TITLE: HIGH-PRESSURE Light scattering Study of low Energy excitations in glasses

SPEAKER: Dr. Masashi Yamaguchi,

TIME: Thursday Feb. 13, 2003 at 4 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Glasses are ubiquitous in technology and nature. Fiber communication technology depends heavily on glassy materials, most of solar cells are made of amorphous silicon and it is considered that most of H2O molecules in the universe exist as a part of amorphous ice rather than liquids, crystal or vapor. However, our understanding of glass properties is far less behind those of crystals. Glasses show distinctly different behavior from crystals in low temperature region (T<10 K). For example, thermal conductivity of glasses exhibits a so-called "plateau region" in its temperature dependence, which does not exist in crystals. In addition, specific heat of glasses has excess contribution in the thermal conductivity in the temperature range similar to the "plateau region". This characteristic behavior to glasses is related to the low energy vibrational excitation, which appears as the "Boson peak" in light scattering spectra. The heat conducting mechanism above the "plateau region" has been a matter of substantial controversy. The relationship between the heat conducting mechanism of glasses above the "plateau region" and low energy excitations has been studied using light scattering experiment. By measuring pressure dependence of light scattering spectra, anharmonic interaction between the acoustic phonons and the low energy excitation was investigated. Anharmonic elastic stiffness coefficients related to longitudinal acoustic phonons and strongly localized modes were theoretically derived. The results show strong experimental support for the phonon-assisted strongly localized mode hopping process that could account for the thermal conductivity above the plateau region.