Vibrational band modes in germanium: Isotopic disorder-induced Raman scattering

Ideal soft mode-type quantum phase transition and phase coexistence at quantum critical point in 18O-exchanged SrTiO3

The quantum ferroelectric phase transition of 18O-exchanged SrTiO3 (x% exchanged SrTiO3 is abbreviated as STO18-x) was investigated by Raman scattering as a function of x. The result indicates the ideal soft mode-type quantum ferroelectric phase transition of STO18-x, where the 18O exchange enhances the softening of the soft mode by the suppression of quantum fluctuation. In the vicinity of the quantum critical point (x approximately xc=33%), the system results in the ferroelectric-paraelectric phase coexistence state, in clear contrast to the homogeneous ferroelectric phase in STO18-x, whose x is sufficiently larger than xc. Simultaneously, the softening of the soft mode becomes strongly rounded with the underdamped oscillation. The present result indicates that the sensitivity of the soft phonon vibration to the mass disorder is dramatically enhanced in the vicinity of the quantum critical point.

Isotope-disorder-induced line broadening of phonons in the Raman spectra of SiC

The width of phonon lines in the Raman spectra of ideal isotopically pure solids is determined by inelastic scattering processes. In solids that contain a mixture of different isotopes of one atomic constituent, elastic scattering due to isotopic mass disorder opens up decay channels that result in additional line broadening. We use different polytypes of SiC with an associated number of Raman active modes in order to experimentally validate the proportionality between linewidth and phonon density of states predicted by a simple elastic scattering theory.