Mixing of magnetic and phononic excitations in incommensurate spin-Peierls systems

Abstract

We analyze the excitation spectra of a spin-phonon coupled chain in the presence of a soliton. This is taken as a microscopic model of a Spin-Peierls material placed in a high magnetic field. We show, by using a semiclassical approximation in the bosonized representation of the spins that a trapped magnetic state obtained in the adiabatic approximation is destroyed by dynamical phonons. Low energy states are phonons trapped by the soliton. When the magnetic gap is smaller than the phonon frequencies the only low energy state is a mixed magneto-phonon state with the energy of the gap. We emphasize that our results are relevant for the Raman spectra of the inorganic Spin-Peierls material CuGeO3.The discovery in 1993 by Hase et al [1] of the first inorganic Spin-Peierls compound CuGeO 3 has opened the possibility of study the physics of this collective phenomena in a deep way. Several experimental proves have given an exhaustive information about the excitation spectra of this system and its evolution with an applied magnetic field. The effect of non-magnetic impurities has been investigated also.Theoretical studies have focused on simplified magnetic model. The excitation spectra in the low temperature phase have been analyzed using a dimerized and frustrated Heisenberg chain as a minimal model for this material [3][4][5]. The logic underlying these studies are: the competition between magnetic and elastic energies resolves in the low temperature phase in the dimerization of the lattice. Once this process takes place, phononic and magnetic excitation completely decouple and the magnetic excitations are the same as the chemical dimerized system. This point of view is based on an adiabatic approximation supposing that the energy scale of the magnetic process are high enough respect to the phononic ones. As it has been recently emphasized [6], this relation is not fulfilled for CuGeO 3 where the phonons relevant for the dimerization process are about one order of magnitude more energetic than the magnetic gap. The adiabatic approximation is questionable for this system. An antiadiabatic approach has been developed. The frustrated interaction arises, in this context, from the integration of the in-chain phonons and the explicit dimerization from the interchain interaction treated in a mean field approximation [7]. The same frustrated-dimerized Hamiltonian is therefore obtained but with a reinterpretation of the parameters. What is clearly missed in these studies is a general understanding on how spin and phonons mix as elementary excitation and how the spectra of Spin-Peierls systems is built as a result of this mixing. Some recent numerical results have partially addressed this question [8].In this paper, we analyze the excitation spectra of onedimensional spin-phonon system by semiclassical techniques on the bosonized representation of the spins subsystem. We focus on the properties of this system in a high magnetic field. In the dimerized phase the system is in a singlet ground state. Coupling with a magnetic field...

Double frustration and magneto-electro-elastic excitations in collinear multiferroic materials

“…The ∆S z = 1 spin excitations induced by a magnetic field on a plateau state are the key to understand the high field plateau border. It is well known that the excitation of M = 0 plateau in one dimensional antiferromagnetic spin chains is not a stable singlet-triplet excitation but decays into two spinons [24,[29][30][31][32]. Each spinon carries spin S z = 1/2 as a topological charge and may be described as a soliton quasiparticle interpolating between different dimerized vacua; spatially, the soliton profile can be seen as a smooth domain wall.…”