DYNAMICS OF NONEQUILIBRIUM BROADBAND PHONON DISTRIBUTIONS (HEAT-PULSE, VIBRONIC-SIDEBAND SPECTROMETER, STRONTIUM FLOURITE, EUROPIAM).

摘要

A spectrometer('(1)) has been used to measure the spectral, spatial and temporal distribution of phonons (the quanta of a crystal lattices' displacement field) propagating in a SrF(,2) crystal. The spectrometer uses as the detecting element the vibronic sidebands associated with certain luminescent transitions at Eu('2+) probe ions placed in dilute concentration in the crystal. Vibronic sidebands arise from the interaction between the time-varying crystal field associated with phonons and an electronic transition localized at the probe ions. The phonons are generated in a constantan thin film through electrical heating and cross the interface between the film and crystal. We have measured the phonon occupation numbers in absolute units, which is the first time this has been achieved in heat pulse experiments, and find for phonon frequencies above 1/2 Terahertz that the distributions are essentially Planck distributions with an effective temperature that is space and time dependent.;References(').;('(1))W. E. Bron and W. Grill, Phys. Rev. B16, 5303, 5315 (1977). ('(2))W. L. Schaich, Solid State Comm., in press. ('(3))Y. B. Levinson, Sov. Phys. JETP 52, 704 (1980). ('(4))T. E. Wilson and W. L. Schaich, Solid State Comm., in press.;It is only recently that theoretical progress has been made in predicting the transport of high-frequency (THz) acoustic phonons emitted from a metal surface film into an insulating substrate in which the phonons in the substrate undergo anharmonic (phonon-phonon) interactions in addition to elastic scattering off the probe ions. Previously, an important assumption in the theoretical work was that anharmonic processes are negligible in the crystal substrate. The results of the experiments performed for this dissertation show that anharmonic processes are indeed important and must be considered in high-frequency phonon dynamics. The experimental observations are found to possess the general features predicted by a theoretical approach which includes elastic and anharmonic processes, although the approximations used in the theory and the constraints of the experimental parameters limit a quantitative comparison.('(2,3,4)).