Comparison of the phonon spectra of 70Ge and natural Ge crystals: Effects of isotopic disorder

Vibrational monitor of early demineralization in tooth enamel after in vitro exposure to phosphoridic liquid

The Raman spectroscopic method has been applied to quantitatively assess the in vitro degree of demineralization in healthy human teeth. Based on previous evaluations of Raman selection rules (empowered by an orientation distribution function (ODF) statistical algorithm) and on a newly proposed analysis of phonon density of states (PDOS) for selected vibrational modes of the hexagonal structure of hydroxyapatite, a molecular-scale evaluation of the demineralization process upon in vitro exposure to a highly acidic beverage (i.e., CocaCola™ Classic, pH=2.5) could be obtained. The Raman method proved quite sensitive and spectroscopic features could be directly related to an increase in off-stoichiometry of the enamel surface structure since the very early stage of the demineralization process (i.e., when yet invisible to other conventional analytical techniques). The proposed Raman spectroscopic algorithm might possess some generality for caries risk assessment, allowing a prompt non-contact diagnostic practice in dentistry.

High-resolution X-ray diffraction and micro-Raman scattering studies of Ge(:Ga) thin films grown on GaAs (001) substrates by MOCVD

Ga-doped Ge thin films grown on GaAs (001) substrates have been studied and compared with unintentionally doped Ge film by using HRXRD and Raman scattering in both surface and cross-section configurations.

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.

Vibrational lifetimes and isotope effects of interstitial oxygen in silicon and germanium

Decay dynamics of local vibrational modes provides unique information about energy relaxation processes to solid-state phonon bath. In this Letter the lifetimes of the asymmetric stretch mode of interstitial 16O and 17O isotopes in Si are measured at 10 K directly by time-resolved, transient bleaching spectroscopy to be 11.5 and 4.5 ps, respectively. A calculation of the three-phonon density of states shows that the 17O mode lies in the highest phonon density resulting in the shortest lifetime. The lifetime of the 16O mode in Ge is measured to be 125 ps, i.e., approximately 10 times longer than in Si. The interaction between the local modes and the lattice vibrations is discussed according to the activity of phonon combinations.

Photoluminescence of isotopically purified silicon: how sharp are bound exciton transitions?

We report the first high resolution photoluminescence studies of isotopically pure Si (99.896% (28)Si). New information is obtained on isotopic effects on the indirect band gap energy, phonon energies, and phonon broadenings, which is in good agreement with calculations and previous results obtained in Ge and diamond. Remarkably, the linewidths of the no-phonon boron and phosphorus bound exciton transitions in the (28)Si sample are much narrower than in natural Si and are not well resolved at our maximum instrumental resolution of approximately 0.014 cm(-1). The removal of the dominant broadening resulting from isotopic randomness in natural Si reveals new fine structure in the boron bound exciton luminescence.

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.