Orbiton

Abstract: A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise1. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state2,3,4,5,6,7,8,9 could in principle give rise to orbital waves, or ‘orbitons’, but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated—such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity—are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important10,11. Orbitally ordered states have been found in several transition-metal compounds12,13, and orbitons have been predicted theoretically for LaMnO3 (refs 4, 5). Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.

Abstract: Temperature-dependent Raman spectra of TbMnO3 from 5 to 300 K in the spectral range of 200-1525 cm(-1) show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first-order Raman mode is nearly constant and high (similar to 0.6) at all temperatures, suggesting an orbiton-phonon mixed nature of the high frequency mode. One of the first-order phonon modes shows anomalous softening below T-N (similar to 46 K), suggesting a strong spin-phonon coupling.

Abstract: In ${\mathrm{FeSc}}_{2}{\mathrm{S}}_{4}$ spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using terahertz and far-infrared spectroscopy we study low-lying excitations in ${\mathrm{FeSc}}_{2}{\mathrm{S}}_{4}$ and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.

Abstract: Temperature-dependent Raman spectra of TbMnO$_3$ from 5 K to 300 K in the spectral range of 200 to 1525 cm$^{-1}$ show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first order Raman mode is nearly constant and high ($\sim$ 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature of the high frequency mode. One of the first order phonon modes shows anomalous softening below T$_N$ ($\sim$ 46 K), suggesting a strong spin-phonon coupling.