Fate of orbitons coupled to phonons

Abstract

The key feature of an orbital wave or orbiton is a significant dispersion, which arises from exchange interactions between orbitals on distinct sites. We study the effect of a coupling between orbitons and phonons in one dimension using continuous unitary transformations. Already for intermediate values of the coupling, the orbiton band width is strongly reduced and the spectral density is dominated by an orbiton-phonon continuum. However, we find sharp features within the continuum and an orbiton-phonon antibound state above. Both show a significant dispersion and should be observable experimentally.where c and b are bosonic operators that represent orbitons and phonons, respectively, orb ex and orb JT =4g 2 / ph denote the contributions to the local orbiton energy from superexchange and from a static Jahn-Teller deformation, 10 J the superexchange coupling constant, ph the phonon energy, and g the coupling between orbitons and phonons. The Jahn-Teller energy orb JT =4g 2 / ph results from the static distortion of the local environment of the transition-metal ion with orbital degeneracy. This distortion lifts the degeneracy such that the PHYSICAL REVIEW B 76, 075108 ͑2007͒

Orbital superexchange and crystal field simultaneously at play in YVO3: Resonant inelastic x-ray scattering at the VLedge and the OKedge

“…A finite dispersion of the orbital excitations is expected to reveal itself via a momentum-dependent line shape of this continuum. 10 A detailed study of the q dependence of the line shape remains a challenge to both theory and experiment.…”

“…However, this complex problem has hardly been addressed thus far. [9][10][11][12] Therefore we focus on the two limiting cases, either a dominant crystal field or dominant superexchange interactions. Both, superexchange and the noncubic crystal field lift the threefold orbital degeneracy of the cubic 3 T 1 ground state, giving rise to low-energy intra-t 2g excitations.…”

“…The excitation spectrum is then dominated by an orbitonphonon continuum. 10 If we consider also the coupling to magnons, 25,71 we have to expect an orbiton-magnon-phonon continuum. A finite dispersion of the orbital excitations is expected to reveal itself via a momentum-dependent line shape of this continuum.…”

“…If both, superexchange interactions and the coupling to the lattice are relevant, the spectral signatures become more difficult to interpret, a complex situation that has hardly been studied thus far. [9][10][11][12] The experimental observation of orbitons at low energies 250 meV has been claimed based on Raman data of LaMnO 3 , RTiO 3 , and RVO 3 with R = rare earth. [13][14][15][16][17] However, the orbiton interpretation of these data has caused controversy.…”

Single and Double Orbital Excitations Probed by Resonant Inelastic X-Ray Scattering

“…The computed orbiton spectrum for the e g orbital ordering of LaMnO 3 shows, besides the orbiton dispersions, also strong momentum dependence of scattering intensity, with in particular a vanishing of it at q=(0,0) and (π,π) for all but one orbiton branch. The orbiton is also expected to have phonon sidebands [19,20], observable in RIXS as well.…”