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arXiv:cond-mat/9712239 · Physical review. B, Condensed matter

Domain excitations in spin-Peierls systems

A. Dobry, David Ibaceta

Published Aug 1, 1998• 7 citations
arXivHTMLPDF

Abstract

We study a model of a spin-Peierls material consisting of a set of antiferromagnetic Heisenberg chains coupled with phonons and interacting among them via an interchain elastic coupling. The excitation spectrum is analyzed by bosonization techniques and the self-harmonic approximation. The elementary excitation is the creation of a localized domain structure where the dimerized order is the opposite to the one of the surroundings. It is a triplet excitation whose formation energy is smaller than the magnon gap. Magnetic internal excitations of the domain are possible and give the further excitations of the system. We discuss these results in the context of recent experimental measurements on the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_{3}.$

DOI
10.1103/physrevb.58.3124

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We study a model of a spin-Peierls material consisting of a set of antiferromagnetic Heisenberg chains coupled with phonons and interacting among them via an interchain elastic coupling. The excitation spectrum is analyzed by bosonization techniques and the self-harmonic approximation. The elementary excitation is the creation of a localized domain structure where the dimerized order is the opposite to the one of the surroundings. It is a triplet excitation whose formation energy is smaller than the magnon gap. Magnetic internal excitations of the domain are possible and give the further excitations of the system. We discuss these results in the context of recent experimental measurements on the inorganic spin-Peierls compound CuGeO3.{\mathrm{CuGeO}}_{3}.CuGeO3​.
We reconsider the excitation spectra of a dimerized and frustrated antiferromagnetic Heisenberg chain. This model is taken as the simpler example of competing spontaneous and explicit dimerization relevant for spin-Peierls compounds. The bosonized theory is a two-frequency sine-Gordon field theory. We analyze the excitation spectrum by semiclassical methods. The elementary triplet excitation corresponds to an extended magnon whose radius diverges for vanishing dimerization. The internal oscillations of the magnon give rise to a series of excited states until another magnon is emitted and a two-magnon continuum is reached. We discuss, for weak dimerization, the manner in which the magnon forms as a result of a spinon-spinon interaction potential.
The role of dynamical magnetoelastic coupling in spin-Peierls chains is investigated by numerical and analytical techniques. We show that a Heisenberg spin chain coupled to dynamical optical phonons exhibits a transition towards a spontaneously dimerized state in a wide range of parameter space. The low-energy excitations are characterized as solitons. No binding between solitons occurs in the isolated spin-phonon chain and the dynamical spin structure factor shows a broad magnon dispersion. However, elastic interchain coupling can lead to the formation of bound states.
We consider the ground state and the elementary excitations of an array of spin-Peierls chains coupled by elastic and magnetic interactions. It is expected that the effect of the magnetic interchain coupling will be to reduce the dimerization amplitude and that of the elastic coupling will be to confine the spin one-half solitons corresponding to each isolated chain. We show that this is the case when these interactions are not frustrated. On the other hand, in the frustrated case we show that the amplitude of dimerization in the ground state is independent of the strength of the interchain magnetic interaction in a broad range of values of this parameter. We also show that free solitons could be the elementary excitations when only nearest neighbor interactions are considered. The case of an elastic interchain coupling is analyzed on a general energetic consideration. To study the effect of the magnetic interchain interaction the problem is simplified to a two-leg ladder which is solved using DMRG calculations. We show that the deconfinement mechanism is effective even with a significantly strong antiferromagnetic interchain coupling.
We study spin dynamics of a spin-Peierls chain with nearest-neighbor and next-nearest-neighbor Heisenberg spin exchange interactions together with a gapped and dispersionless phonon. The dynamical spin correlation function and phonon excitation spectrum are calculated at zero temperature by using dynamical density-matrix renormalization-group method. We find a new spin excitation assisted by non-softening phonon. The excitation is located above phonon in energy and shows a dispersive feature with strong intensity near the momentum πππ. The phonon excitation spectrum is also influenced by the spin-phonon interaction. We discuss the possibility of observing the spin-phonon coupled features in inorganic spin-Peierls compound CuGeO3_33​.
Abstract The role of dynamical magneto-elastic coupling in spin–Peierls chains is investigated by various numerical techniques. We show that, generically, a Heisenberg spin chain coupled to dynamical optical phonons exhibits a transition towards a spontaneously dimerized state. The low energy excitations are characterized as topological solitons. No binding between solitons occurs in the isolated spin–phonon chain. However, elastic interchain coupling can lead to the formation of bound states.
Motivated by recent measurements on CuGeO3{\mathrm{O}}_{3}O3​, the spectrum of magnetic excitations of an antiferromagnetic S=12S=\frac{1}{2}S=21​ chain with alternating coupling strength is investigated. Wave-vector-dependent magnons and a continuum with square-root behavior at the band edges are found. The spectral density of the continua is calculated. Spin-rotation symmetry fixes the gap of the continuum to be twice the elementary magnon gap. This is in excellent agreement with experimental results. The existence of (anti)bound states of two magnons is predicted: below the continuum a singlet and a triplet, above the continuum a quintuplet.