Experimental quest for orbital waves

Abstract

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

Orbiton-mediated multiphonon scattering inLa1−xSrxMnO3

“…9,14 Here we point out that the FC mechanism is consistent with the simultaneous observation of similar multi-phonon features by optical conductivity and Raman scattering measurements despite completely different selection rules. 2 Originally, infrared inactive modes become infrared active in the multi-phonon scattering owing to nonlinear local oxygen displacements. In this case, the weak intensity of the multi-phonon modes seen in the optical conductivity can be ascribed to orbiton-assisted parity breaking.…”

“…5 Interestingly, in spite of mutually exclusive selection rules an infrared absorption study shows similar features as Raman scattering measurements at about 125 meV (1000 cm −1 ), 145 meV (1160 cm −1 ), and 160 meV (1280 cm −1 ). 2 This was attributed to multi-phonon scattering instead of orbital excitations. On the other hand, orbiton-phonon mixed modes are predicted to appear as satellite structures in the phonon spectrum due to electron-phonon coupling.…”

“…1 Despite pronounced temperature-and symmetry-dependent properties of the modes, the assignment has been confronted with criticism and alternative concepts. 2,3,4,5 For instance, multi-phonon scattering has been theoretically predicted to arise from the Franck-Condon (FC) process via self-trapped orbitons. 5 Interestingly, in spite of mutually exclusive selection rules an infrared absorption study shows similar features as Raman scattering measurements at about 125 meV (1000 cm −1 ), 145 meV (1160 cm −1 ), and 160 meV (1280 cm −1 ).…”

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

“…[13][14][15][16][17] However, the orbiton interpretation of these data has caused controversy. [14][15][16][17][18][19][20][21][22] For manganites with partially occupied e g states in a predominantly octahedral crystal field, it is meanwhile well established that the orbital degree of freedom is quenched by the strong crystal-field splitting of the order of 1 eV. 23,24 Recently, it has been shown by resonant inelastic x-ray scattering (RIXS) on 1D cuprate chains that such high-energy orbital excitations may show a significant dispersion.…”

Orbital wave in the Raman scattering cross section of LaMnO3

“…They claimed that a weak signal near 150 meV is a new elementary excitation that differs from the simple phonon. However, other researchers suspected that the weak signal is due to the first overtones of fundamental phonon modes [5]. Until recently, studies on the Raman scattering signal of LaMnO 3 remained controversial despite the accumulation of precise spectroscopic studies for high-quality crystals of LaMnO 3 and its relatives [6][7][8].…”