arXiv subjects

Jun Ohara

Publications and source records attributed to Jun Ohara.

14 recordsLinked to original sources

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↗

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 N\'eel-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↗

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↗

arXiv2013

Optical manipulation of magnetism in spin-charge coupled correlated electron system

Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than change into the ferromagnetic metallic characteristic, which are expected from the conventional double exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a new principle for optically manipulating magnetism.

cond-mat.str-el↗

arXiv2012

Photoinduced magnetic bound state in itinerant correlated electron system with spin-state degree of freedom

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photo-irradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.

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

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↗

arXiv2007

Optical characterization of platinum-halide ladder compounds

New varieties of quasi-one-dimensional halogen (X)-bridged transition-metal (M) complexes, (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_X(ClO_4_)_3_\cdotH_2_O (X=Br,Cl) and (C_10_H_8_N_2_)[Pt(C_4_H_13_N_3_)Br]_2_Br_4_\cdot2H_2_O, comprising two-leg ladders of mixed-valent platinum ions, are described in terms of a multiband extended Peierls-Hubbard Hamiltonian. The polarized optical conductivity spectra are theoretically reproduced and the ground-state valence distributions are reasonably determined. The latter variety, whose interchain valence arrangement is out of phase, is reminiscent of conventional MX single-chain compounds, while the former variety, whose interchain valence arrangement is in phase, reveals itself as a novel d-p-π-hybridized multiband ladder material.

cond-mat.str-el↗

arXiv2005

Photoinduced infrared absorption of quasi-one-dimensional halogen-bridged binuclear transition-metal complexes

We investigate the optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear transition-metal MMX complexes with particular emphasis on a comparison among the three distinct groups: A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I; A=Na,K,NH_4_,...), Pt_2_(RCS_2_)_4_I (R=C_n_H_2n+1_) and Ni_2_(CH_3_CS_2_)_4_I, which exhibit a mixed-valent ground state with the X sublattice dimerized, that with the M_2_ sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively. Soliton-induced absorption spectra for A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O should split into two bands, while that for Pt_2_(RCS_2_)_4_I and Ni_2_(CH_3_CS_2_)_4_I should consist of a single band. The excitonic effect is significant in Ni_2_(CH_3_CS_2_)_4_I.

cond-mat.str-el↗

arXiv2005

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds

In order to reveal the relaxation mechanism of photogenerated charge-transfer excitations in quasi-one-dimensional halogen-bridged diplatinum complexes, we calculate the low-lying adiabatic potential energy surfaces of a one-dimensional extended Peierls-Hubbard model. High-energy excitations above the electron-hole continuum may relax into polarons, while excitons pumped within the optical gap are self-localized and then either decay by luminescence or divide into solitons. Neutral solitons, charged solitons, and polarons may be simultaneously photogenerated in a diplatinum-halide chain, which has never been observed in any conventional platinum-halide chain. Optical conductivity is also simulated along the decay paths for experimental verification.

cond-mat.str-el↗

arXiv2004

Photoinduced Absorption Spectra of Halogen-Bridged Binuclear Metal Complexes: Possible Contrast between R_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O and Pt_2_(CH_3_CS_2_)_4_I

The optical conductivity of photogenerated solitons in quasi-one-dimensional halogen-bridged binuclear metal (MMX) complexes is investigated with particular emphasis on a comparison between the two family compounds R_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I;R=NH_4_,Na,K,...; pop=diphosphonate=P_2_O_5_H_2_^2-^) and Pt_2_(dta)_4_I (dta=dithioacetate=CH_3_CS_2_^-^). Soliton-induced absorption spectra for the pop complexes should split into two bands, while those for the dta complex should consist of a single band.

cond-mat.str-el↗

arXiv2004

Soliton-induced optical absorption of halogen-bridged mixed-valence binuclear metal complexes

Employing the one-dimensional single-band extended Peierls-Hubbard model, we investigate optical conductivity for solitonic excitations in halogen-bridged binuclear metal (MMX) complexes. Photoinduced soliton absorption spectra for MMX chains possibly split into two bands, forming a striking contrast to those for conventional mononuclear metal (MX) analogs, due to the broken electron-hole symmetry combined with relevant Coulomb and/or electron-phonon interactions.

cond-mat.str-el↗