arXiv subjects

Shoji Yamamoto

Publications and source records attributed to Shoji Yamamoto.

At least 19 recordsLinked to original sources

arXiv2026

Superconfined Antiferromagnons on the Two-Dimensional Penrose Lattice

We find novel confined states in the spin-$S$ nearest-neighbor antiferromagnetic Heisenberg model on the two-dimensional Penrose lattice. Linear spin waves have massively degenerate eigenstates strictly confined to tricoordinated sites. They contrast with the well-known itinerant analogs in the tight-binding model, where electrons are confined but extended to both tricoordinated and pentacoordinated sites. It is the site potentials in the spin-wave Hamiltonian, originating from Coulomb interactions between electrons, that confine spin waves to minimally coordinated sites only. Confined states in the tight-binding Hamiltonian consist of six types of building blocks, whereas those in the spin-wave Hamiltonian consist of only four of them. Confined spin waves are robust against $1/S$ corrections. Emergent $O(S^{0})$ interactions further confine -- superconfine -- spin waves into two separate groups within tricoordinated sites.

cond-mat.str-el↗

arXiv2026

Raman Characterization of Two-Dimensional Quasiperiodic Antiferromagnets on Various Lattices: Spin-Orbit Mechanism

We study first-order (single-magnon) inelastic light scatterings in spin-$\frac{1}{2}$ two-dimensional quasiperiodic antiferromagnets in comparison with those emergent on periodic lattices. Unlike second-order (two-magnon) Raman scatterings based on an exchange interaction between neighboring spins, the present observations involve an indirect electric-dipole coupling which proceeds through a spin-orbit interaction. We discuss the nearest-neighbor antiferromagnetic XXZ Hamiltonian on various quasiperiodic and periodic bipartite lattices. The first-order spectra, of our present interest, consist only of rotation-invariant and mirror-symmetric magnons, while the second-order ones cannot select any particular magnon. With the exchange anisotropy moving away from the Ising limit toward the Heisenberg isotropic point, every initial delta-function peak bifurcates or divides into more in each individual manner on quasiperiodic lattices, while it remains singly peaked all the way on periodic lattices. Such splittings depend on how many types of isocoordinated sites for each coordination number and relative positions between those of the same type. A perpendicular-space representation of the first-order Raman spectrum serves as a fingerprint of each quasiperiodic tiling.

cond-mat.str-el↗

arXiv2025

Breakdown of the conventional spin-wave dynamics and its double-constraint modification in the spin-$\mathbf{\frac{1}{2}}$ triangular-prism Heisenberg antiferromagnet

Spontaneous magnon decays in an $S=\frac{1}{2}$ Heisenberg antiferromagnet on the equilateral triangular prism are investigated in terms of modified magnon Green's functions. In one dimension, the so-called infrared divergence prevents us from calculating any -- whether static or dynamic -- structure factor within the conventional spin-wave theory even at zero temperature. The well-known modified spin-wave theory initiated by Takahashi completely fails to treat anharmonicities to cause transverse-to-longitudinal coupling which are quite characteristic of noncollinear antiferromagnets. We propose imposing a double-constraint condition on spin waves to solve all these difficulties and get a full view of the nonlinear spin-wave dynamics in one-dimensional frustrated noncollinear antiferromagnets. We reveal a novel instability of the single-particle spectrum in the absence of any well-defined magnetically ordered ground state.

cond-mat.str-el↗

arXiv2024

Magnon Confinement on the Two-Dimensional Penrose Lattice: Perpendicular-Space Analysis of the Dynamic Structure Factor

Employing the spin-wave formalism within and beyond the harmonic-oscillator approximation, we study the dynamic structure factors of spin-$\frac{1}{2}$ nearest-neighbor quantum Heisenberg antiferromagnets on two-dimensional quasiperiodic lattices with particular emphasis on a magnetic analog to the well-known confined states of a hopping Hamiltonian for independent electrons on a two-dimensional Penrose lattice. We present comprehensive calculations on the $\mathbf{C}_{5\mathrm{v}}$ Penrose tiling in comparison with the $\mathbf{C}_{8\mathrm{v}}$ Ammann-Beenker tiling, revealing their decagonal and octagonal antiferromagnetic microstructures. Their dynamic spin structure factors both exhibit linear soft modes emergent at magnetic Bragg wavevectors and have nearly or fairly flat scattering bands, signifying magnetic excitations localized in some way, at several different energies in a self-similar manner. In particular, the lowest-lying highly flat mode is distinctive of the Penrose lattice, which is mediated by its unique antiferromagnons confined within tricoordinated sites only, unlike their itinerant electron counterparts involving pentacoordinated as well as tricoordinated sites. Bringing harmonic antiferromagnons into higher-order quantum interaction splits the lowest-lying nearly flat scattering band in two, each mediated by further confined antiferromagnons, which is fully demonstrated and throughly visualized in the perpendicular as well as real spaces. We disclose superconfined antiferromagnons on the two-dimensional Penrose lattice.

cond-mat.str-el↗

arXiv2023

Thermal features of Heisenberg antiferromagnets on edge- versus corner-sharing triangular-based lattices: A message from spin waves

We construct modified spin-wave thermodynamics for frustrated noncollinear antiferromagnets for the first time. The well-known modified spin-wave theory for collinear antiferromagnets diagonalizes a bosonic Hamiltonian subject to the constraint that the total staggered magnetization be zero. Applying this scheme as it is to frustrated noncollinear antiferromagnets ends in a poor thermodynamics, missing the optimal ground state and breaking the local U(1) rotational symmetry. We find a new double-constraint modification scheme to overcome this difficulty, which is tuned especially to frustrated spiral magnets but spontaneously goes back to the standard single-constraint condition at the onset of a collinear Néel-ordered classical ground state. We apply this new scheme to triangular-based polyhedral and planar antiferromagnets with particular interest in a possible contrast between edge- versus corner-sharing geometries. Under such circumstances that very few methods are available to calculate finite-temperature properties of frustrated noncollinear quantum magnets in the thermodynamic limit, our newly developed modified spin-wave theory predicts that the specific heat of the kagome-lattice Heisenberg antiferromagnet in the corner-sharing geometry remains having both mid-temperature broad maximum and low-temperature narrow peak in the thermodynamic limit, while the specific heat of the triangular-lattice Heisenberg antiferromagnet in the edge-sharing geometry retains a low-temperature sharp peak followed by a mid-temperature weak anormaly in the thermodynamic limit.

cond-mat.stat-mech↗

arXiv2022

Topological Characterization of Kitaev Spin Nanoribbons with Ordered Flux Configurations

We demonstrate topological characterization of $S=\frac{1}{2}$ Kitaev quantum spin liquids on a series of one-dimensional honeycomb nanoribbon lattices with zigzag and armchair terminated edges. We draw their Majorana spinon phase diagrams with varying nearest-neighbor exchange couplings not only at the sector of the ground flux configuration but also at some sectors of excited flux configurations. In the ground states of the zigzag and armchair nanoribbons, there occur a single and multiple phase transitions, respectively, the former and latter of which are insensitive and subject to the background flux configuration, respectively. Topological phases each have a winding number as their invariant. On each phase boundary, the Majorana spinon dispersion relation reflects both of the change in the winding number and the background flux configuration.

cond-mat.str-el↗

arXiv2022

Polarized Raman Response of Two-Dimensional Quasiperiodic Antiferromagnets: Configuration-Interaction versus Green's Function Approaches

We study Raman response of Heisenberg antiferromagnets on the $\mathbf{C}_\mathrm{5v}$ Penrose and $\mathbf{C}_\mathrm{8v}$ Ammann-Beenker lattices within and beyond the Loudon-Fleury second-order perturbation scheme intending to explore optical features peculiar to quasiperiodic magnets. Within the Loudon-Fleury mechanism, we find one and only Raman-active mode of $\mathrm{E}_2$ symmetry without any dependence on linear incident and scattered polarizations. Beyond the Loudon-Fleury mechanism, two more symmetry species $\mathrm{A}_1$ and $\mathrm{A}_2$ are activated via dynamic ring exchange and chiral spin fluctuations, respectively, which can be extracted by the use of circular as well as linear polarizations. We employ Green's functions on one hand and configuration-interaction wavefunctions on the other hand to calculate the multimagnon contributions to inelastic light scatterings. Demonstrating the great advantage of the configuration-interaction scheme, we reveal that a major portion of the Shastry-Shraiman fourth-order Raman intensity is mediated by multimagnon fluctuations.

cond-mat.str-el↗

arXiv2020

Photoinduced Bidirectional Magnetism against Monodirectional Electronics in Square-Antiprismatic Octacyanometalates

Irradiating ${\rm Cu}_2{\rm Mo}({\rm CN})_8\cdot 8{\rm H}_2{\rm O}$ with blue light induces a global magnetization, whereas succeeding irradiations with red or longer-wavelength light demagnetize this material. We solve the time-dependent Schrödinger equation for an extended Hubbard model to reproduce the photoreversible magnetism. Monitoring the photoinduced optical-conductivity and angle-resolved-photoemission spectra, we reveal that the magnetic round trip by way of ferromagnetism is far from a return in terms of electronics. While visible-light-induced magnetization has never been observed in the tungsten analog ${\rm Cu}_2{\rm W}({\rm CN})_8\cdot 5{\rm H}_2{\rm O}$, infrared-light irradiation may magnetize this material as well.

cond-mat.str-el↗

arXiv2020

Projective-symmetry-group analysis of inelastic light scattering in Kitaev spin balls

Projective symmetry groups are applied to Raman observations of the Kitaev quantum spin liquids in spherical lattice geometries realized by Platonic and Archimedean polyhedra. Parton single excitations in Kitaev spin polyhedra are characterized by double-valued irreducible representations of their belonging projective symmetry groups, whereas parton geminate excitations relevant to Raman scattering are decomposed into single-valued irreducible representations of the corresponding point symmetry groups. We combine a standard point-symmetry-group analysis of the Loudon-Fleury vertices and an elaborate projective-symmetry-group analysis of itinerant spinons against the ground gauge fields to reveal $hidden$ $selection$ $rules$ for Raman scattering in $\mathbb{Z}_2$ spin liquids.

cond-mat.str-el↗

arXiv2020

Raman Scattering Polarization and Single Spinon Identification in Two-Dimensional Kitaev Quantum Spin Liquids

Perfect deporalization of the Loudon-Fleury inelastic visible-light scattering in the Kitaev honeycomb model is well known. Though it happens in Heisenberg Kagome and triangular antiferromagnets as well, yet we prove it to be of geometric origin rather than peculiar to quantum spin liquids. A Kitaev spin liquid in the square planar geometry indeed exhibits polarized Raman spectra containing defferent symmetry species, each brought by symmetry-compatible spinon geminate excitations, i.e. arising from symmetry-compatible direct-product representations made of double-valued irreducible representations of mediating-spinon-belonging gauged $\bm{k}$-point symmetry groups. We combine a standard point-symmetry-group analysis of the Raman vertex in the real space and an elaborate projective-symmetry-group analysis of Raman-scattering-mediating Majorana spinons in the reciprocal space to identify emergent spinons singly.

cond-mat.str-el↗

arXiv2020

Optical observation of quasiperiodic Heisenberg antiferromagnets in two dimensions

We calculate magnetic Raman spectra of Heisenberg antiferromagnets on the two-dimensional Penrose lattice. We follow the Shastry-Shraiman formulation of Raman scattering in a strongly correlated Hubbard system and obtain the second- and fourth-order effective Raman operators. The second-order Raman intensity comes from the E2 mode, and it is invariant under an arbitrary rotation of polarization vectors. The fourth-order Raman intensities consist of A1 and A2, as well as E2, modes and therefore yield strong polarization dependence. In particular, the A2 mode intensity directly detects the dynamical spin-chirality fluctuations. Employing linearly and circularly polarized lights, we can separately extract every irreducible representation from the observations. We further discuss effects of magnon-magnon interactions on the magnetic Raman scattering. Our theory provides a reasonable explanation for the two-magnon scattering process.

cond-mat.str-el↗

arXiv2019

Spin-wave thermodynamics of square-lattice antiferromagnets revisited

Modifying the conventional spin-wave theory in a novel manner based on the Wick decomposition, we present an elaborate thermodynamics of square-lattice quantum antiferromagnets. Our scheme is no longer accompanied by the notorious problem of an artificial transition to the paramagnetic state inherent in modified spin waves in the Hartree-Fock approximation. In the cases of spin $\frac{1}{2}$ and spin $1$, various modified-spin-wave findings for the internal energy, specific heat, static uniform susceptibility, and dynamic structure factor are not only numerically compared with quantum Monte Carlo calculations and Lanczos exact diagonalizations but also analytically expanded into low-temperature series. Modified spin waves interacting via the Wick decomposition provide reliable thermodynamics over the whole temperature range of absolute zero to infinity. Adding higher-order spin couplings such as ring exchange interaction to the naivest Heisenberg Hamiltonian, we precisely reproduce inelastic-neutron-scattering measurements of the high-temperature-superconductor-parent antiferromagnet $\mathrm{La}_2\mathrm{CuO}_4$. Modifying Dyson-Maleev bosons combined with auxiliary pseudofermions also yields thermodynamics of square-lattice antiferromagnets free from thermal breakdown, but it is less precise unless temperature is sufficiently low. Applying all the schemes to layered antiferromagnets as well, we discuss the advantages and disadvantages of modified spin-wave and combined boson-pseudofermion representations.

cond-mat.str-el↗

arXiv2011

Photoinduced Structural Phase Transitions in Polyacene

There exist two types of structural instability in polyacene: double bonds in a cis pattern and those in a trans pattern. They are isoenergetic but spectroscopically distinct. We demonstrate optical characterization and manipulation of Peierls-distorted polyacene employing both correlated and uncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and trans-distorted isomers, elucidate their optical-conductivity spectra, and then explore their photoresponses. There occurs a photoinduced transformation in the polyacene structure, but it is one-way switching: The trans configuration is well convertible into the cis one, whereas the reverse conversion is much less feasible. Even the weakest light irradiation can cause a transition of uncorrelated electrons, while correlated electrons have a transition threshold against light irradiation.

cond-mat.str-el↗

arXiv2010

Ground-state properties of a Peierls-Hubbard triangular prism

Motivated by recent chemical attempts at assembling halogen-bridged transition-metal complexes within a nanotube, we model and characterize a platinum-halide triangular prism in terms of a Peierls-Hubbard Hamiltonian. Based on a group-theoretical argument, we reveal a variety of valence arrangements, including heterogeneous or partially metallic charge-density-wave states. Quantum and thermal phase competitions are numerically demonstrated with particular emphasis on novel insulator-to-metal and insulator-to-insulator transitions under doping, the former of which is of the first order, while the latter of which is of the second order.

cond-mat.str-el↗

arXiv2009

Competing Ground States of a Peierls-Hubbard Nanotube

Motivated by iodo platinum complexes assembled within a quadratic-prism lattice, [Pt(C$_2$H$_8$N$_2$)(C$_{10}$H$_8$N$_2$)I]$_4$(NO$_3$)$_8$, we investigate the ground-state properties of a Peierls-Hubbard four-legged tube. Making a group-theoretical analysis, we systematically reveal a variety of valence arrangements, including half-metallic charge-density-wave states. Quantum and thermal phase competition is numerically demonstrated with particular emphasis on doping-induced successive insulator-to-metal transitions with conductivity increasing stepwise.

cond-mat.str-el↗

arXiv2008

Optical characterization of ground states of polyacene

We investigate the ground-state properties of polyacene in terms of an extended Peierls-Hubbard Hamiltonian with particular emphasis on its structural instability of two types: double bonds in a "cis" pattern and those in a "trans" pattern. Calculating the polarized optical conductivity spectra within and beyond the Hartree-Fock scheme, we reveal a striking contrast between the "cis" and "trans" configurations. The two Peierls-distorted states are almost degenerate in their energetics but quite distinct in their optics.

cond-mat.str-el↗

arXiv2008

Photoproduction of spin and charge carriers in halogen-bridged binuclear platinum chain complexes

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds is theoretically investigated within a one-dimensional extended Peierls-Hubbard model. We first illuminate the whole relaxation scenario in terms of variational wave functions and then visualize each relaxation channel numerically integrating the Schrödinger equation. High-energy excitations above the electron-hole continuum tend to relax into polarons, while excitons pumped within the optical gap, unless luminescent, turn into solitonic states nonradiatively. Neutral and charged solitons coexist as stable photoproducts, which has never been observed in conventional platinum-halide chains, and they are highly resonant on the occasion of their birth and geminate recombination.

cond-mat.str-el↗

arXiv2007

Low-energy structure of the intertwining double-chain ferrimagnets A_3_Cu_3_(PO_4_)_4_ (A=Ca,Sr,Pb)

Motivated by the homometallic intertwining double-chain ferrimagnets A_3_Cu_3_(PO_4_)_4_ (A=Ca,Sr,Pb), we investigate the low-energy structure of their model Hamiltonian H=\sum_n_[J_1_(S_{n :1}_+S_{n :3}_) +J_2_(S_{n+1:1}+S_{n-1:3}_)]\cdotS_{n:2}_, where S_{n:l}_ stands for the Cu^{2+}^ ion spin labeled l in the nth trimer unit, with particular emphasis on the range of bond alternation 0<J_2/J_1<1. Although the spin-wave theory, whether up to O(S^1^) or up to O(S^0^), claims that there exists a flat band in the excitation spectrum regardless of bond alternation, a perturbational treatment as well as the exact diagonalization of the Hamiltonian reveals its weak but nonvanishing momentum dispersion unless J_2_=J_1_ or J_2_=0. Quantum Monte Carlo calculations of the static structure factor further convince us of the low-lying excitation mechanism, elucidating similarities and differences between the present system and alternating-spin linear-chain ferrimagnets.

cond-mat.str-el↗