-, and three-dimensional orthogonal-dimer lattices. Eigensystem embedding demonstrates that a quantum ferromagnet can be obtained for an arbitrary spin S >= 2 in any dimension and for any lattice in which anomalous ground states are realized in a spin-1/2 Heisenberg model.

cond-mat.str-el↗

arXiv2023

Multiple Magnetoelectric Plateaux in Polar Magnet Fe$_2$Mo$_3$O$_8$

The magnetization and electric polarization of a polar antiferromagnet Fe$_2$Mo$_3$O$_8$ are studied up to 66 T for spin-saturation magnetic fields applied along the polar axis. The magnetization process at 1.4 K exhibited multistep structures below the saturation field of 65 T. The electric polarization along the polar axis exhibits a similar multistep behavior with a total change of 1.2 $\rmμC/cm^{2}$. A combined triangular-lattice antiferromagnetic model with strong Ising-type spin anisotropy reproduces this multistep magnetoelectric (ME) effect. The exchange striction mechanism explains the remarkable ME response in the two sub-lattice type-I multiferroic materials. These results and interpretation demonstrate a method for realizing multistage magnetoelectric effects in hybrid spin systems.

cond-mat.str-el↗

arXiv2023

Theory of Fractionally-magnetized Quantum Ferromagnet

We present a theory to realize entangled quantum spin states with fractional magnetization. The origin of magnetization reduction is partly emergent antiferromagnetism, that is, spin-liquefaction of ferromagnetism. We study a ferromagnetic bilinear coupling region of the spin-$S$ $({\geqq} 1)$ bilinear-biquadratic spin chain based on (i) a rigorous eigenstate correspondence between the spin-$S$ model and spin-$\frac12$ model and (ii) a numerical exact-diagonalization calculation up to $S=3$. As a result, we obtain a fractional magnetized $M=1-1/(2S)$ phase, where ground states have quantum entanglement-reflecting corresponding spin-$\frac12$ antiferromagnetic ground states in a ferromagnetic background. This spin-liquefaction theory of ferromagnets can be generalized to any-dimensional lattices even under a magnetic field. This fractional ferromagnetism opens the new research field of quantum ferromagnets.

cond-mat.str-el↗

arXiv2023

Theory of absorption in a frustrated spin gap system ${\rm SrCu_2(BO_3)_2}$

We achieve a comprehensive understanding of the magnetic excitations observed by electric spin resonance and far-infrared spectroscopy in a frustrated spin gap system ${\rm SrCu_2(BO_3)_2}$ by considering the effects of magnetoelectric couplings and Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland model. The transitions from the dimer singlet ground state to the triplon and the bound states of two triplons are electroactive through the magnetoelectric couplings, even in the Shastry-Sutherland Heisenberg model. The results indicate that an electro-triplon and a novel electroactive magnetic excitation, as with an electromagnon in the multiferroics, can be realized in the conventional spin-gapped singlet and anomalous spin-liquid states. Clarifying these electroactive magnetic excitations in various spin systems will help analyze a broad range of quantum magnets, {\it e.g.}, quantum spin liquids and spin nematics.

cond-mat.str-el↗

arXiv2019

Direct coupling of ferromagnetic moment and ferroelectric polarization in BiFeO$_3$

The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO$_{3}$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis (${\bf P}_{\rm t}$) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\bf P}_{\rm t}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

cond-mat.str-el↗

arXiv2018

Fractionally quantized Berry phases of magnetization plateaux in spin-$1/2$ Heisenberg multimer chains

We study the fractionally quantized $Z_N$ Berry phase, $γ_N=0, {2π\over N}, {4π\over N},\ldots, {2(N-1)π\over N}$, to characterize local $N$-mer spin structures at magnetization plateaux in spin-1/2 Heisenberg multimer ($N$-mer) models, i.e., highly frustrated $N$-leg ladder models, which are generalizations of an orthogonal dimer chain and have exact ground states in the strong multimer coupling region. We demonstrate that all $N$ types of Berry phases, which characterize magnetization-plateau phases, appear in a magnetic phase diagram when $N=2$ and $4$. We show that magnetization plateau with magnetization $\langle m\rangle$ and $D$-fold degenerated states has $γ_N=π(\langle m\rangle-1) D$, except for the Haldane phase with $γ_N=0$. In addition, we find that a complementary $Z_N$ Berry phase becomes non-zero in the $S=N/2$ Haldane phase for $N=2$ and 4. Because the exact quantization of the $Z_N$ Berry phases is protected by the translational (or rotational) symmetry along the rung direction, the $Z_N$ Berry phase has the potential to be applied for a wide class of magnetization plateaux in coupled multimer systems.

cond-mat.str-el↗

arXiv2017

Successive field-induced transitions in BiFeO$_{3}$ around room temperature

The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO$_3$ were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of $2\times 10^{-5}$, during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a large magnetoelectric effect was observed. Neutron diffraction measurements clarified that incommensurate spin modulation along [110] direction in the cycloidal phase becomes commensurate in the intermediate phase. Theoretical calculations based on the standard spin Hamiltonian of this material suggest an antiferromagnetic cone-type spin order in the intermediate phase.

cond-mat.mtrl-sci↗

arXiv2013

The Spin State and Spectroscopic Modes of Multiferroic BiFeO3

Spectroscopic modes provide the most sensitive probe of the very weak interactions responsible for the properties of the long-wavelength cycloid in the multiferroic phase of \BF below $\TN \approx 640$ K. Three of the four modes measured by THz and Raman spectroscopies were recently identified using a simple microscopic model. While a Dzyaloshinskii-Moriya (DM) interaction $D$ along $[-1,2,-1]$ induces the cycloid with wavevector $(2π/a)(0.5+δ, 0.5, 0.5-δ)$ ($δ\approx 0.0045$), easy-axis anisotropy $K$ along the $[1,1,1]$ direction of the electric polarization ${\bf P}$ induces higher harmonics of the cycloid, which split the $Ψ_1$ modes at 2.49 and 2.67 meV and activate the $Φ_2$ mode at 3.38 meV. However, that model could not explain the observed low-frequency mode at about 2.17 meV. We now demonstrate that an additional DM interaction $D'$ along $[1,1,1]$ not only produces the observed weak ferromagnetic moment of the high-field phase above 18 T but also activates the spectroscopic matrix elements of the nearly-degenerate, low-frequency $Ψ_0$ and $Φ_1$ modes, although their scattering intensities remain extremely weak. Even in the absence of easy-axis anisotropy, $D'$ produces cycloidal harmonics that split $Ψ_1 $ and activate $Φ_2$. However, the observed mode frequencies and selection rules require that both $D'$ and $K$ are nonzero. This work also resolves an earlier disagreement between spectroscopic and inelastic neutron-scattering measurements.

cond-mat.mtrl-sci↗

arXiv2012

Identifying the Magnetoelectric Modes of Multiferroic BiFeO$_3$

We have identified three of the four magnetoelectric modes of multiferroic BiFeO$_3$ measured using THz spectroscopy. Excellent agreement with the observed peaks is obtained by including the effects of easy-axis anisotropy along the direction of the electric polarization. By distorting the cycloidal spin state, anisotropy splits the $Ψ_{\pm 1}$ mode into peaks at 20 and 21.5 cm$^{-1}$ and activates the lower $Φ_{\pm 2}$ mode at 27 cm$^{-1}$ (T=200 K). An electromagnon is identified with the upper $Ψ_{\pm 1}$ mode at 21.5 cm$^{-1}$. Our results also explain recent Raman and inelastic neutron-scattering measurements.

cond-mat.str-el↗

arXiv2012

Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets

We investigate a dynamical magnetoelectric effect due to a magnetic resonance in helical spin structures through the coupling between magnetization and electric polarization via a spin current mechanism. We show that the magnon has both the dynamical magnetic moment $ΔM^ω$ and the electric moment $ΔP^ω$ ($\perp ΔM^ω$), i.e., a dynamical toroidal moment, under external magnetic fields, and thus it is named the {\em toroidalmagnon}. The toroidalmagnon exists in most conical spin structures owing to the generality of the spin current mechanism. In the absorption of electromagnetic waves, the toroidalmagnon excitation process generally induces a nonreciprocal directional dichroism as a consequence of an interference of the magnetic and electric responses.

cond-mat.str-el↗

arXiv2011

Chirality of Matter Shows Up via Spin Excitations

Right- and left-handed circularly polarized light interact differently with electronic charges in chiral materials. This asymmetry generates the natural circular dichroism and gyrotropy, also known as the optical activity. Here we demonstrate that optical activity is not a privilege of the electronic charge excitations but it can also emerge for the spin excitations in magnetic matter. The square-lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$ offers an ideal arena to test this idea, since it can be transformed to a chiral form by application of external magnetic fields. As a direct proof of the field-induced chiral state, we observed large optical activity when the light is in resonance with spin excitations at sub-terahertz frequencies. In addition, we found that the magnetochiral effect, the absorption difference for the light beams propagating parallel and anti-parallel to the applied magnetic field, has an exceptionally large amplitude close to 100%. All these features are ascribed to the magnetoelectric nature of spin excitations as they interact both with the electric and magnetic components of light.

cond-mat.str-el↗

arXiv2011

Theory of magnetoelectric resonance in two-dimensional $S=3/2$ antiferromagnet ${\rm Ba_2CoGe_2O_7}$ via spin-dependent metal-ligand hybridization mechanism

We investigate magnetic excitations in an $S=3/2$ Heisenberg model representing two-dimensional antiferromagnet ${\rm Ba_2CoGe_2O_7}$. In terahertz absorption experiment of the compound, Goldstone mode as well as novel magnetic excitations, conventional magnetic resonance at 2 meV and both electric- and magnetic-active excitation at 4 meV, have been observed. By introducing a hard uniaxial anisotropy term $Λ(S^z)^2$, three modes can be explained naturally. We also indicate that, via the spin-dependent metal-ligand hybridization mechanism, the 4 meV excitation is an electric-active mode through the coupling between spin and electric-dipole. Moreover, at 4 meV excitation, an interference between magnetic and electric responses emerges as a cross correlated effect. Such cross correlation effects explain the non-reciprocal linear directional dichroism observed in ${\rm Ba_2CoGe_2O_7}$.

cond-mat.str-el↗

arXiv2007

Ising phases of Heisenberg ladders in a magnetic field

We examine the influence of weak anisotropic interactions on the T=0 phase diagram of the frustrated two-leg Heisenberg ladder, a well-studied spin model exhibiting integer and fractional magnetization plateaux separated by gapless incommensurate states. We find that the Dzyaloshinskii--Moriya coupling may substantially modify the phase diagram so that the half-integer plateau and the surrounding gapless phases merge into a single Ising-ordered phase breaking the translational symmetry of the lattice. A different Ising order is found for a weakly frustrated ladder. Implications for experimental ladder and dimer systems are discussed.

cond-mat.str-el↗

arXiv2005

Flat-Bands on Partial Line Graphs -- Systematic Method for Generating Flat-Band Lattice Structures

We introduce a systematic method for constructing a class of lattice structures that we call ``partial line graphs''.In tight-binding models on partial line graphs, energy bands with flat energy dispersions emerge.This method can be applied to two- and three-dimensional systems. We show examples of partial line graphs of square and cubic lattices. The method is useful in providing a guideline for synthesizing materials with flat energy bands, since the tight-binding models on the partial line graphs provide us a large room for modification, maintaining the flat energy dispersions.

cond-mat.str-el↗

arXiv2005

Non-Degenerate Ground State in the Antiferromagnetic Double-Exchange Model on a Triangular Lattice

In order to study effects of frustration in an itinerant electron system, we investigate ground states of the antiferromagnetic double-exchange model on a triangular lattice. In this model, pseudo-spins are coupled to electron transfer integrals in such a way that antiparallel configurations of pseudo-spins gain kinetic energies. Although the antiferromagnetic Ising model on a triangular lattice shows macroscopic dengenerate ground states, the present model shows that the degeneracy is lifted due to long-range natures of the double exchange interactions. Spin ordering at the ground state is also discussed.

cond-mat.str-el↗

arXiv2005

Frustration-induced Dodecamer Ordering in the Double-Exchange Spin Ice Model on the Kagomé Lattice

We investigate a detail of a dodecamer cluster ordering in a double-exchange spin ice model on a kagomé lattice. In frustrated systems, ordinary spin orderings are suppressed and macroscopic degeneracy remains down to low temperatures. In some frustrated systems, the degeneracy is lifted due to residual interactions and cluster orderings are stabilized. In the present model, the spin ice state is first formed at intermediate temperatures, and further entropies are released at lower temperatures as the dodecamer phase emerges. Since the spin symmetry is not broken in the dodecamer phase, there still exists macroscopic degeneracy. At further low temperatures, a possible spin ordering due to inter-dodecamer interactions is proposed. We discuss that such a multiple-site clustering larger than a bond-pair might be generic to frustrated systems where macroscopic degeneracy is lifted by residual interactions.

cond-mat.str-el↗

arXiv2005

Mechanism for lifting the degeneracy in the double-exchange spin ice model on a kagomé lattice: Dodecamer formation

We investigated the double-exchange spin ice model on a kagomé lattice by Monte Carlo simulation in order to study a mechanism for lifting the degeneracy in frustrated electron systems. We show specific heat and vector spin chirality data on a finite lattice. Specific heat has a double-peak structure: A broad peak and a sharp peak are at $k_{\rm B}T/t \sim 0.15$ and 0.015, respectively, where $t$ is the transfer integral of electrons. The broad peak corresponds to a crossover to the spin ice-like state, on the other hand, the sharp one a transition to a dodecagonal spin cluster (dodecamer) state. We discuss the interplay between the formation of the dodecamer state and the lifting of the macroscopic degeneracy.

cond-mat.str-el↗