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Shoji Yamamoto

Publications and source records attributed to Shoji Yamamoto.

At least 55 records · Page 3Linked to original sources
arXiv2003

Recent Progress of the Low-Dimensional Spin-Wave Theory

A modified spin-wave theory is developed and applied to low-dimensional quantum magnets. Double-peaked specific heat for one-dimensional ferrimagnets, nuclear spin-lattice relaxation in ferrimagnetic chains and clusters, and thermal behavior of Haldane-gap antiferromagnets are described within the scheme. Mentioning other bosonic and fermionic representations as well, we demonstrate that spin waves are still effective in low dimensions.

cond-mat.stat-mech↗
arXiv2003

Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions

Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by Caneschi et al., both of which consist of alternating manganese(II) ions and nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation rate 1/T_1 employing a recently developed modified spin-wave theory. 1/T_1 as a function of temperature drastically varies with the location of probe nuclei in both clusters and chains, though the relaxation time scale is much larger in zero dimension than in one dimension. 1/T_1 as a function of an applied field in long chains forms a striking contrast to that in finite clusters, diverging with decreasing field like inverse square root at low temperatures and logarithmically at high temperatures.

cond-mat.str-el↗
arXiv2003

Spin-Wave Description of Haldane-gap antiferromagnets

Modifying the conventional antiferromagnetic spin-wave theory which is plagued by the difficulty of the zero-field sublattice magnetizations diverging in one dimension, we describe magnetic properties of Haldane-gap antiferromagnets. The modified spin waves, constituting a grand canonical bosonic ensemble so as to recover the sublattice symmetry, not only depict well the ground-state correlations but also give useful information on the finite-temperature properties.

cond-mat.stat-mech↗
arXiv2002

Spin-Wave Description of Nuclear Spin-Lattice Relaxation in Mn_{12}O_{12} Acetate

In response to recent nuclear-magnetic-resonance (NMR) measurements on the molecular cluster Mn_{12}O_{12} acetate, we study the nuclear spin-lattice relaxation rate 1/T_1 developing a modified spin-wave theory. Our microscopic new approach, which is distinct from previous macroscopic treatments of the cluster as a rigid spin of S=10, not only excellently interprets the observed temperature and applied-field dependences of 1/T_1 for ^{55}Mn nuclei but also strongly supports the ^{13}C NMR evidence for spin delocalization over the entire molecule.

cond-mat.stat-mech↗
arXiv2002

Quantum and thermal phase transitions in the Bechgaard salts and their analogs

In order to investigate quantum and thermal phase transitions in the Bechgaard salts and their sulfur analogs, we perform finite-temperature Hartree-Fock calculations in one dimension with particular emphasis on the interplay between charge ordering and lattice instability. The coexisting charge- and spin-density-wave state as well as its precursor fluctuations in (TMTSF)2PF6 and the lattice tetramerization in (TMTTF)2ReO4 are well interpreted.

cond-mat.str-el↗
arXiv2002

Local excitations in mixed-valence bimetal chains

We investigate localized intragap states in mixed-valence bimetal chains in an attempt to encourage further explorations into quasi-one-dimensional halogen-bridged binuclear metal (MMX) complexes. Within a coupled electron-phonon model, soliton and polaron excitations in the two distinct ground states of MMX chains are numerically calculated and compared. Making a continuum-model analysis as well, we reveal their scaling properties with particular emphasis on the analogy between MMX chains and trans-polyacetylene.

cond-mat.str-el↗
arXiv2002

Intrinsic double-peak structure of the specific heat in low-dimensional quantum ferrimagnets

Motivated by recent magnetic measurements on A3Cu3(PO4)4 (A=Ca,Sr) and Cu(3-Clpy)2(N3)2 (3-Clpy=3-Chloropyridine), both of which behave like one-dimensional ferrimagnets, we extensively investigate the ferrimagnetic specific heat with particular emphasis on its double-peak structure. Developing a modified spin-wave theory, we reveal that ferromagnetic and antiferromagnetic dual features of ferrimagnets may potentially induce an extra low-temperature peak as well as a Schottky-type peak at mid temperatures in the specific heat.

cond-mat.str-el↗
arXiv2002

Re-Entrant Quantum Phase Transitions in Antiferromagnetic Spin-1 Ladders

In response to recent chemical attempts to construct higher-spin ladder materials from organic polyradicals, we study the ground-state properties of a wide class of antiferromagnetic spin-1 ladders. Employing various numerical tools, we reveal the rich phase diagram and correct a preceding nonlinear-sigma-model prediction. A variational analysis well interprets the phase competition with particular emphasis on the {\it re-entrant phase boundary} as a function of the rung interaction.

cond-mat.str-el↗
arXiv2001

Soliton excitations in halogen-bridged mixed-valence binuclear metal complexes

Motivated by recent stimulative observations in halogen (X)-bridged binuclear transition-metal (M) complexes, which are referred to as MMX chains, we study solitons in a one-dimensional three-quarter-filled charge-density-wave system with both intrasite and intersite electron-lattice couplings. Two distinct ground states of MMX chains are reproduced and the soliton excitations on them are compared. In the weak-coupling region, all the solitons are degenerate to each other and are uniquely scaled by the band gap, whereas in the strong-coupling region, they behave differently deviating from the scenario in the continuum limit. The soliton masses are calculated and compared with those for conventional mononuclear MX chains.

cond-mat.str-el↗
arXiv2001

Pressure-induced phase transitions of halogen-bridged binuclear metal complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O

Recent contrasting observations for halogen (X)-bridged binuclear platinum complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O, that is, pressure-induced Peierls and reverse Peierls instabilities, are explained by finite-temperature Hartree-Fock calculations. It is demonstrated that increasing pressure transforms the initial charge-polarization state into a charge-density-wave state at high temperatures, whereas the charge-density-wave state oppositely declines with increasing pressure at low temperatures. We further predict that higher-pressure experiments should reveal successive phase transitions around room temperature.

cond-mat.str-el↗
arXiv2001

Quantum and Thermal Phase Transitions of Halogen-Bridged Binuclear Transition-Metal Complexes

Aiming to settle the controversial observations for halogen-bridged binuclear transition-metal (MMX) complexes, finite-temperature Hartree-Fock calculations are performed for a relevant two-band Peierls-Hubbard model. Thermal, as well as quantum, phase transitions are investigated with particular emphasis on the competition between electron itinerancy, electron-phonon interaction and electron-electron correlation. Recently observed distinct thermal behaviors of two typical MMX compounds Pt_2(CH_3CS_2)_4I and (NH_4)_4[Pt_2(P_2O_5H_2)_4I]2H_2O are supported and further tuning of their electronic states is predicted.

cond-mat.str-el↗
arXiv2001

Low-Temperature Properties of Quasi-One-Dimensional Molecule-Based Ferromagnets

Quantum and thermal behaviors of low-dimensional mixed-spin systems are investigated with particular emphasis on the design of molecule-based ferromagnets. One can obtain a molecular ferromagnet by assembling molecular bricks so as to construct a low-dimensional system with a magnetic ground state and then coupling the chains or the layers again in a ferromagnetic fashion. Two of thus-constructed quasi-one-dimensional bimetallic compounds are qualitatively viewed within the spin-wave treatment, one of which successfully grows into a bulk magnet, while the other of which ends in a singlet ground state. Then, concentrating on the ferrimagnetic arrangement on a two-leg ladder which is well indicative of general coupled-chain ferrimagnets, we develop the spin-wave theory and fully reveal its low-energy structure. We inquire further into the ferromagnetic aspect of the ferrimagnetic ladder numerically calculating the sublattice magnetization and the magnetic susceptibility. There exists a moderate coupling strength between the chains in order to obtain the most ferromagnetic ferrimagnet.

cond-mat.stat-mech↗
arXiv2001

Characterization of halogen-bridged binuclear metal complexes as hybridized two-band materials

We study the electronic structure of halogen-bridged binuclear metal (MMX) complexes with a two-band Peierls-Hubbard model. Based on a symmetry argument, various density-wave states are derived and characterized. The ground-state phase diagram is drawn within the Hartree-Fock approximation, while the thermal behavior is investigated using a quantum Monte Carlo method. All the calculations conclude that a typical MMX compound Pt_2(CH_3CS_2)_4I should indeed be regarded as a d-p-hybridized two-band material, where the oxidation of the halogen ions must be observed even in the ground state, whereas another MMX family (NH_4)_4[Pt_2(P_2O_5H_2)_4X] may be treated as single-band materials.

cond-mat.str-el↗
arXiv2000

Significance of the direct relaxation process in the low-energy spin dynamics of a one-dimensional ferrimagnet NiCu(C_7H_6N_2O_6)(H_2O)_3 2H_2O

In response to recent nuclear-magnetic-resonance measurements on a ferrimagnetic chain compound NiCu(C_7H_6N_2O_6)(H_2O)_3 2H_2O [Solid State Commun. {\bf 113} (2000) 433], we calculate the nuclear spin-lattice relaxation rate 1/T_1 in terms of a modified spin-wave theory. Emphasizing that the dominant relaxation mechanism arises from the direct (single-magnon) process rather than the Raman (two-magnon) one, we explain the observed temperature and applied-field dependences of 1/T_1. Ferrimagnetic relaxation phenomena are generally discussed and novel ferrimagnets with extremely slow dynamics are predicted.

cond-mat.str-el↗
arXiv2000

Coexistent quantum and classical aspects of magnetization plateaux in alternating-spin chains

Magnetization process of ferrimagnetic Heisenberg chains of alternating spins are theoretically studied. The size scaling analysis with the exact diagonalization of finite systems for ($S$,$s$)=(3/2,1) and (2,1) indicates a multi-plateau structure in the ground-state magnetization curve for $S$ and $s$ $>1/2$. The first plateau at the spontaneous magnetization can be explained by a classical origin, that is the Ising gap. In contrast, the second or higher one must be originated to the quantization of the magnetization. It is also found that all the $2s$ plateaux, including the classical and quantum ones, appear even in the isotropic case with no bond alternation.

cond-mat.stat-mech↗
arXiv2000

Multi-plateau magnetization curves of one-dimensional Heisenberg ferrimagnets

Ground-state magnetization curves of ferrimagnetic Heisenberg chains of alternating spins $S$ and $s$ are numerically investigated. Calculating several cases of $(S,s)$, we conclude that the spin-$(S,s)$ chain generally exhibits $2s$ magnetization plateaux even at the most symmetric point. In the double- or more-plateau structure, the initial plateau is generated on a classical basis, whereas the higher ones are based on a quantum mechanism.

cond-mat.stat-mech↗
arXiv2000

Modified spin-wave description of the nuclear spin relaxation in ferrimagnetic Heisenberg chains

We make a modified spin-wave description of the nuclear spin relaxation in Heisenberg alternating-spin chains with antiferromagnetic exchange coupling. In contrast with the conventional one-dimensional antiferromagnetic spin-wave theory, which is plagued with the divergence of the sublattice magnetization even in the ground state, the present spin-wave description is highly successful over a wide temperature range. The temperature dependence of the relaxation rate $T_1^{-1}$ significantly varies with the crystalline structure, exhibiting both ferromagnetic and antiferromagnetic aspects. $T_1^{-1}$ further shows a unique dependence on the applied field, which turns out an indirect observation of the quadratic dispersion relations.

cond-mat.stat-mech↗