K. E. O'Hara and J. P. Wolfe
"Relaxation kinetics of excitons in cuprous oxide." Physical Review B 62, 12909-12922 (2000).
Abstract: Due to a simple band structure, the excitons in cuprous oxide (Cu₂O) are a model system for kinetic studies. Cuprous oxide appeared to be a host for a Bose-Einstein condensate of excitons, as the excitons showed transient kinetic energy distributions which matched those expected for a Bose gas near the critical density for Bose-Einstein condensation. However, recent absolute measurements of the exciton density made it clear that two-exciton annihilation is limiting the exciton density to far below the quantum density. This paper reconciles the measured exciton density with the observed exciton energy distributions by using a Boltzmann equation approach. We include experimentally determined rates for acoustic- and optical-phonon emission, conversion between exciton spin states, and two-exciton annihiliation, and use recent diffusion-Monte-Carlo estimates of the exciton-exciton elastic scattering cross section. Many experiments intending to produce a dense exciton gas in Cu₂O used surface photoexcitation, and we found it important to include the resulting spatial inhomogeneities in the model by following the exciton occupation numbers as functions of space as well as momentum and time. A detailed but straightforward numerical integration of the resulting Boltzmann equation does in fact match the experimental results, without the assumption of quantum statistics for the excitons.

J.D. Short and J.P. Wolfe
"Evidence for large gap anisotropy in superconducting Pb from phonon imaging." Physical Review Letters 85, 5198-5201 (2000).
Abstract: We report the first ballistic phonon images of superconducting Pb. Unusual absorption lines are observed for phonon wave vectors in {111} planes. We show that a highly anisotropic energy gap can lead to sharply defined directions of phonon attenuation. Overhauser and Daemen [Phys. Rev. Lett. 61, 1885 (1988)] postulated a spin-density-wave ground state for Pb that leads to directions of strongly reduced gap. By applying their idea to the actual Fermi surface of Pb, we predict phonon attenuation directions consistent with the data.

K. E. O'Hara, L. Ó Súilleabháin, and J. P. Wolfe
"Strong nonradiative recombination of excitons in Cu₂O and its impact on Bose-Einstein statistics." Physical Review B 60, 10565-10568 (1999).
Abstract: We have reproduced the spectroscopic evidence for Bose-Einstein statistics of excitons in Cu2O under a variety of conditions. However, our current measurements of the volume of the gas, its brightness, and the dependence of the energy distribution on crystal temperature, are inconsistent with a homogeneous gas of density sufficient to reach the quantum-statistical regime.

J.T. Warren, K.E. O'Hara, and J.P. Wolfe
"Two-body decay of thermalized excitons in Cu₂O." Physical Review B61, 8215-8233.
Abstract: We have examined the decay of thermalized excitons in cuprous oxide (Cu2O) and determined their lifetime against two-body decay (i.e., Auger recombination). The experiments are conducted at T = 70 K with near-resonant picosecond excitation to ensure a thermal equilibrium between orthoexcitons, paraexcitons, and the crystal lattice. Time-resolved spectroscopy reveals the gas reaching equilibrium with the lattice temperature in 0.5 ns. The wavelength of the excitation photons and the spatial distribution of the laser beam are selected to produce a well defined spatial distribution of excitons. Time-resolved photoluminescence imaging measures the diffusion of excitons. From absolute measurements of the gas volume and the luminescence intensity, we determine the instantaneous gas density. At high excitation levels, a rapid nonexponential decay of the excitonic gas is observed. The decay curve is well explained by assuming that the local exciton density is governed by the rate equation dn/dt = -An2-n/, with an Auger constant A = 0.6×10-16 cm3/ns and a residual decay time = 300 ns. This value of the Auger constant is comparable to that estimated previously for a nonthermalized exciton gas at a lattice temperature of 2 K, indicating that the Auger lifetime of an exciton is only weakly dependent on its kinetic energy. The Auger process characterized here defines the practical limits for exciton densities in cuprous oxide.

Hauser MR. Gaitskell R. Wolfe JP.
"Imaging nonequilibrium phonons in Nb." Physica B 263-264, 87-89 (1999).
Abstract: Phonon imaging techniques are applied to superconducting Nb. Due to the superconducting energy gap, the electronic scattering of phonons in metals is greatly reduced and ballistic phonon transport is observed. The non-equilibrium phonons which are generated in the experiment are accompanied by non-equilibrium quasiparticles. These quasiparticles modify the observed phonon propagation and also recombine to generate additional phonons which are detected. Thus, our experiments provide information not only about phonon propagation in a Nb lattice, but also about the phonon-quasiparticle interactions and the dynamics of the non-equilibrium quasiparticles. (6 References).

Wolfe JP.
Imaging phonons. Acoustic wave propagation in solids. (Cambridge, UK, Cambridge University Press, 1998) pp.xiii+411.
Abstract: The book considers the vibrational properties of crystalline solids, elucidated by new imaging techniques. From the megahertz vibrations of ultrasound to the near-terahertz vibrations associated with heat, the underlying elastic anisotropy of the crystal asserts itself. Phonons-the elementary packets of heat display unique patterns of "caustics". The book is of interest to graduates and researchers in condensed matter physics, materials science, and acoustics. (486 References).

Kim JC. Wolfe JP.
"Bose-Einstein statistics of an excitonic gas in two dimensions: Excitons and biexcitons in a GaAs quantum well." Physical Review B 57, 9861-9868 (1998).
Abstract: Intense photoexcitation of a GaAs quantum well produces a dense gas of free excitons that can pairwise combine to form biexcitons. At sufficiently high density where interparticle spacing is comparable to the thermal deBroglie wavelength, such a two-component gas confined to two-dimensional motion may exhibit quantum-statistical behavior. In this paper, we theoretically show how quantum statistics modifies the equilibrium-density relationship between excitons and biexcitons from that of a classical square law. We also experimentally examine a gas of excitons and biexcitons in GaAs quantum wells and find the predicted signature of Bose-Einstein statistics: a saturation of the exciton density with a continued growth of the biexciton density, as the pair density is increased. For a two-dimensional excitonic gas at a temperature of 5 K inside a 100-AA GaAs quantum well, the calculated pair density at the onset of excitonic saturation is only about 1*10/sup 11/cm/sup -2/, a density readily attained by photoexcitation with 5-ps laser pulses focused to a 3- mu m spot. Concurrent with the saturation behavior with increasing density, we observe a gradual broadening and blueshifting of the luminescence peaks, indicating the onset of many-particle effects. Also, the deduced ratio of the total radiative rate of a biexciton to that of an exciton is considerably smaller than what might be expected from simple kinetic arguments. Thus, it seems that a rigorous understanding of the spectral line shapes and luminescence intensities is needed to support our interpretation of Bose-Einstein statistics in this system. (26 References).

Msall ME. Wolfe JP.
"Phonon production in weakly photoexcited semiconductors: quasidiffusion in Ge, GaAs, and Si." Physical Review B 56, 9557-9564 (1997).
Abstract: Nonequilibrium phonon propagation is observed in Ge, Si, and GaAs at T=2.2 K when these materials are weakly photoexcited with a pulsed Ar/sup +/ laser (typically a defocused 10 ns, nanojoule pulse). The overall shapes of the experimental heat pulses agree well with computer simulations incorporating anisotropic phonon propagation (phonon focusing effects), isotope scattering, and anharmonic decay. Quantitative comparisons between the experimental and theoretical decay rates lead us to conclude that (1) although surface reflections near the excitation point play a key role in determining the character of the detected phonon energy, reflections from the sample sidewalls appear to have a smaller effect at times of interest in the experiment; (2) the agreement between the experimental data and our theoretical models of quasidiffusion is better for silicon than for germanium or gallium arsenide. These moderate discrepancies (factors of about 2 in decay rates) are attributed to uncertainties in the elastic-scattering rate and/or anharmonic decay rate. Overall, the basic model of quasidiffusive phonon propagation is quite successful in predicting the principle features of nonequilibrium phonon propagation following direct photoexcitation of a semiconductor. (14 References).

Wolfe JP. Vines RE.
"Acoustic wavefront imaging of carbon-fiber/epoxy composites." IEEE Ultrasonics Symposium Proceedings (Cat. No.96CH35993). IEEE. Part vol.1, 607-14 (1996).
Abstract: Acoustic wavefront imaging using point-focus immersion transducers has enabled us to observe propagation of acoustic waves in anisotropic solids. This technique has been useful for studying both bulk-wave and surface-wave propagation in a variety of materials. By using line-focus transducers, we can image surface acoustic wave propagation based on wave-vector, rather than group-velocity, direction. In this paper we apply these techniques to carbon-fiber/epoxy composite materials. (9 References).

Tamura S. Vines RE. Wolfe JP.
"Effects of liquid loading on surface acoustic waves in solids." Physical Review B 54, 5151-5163 (1996).
Abstract: We study the effect of liquid loading on the characteristics of surface acoustic waves propagating along the (100) surface of silicon. A nonviscous liquid layer of infinite thickness in contact with a solid substrate causes changes in the wave velocity and the attenuation of surface waves along the liquid-solid boundary. More importantly, we examine an additional branch of the Rayleigh surface that is induced by the liquid loading. We also find that the pseudosurface wave, which is restricted only to certain directions on the free surface, extends to all directions on the liquid-loaded (100) surface. Calculating the acoustic Poynting vectors associated with these surface-related acoustic modes, we find that the Rayleigh wave and the induced Rayleigh wave emit energy only into the liquid, whereas the pseudosurface wave emits energy both into the liquid and solid substrate. (24 References).

Yoon HW. Wake DR. Wolfe JP.
"Effect of exciton-carrier thermodynamics on the GaAs quantum well photoluminescence." Physical Review B 54, 2763-2774 (1996).
Abstract: In order to explain the power-dependent temporal behavior of the photoluminescence from free excitons in a GaAs quantum well following a short optical pulse, we consider the excitons and free carriers to be a nearly ideal gas in thermodynamic equilibrium. The temperature of the gas, which decreases in time after the pulse due to cooling by phonon emission, can be measured experimentally from the free-carrier recombination luminescence. Because only excitons with near-zero kinetic energy can luminesce, the photoluminescence intensity depends on the excitonic-gas temperature. Due to the law of mass action between excitons and free carriers, the photoluminescence intensity of excitons also depends on the density of the gas, which decays in time. Using the known binding energy of excitons, we find that the density-dependent temporal behavior of the photoluminescence is consistent with a simple thermodynamic equilibrium between excitons and free carriers. (29 References).

Msall ME. Esipov SE. Wolfe JP.
"Photo-produced phonons in semiconductors." Physica B 219-220, 738-740 (1996).
Abstract: The techniques of phonon imaging are used to study optically generated phonon sources in Si, Ge and GaAs at 1.7 K. While quasi-diffusive theory applies to all 3 systems under very weak photoexcitation, significant differences in ballistic phonon production occur under strong photoexcitation. Beginning at moderate excitation density, an extra component of low- nu phonons is observed. This component is a small fraction of the total detected phonon energy (~1% in GaAs, 5-10% in Si and Ge) but is spatially and temporally concentrated due to the phonon focusing effect. Measurements of the photoluminescence of Si and Ge verify the presence of a dense e-h liquid phase at these densities. We postulate that acoustic phonons are emitted directly by the e-h plasma in all 3 systems, bypassing the slower production via anharmonic decay which results in quasi-diffusion. For large populations of low- nu phonons coalescence processes may be initiated. This may explain the reduction in ballistic phonon production in Ge at the highest excitation levels. (4 References).

Ikehata A. Vines RE. Tamura S. Wolfe JP.
"Images of surface acoustic waves on the (110) face of cubic crystals with liquid loading." Physica B 219-220, 710-713 (1996).
Abstract: We study the effect of liquid loading on surface acoustic waves propagating on the (110) face of cubic crystals. A semi-infinite nonviscous liquid in contact with solid substrate induces new branches of Rayleigh and pseudo-surface waves which are not supported on the free surface. The group velocities are calculated and compared with ultrasound images obtained experimentally. (3 References).

Wolfe JP. Hauser MR.
"Acoustic wavefronts and internal diffraction." Physica B 219-220, 702-705 (1996).
Abstract: We have developed new imaging methods for the study of ultrasonic waves in solids. The signal from a point source of ultrasound is detected as a function of space and time, revealing the acoustic wavefront (or group-velocity surface) striking a surface of the solid. By Fourier transforming the x-y-t "data cube" resulting from a short pulse on silicon, the internal diffraction of acoustic waves is observed. (4 References).

Wake DR. Yoon HW. Wolfe JP.
"Thermodynamics of excitons and free carriers in a GaAs MQW." 22nd International Conference on the Physics of Semiconductors (World Scientific, Singapore, 1995) pp.1416-1419.
Abstract: We examine the role of thermodynamic equilibrium between free electron-hole populations and excitons for time-resolved photoluminescence in a GaAs multiple quantum well (MQW). The carrier temperature has been directly determined by band-to-band electron-hole recombination spectra. Following excitation with a 3-ps laser pulse with variable energy above the band edge of a GaAs MQW structure, we find density-dependent and temperature-dependent thermodynamic effects are needed to successfully model the time-dependent behavior of the exciton luminescence intensity. The carrier-density dependence is anomalous in this context, but the delay in peak luminescence intensity with increasing carrier density is attributed to slower cooling due to LO phonon reabsorption at high carrier density. (8 References).

Kim JC. Wake DR. Wolfe JP.
"Thermodynamics of biexcitons in a GaAs quantum well." 22nd International Conference on the Physics of Semiconductors (World Scientific, Singapore, 1995) pp.1368-1371.
Abstract: For GaAs multiple quantum wells, emission from radiatively recombining free excitons dominates the photoluminescence at low temperature and moderate pair-density. At high pair-density, a new component appears just below the lowest (heavy-hole) excitonic emission. Previous studies have attributed this feature to the biexciton because it appears below the exciton line and grows super-linearly with respect to the exciton intensity. In this paper, we considerably strengthen this identification by analyzing time- and space-resolved photoluminescence data that imply a dynamic chemical equilibrium between the excitons and biexcitons, i.e., a law of mass action. We find, as the excitons and biexcitons decay, the law of mass action is verified over 2 orders of magnitude in pair-density. (8 References).

Yoon HW. Wake DR. Wolfe JP.
"Field modulated diffusivity of excitons in coupled asymmetric quantum wells." 22nd International Conference on the Physics of Semiconductors (World Scientific, Singapore, 1995) pp.1091-1094.
Abstract: Using time- and space-resolved photoluminescence spectroscopy, we have measured the lateral in-plane transport of spatially direct and indirect excitons in biased coupled asymmetric GaAs quantum wells. At 4.2 K, spatial transport measurements at zero electrical bias show that the direct excitons become localized (immobile) and display an 0.8 meV spectral shift to lower energies. With increasing bias, the localization is reduced and eventually disappears under high bias as the excitons increasingly become spatially indirect. At higher bath temperatures, we find that the spatial localization is absent and the exciton peak does not shift to lower energies. Furthermore, the spatial transport properties are not affected by the changes in the field bias. We discuss our results in terms of interface-roughness scattering arising from the different interfacial qualities on either side of the GaAs quantum well. (7 References).

Kavoulakis GM. Baym G. Wolfe JP.
"Quantum saturation and condensation of excitons in Cu₂O: a theoretical study." Physical Review B 53, 7227-43 (1996).
Abstract: Recent experiments on high density excitons in Cu₂O provide evidence for degenerate quantum statistics and Bose-Einstein condensation of this nearly ideal gas. We model the time dependence of this bosonic system including exciton decay mechanisms, energy exchange with phonons, and interconversion between ortho (triplet-state) and para (singlet-state) excitons, using parameters for the excitonic decay, the coupling to acoustic and low-lying optical phonons, Auger recombination, and ortho-para interconversion derived from experiment. The single adjustable parameter in our model is the optical-phonon cooling rate for Auger and laser-produced hot excitons. We show that the ortho excitons move along the phase boundary without crossing it (i.e., they exhibit a "quantum saturation"), as a consequence of the balance of entropy changes due to cooling of excitons by phonons and heating by the nonradiative Auger two-exciton recombination process. The Auger annihilation rate for para-para collisions is much smaller than that for ortho-para and ortho-ortho collisions, explaining why, under the given experimental conditions, the para excitons condense while the ortho excitons fail to do so. (29 References).

Vines RE. Hauser MR. Wolfe JP.
"Imaging of surface acoustic waves." Zeitschrift fuer Physik B 98, 255-271 (1995).
Abstract: A new experimental method has been devised that directly determines the group velocities of surface acoustic waves. A point source and a point detector are employed to measure the ultrasonic transmission across a solid surface as a continuous function of the propagation direction. Results for single pulses give the times-of-flight for both Rayleigh surface waves (RSW's) and pseudo-surface-waves (PSW's). Calculations and measurements of the group velocities of the surface waves on silicon show some unanticipated behavior: fluid loading qualitatively changes the group velocity curves for both RSW and PSW. In particular, the RSW branch gains an additional component which we denote here as an induced Rayleigh wave (IRW). If a wave train is employed in the experiment, the analog of phonon focusing is observed for the ultrasonic waves, modified by "internal-diffraction" effects. Systematic measurements of the wave intensities on silicon as a function of propagation distance are consistent with expected acoustic losses into the surrounding water: the attenuation length of a wave depends on the mode and frequency. A survey of surface-wave images on other crystals is included in this study. (20 References).

Yoon HW. Wake DR. Wolfe JP. Salvador A. Morkoc H.
"Field-modulated diffusivity of excitons in coupled asymmetric quantum wells." Physical Review B 51,17689-17697 (1995).
Abstract: Using time- and space-resolved photoluminescence spectroscopy, we have measured the lateral in-plane transport of spatially direct and indirect excitons in coupled asymmetric GaAs quantum wells. The quantum wells were grown in the intrinsic region of a p-i-n structure to allow the application of an electric field normal to the growth plane. At 4.2 K, spatial transport measurements at zero electrical bias show that the direct excitons become localized (immobile) and display a 0.8-meV spectral shift to lower energies. With increasing bias, the localization is reduced and eventually disappears as the excitons increasingly become spatially indirect. At higher bath temperatures (20-60 K), we find no spatial localization, nor does the exciton peak shift to lower energies. Furthermore, the spatial transport properties are not affected by the changes in the field bias. We discuss our results in terms of interface-roughness scattering arising from the different interfacial qualities on either side of the GaAs quantum well. (25 References).

Author Ramsbey MT. Wolfe JP.
"Dispersive phonon imaging of InAs." Zeitschrift fuer Physik B 97, 413-424 (1995).
Abstract: The phonon-focusing patterns of ballistic phonons in InAs are measured in the frequency range 0.1 to 1 THz, in an effort to test the global validity of lattice dynamics models for this semiconductor. Phonon caustic patterns depend sensitively on the shapes of constant frequency surfaces. Several tunnel-junction detectors with sensitivity onsets in this frequency range are used to measure dispersive shifts in the phonon caustics. The measured caustic positions are compared to those predicted by rigid-ion and bond-charge models. Similar to the case of InSb studied by Hebboul and Wolfe, a 6-parameter bond-charge model (BCM) - with force constants determined by neutron, X-ray, and Raman scattering - reproduces the phonon-imaging data both qualitatively and quantitatively. Comparisons of the focusing patterns with an 11-parameter rigid-ion model (RIM) do not show good agreement. New structures are predicted in the phonon-focusing patterns at frequencies above about 1.2 THz-presently outside our experimental range-which are highly sensitive to the theoretical modeling. (32 References).

Wolfe JP. Hauser MR.
"Acoustic wavefront imaging." Annalen der Physik 4, 99-126 (1995).
Abstract: We introduce a new class of experiments which provide graphic insights into the propagation of acoustic waves in anisotropic media. Simply stated, we have devised a means of observing the expanding acoustic wavefront from a point disturbance in a solid. The data may be viewed as a movie or a series of snapshots. The observed wavefronts represent the group-velocity surfaces of acoustic waves, which reflect the basic elastic anisotropy of the solid. The technique has been applied to coherent acoustic waves with frequencies in the megahertz range (at ambient temperatures) and to incoherent heat pulses in the hundred-gigahertz range (at liquid-helium temperatures). In this article, we first provide a pedagogical introduction to wave propagation in elastically anisotropic media, reviewing some early methods for visualizing acoustic waves. Next, we describe the "acoustic wavefront imaging" method and give representative results in crystals and composite materials. Finally, we show how this method relates to recent advances in phonon imaging and internal diffraction of ultrasound. (32 References).

Author Vines RE. Tamura S-I. Wolfe JP.
"Surface acoustic wave focusing and induced Rayleigh waves." Phys. Rev. Lett. 74, 2729-2732 (1995).
Abstract: We introduce a new method for observing the anisotropic propagation of surface acoustic waves on solids. Point source and point detection of ultrasonic waves, scanned as a function of propagation angle, give the group velocities and intensities of Rayleigh and pseudosurface waves. Group-velocity focusing and internal diffraction are observed for Si, and a new Rayleigh mode, induced by water loading by the surface, is discovered. Such effects are predicted for most anisotropic solids. (10 References).

Kim JC. Wake DR. Wolfe JP.
"Thermodynamics of biexcitons in a GaAs quantum well." Physical Review B 50, 15099-15107 (1994).
Abstract: Photoluminescence from GaAs quantum wells displays sharp spectral features associated with the free exciton. At high excitation density and low temperature, an additional component appears just below the lowest (heavy-hole) excitonic emission. Previous studies have attributed this feature to the biexciton, consisting of two electrons and two holes, because it appears below the exciton line and grows superlinearly with respect to the exciton intensity. In this paper, we considerably strengthen this identification by analyzing time- and space-resolved photoluminescence data that imply a dynamic chemical equilibrium between the excitons and biexcitons, i.e., a law of mass action. This thermodynamic (complementary to spectroscopic) evidence for biexcitons is based on the interdependent temporal decay and spatial transport behavior of the two components on a picosecond time scale. As the excitons recombine, the law of mass action describes the equilibrium over more than two orders of magnitude in exciton density as determined from the measured photoluminescence intensity and volume of the exciton gas. (16 References).

Shields JA. Wolfe JP.
"Phonon scattering from residual defects in GaAs: observation of optically induced metastability by phonon imaging." Physical Review B 50, 8297-8304 (1994).
Abstract: Phonon imaging with frequency-selective detectors is used to probe the scattering from residual defects in semi-insulating GaAs. As in previous heat-pulse studies, we find that the transmission of nonequilibrium phonons with longitudinal and transverse polarizations can be modified by near-infrared excitation of the crystal at low temperatures. These effects have been attributed to metastable states of the ubiquitous EL2 defect in liquid-encapsulated-Czochralski-grown GaAs. In the present work we use the dispersive shifts in the phonon-focusing pattern to gain information about the frequency dependence of these optically induced changes. For particular polarizations and propagation directions, near-infrared excitation causes a huge decrease in the scattering of phonons with frequencies above 700 GHz. Our results show that, unlike the elastic scattering of simple mass defects, the residual defects are effective inelastic scatterers of phonons. An analysis of our data based on the theory of Nowick and Berry indicates that the change in the transmission of phonons following photoexcitation cannot be attributed to a unique symmetry change of a single defect. (16 References).

Ramsbey MT. Szafranek I. Stillman G. Wolfe JP.
"Optical detection and imaging of nonequilibrium phonons in GaAs using excitonic photoluminescence." Physical Review B 49, 16427-16440 (1994).
Abstract: We have developed a time- and space-resolved optical detector of nonequilibrium phonons utilizing excitonic photoluminescence at low temperatures. An epilayer of GaAs is "sensitized" by excitation with a low-power, focused "probe" beam, which creates free excitons (FE's) and bound excitons (BE's) in the layer. The photoluminescence intensity from these species is reduced by a flux of nonequilibrium phonons, created on the opposite side of the GaAs substrate by optical excitation of a metal film with a "pump" beam. A simple model is proposed for the phonon-induced change in photoluminescence intensity of the FE's; previous work (e,g., by Blank et al. (Sov. Phys. Semicond. 25, 39 (1991))) has treated the case of BE's. Both time-resolved heat pulses and space-resolved heat flux associated with phonon focusing are observed. The heat pulses are characterized by a broad temporal distribution, the exact origin of which is not determined. If the sensitivity of this detection technique could be raised sufficiently, space and time resolutions of 1 mu m and 1 ns, respectively, would be feasible. (76 References).

Esipov SE. Msall ME. Cabrera B. Wolfe JP.
"Ballistic phonon emission from electron-hole droplets: application to the nuclear recoil problem." Journal of Low Temperature Physics 93, 377-385 (1993).
Abstract: The production of phonons after photoexcitation of Si is greatly influenced by electron-hole droplets. At moderate excitation densities droplets are formed and the process of quasidiffusion is bypassed by emission of ballistic phonons from droplets. We review these ideas and discuss the possibility of droplet formation in the course of plasma cooling which follows the ionization process of a nuclear recoil. (14 References).

Weilert MA. Msall ME. Anderson AC. Wolfe JP.
"Phonon scattering from ferroelectric domain walls: phonon imaging in KDP." Physical Review Letters 71, 735-738 (1993).
Abstract: The ferroelectric domain wall is an ideal interface for studies of the boundary scattering of high frequency acoustic phonons. These internal interfaces have no contamination and can be reversibly created and destroyed by application of an electric field at the phase transition-122 K in KDP. Images of phonons in single- and multiple-domain cases show remarkable differences in the ballistic heat flux. Theoretical modeling based on acoustic wave reflection reveals that the domain walls act as nearly perfect grain boundaries. (14 References).

Lin JL. Wolfe JP.
"Bose-Einstein condensation of paraexcitons in stressed Cu₂O." Physical Review Letters 71, 1222-1225 (1993).
Abstract: Reducing the multiplicity of the orthoexciton ground state in Cu/sub 2/O by applying uniaxial stress greatly increases the quantum degeneracy of the exciton gas produced by intense pulsed excitation. Simultaneously, the paraexciton luminescence spectrum develops an extra component at low energy which is interpreted as a Bose-Einstein condensate of paraexcitons. (15 References).

Shields JA. Msall ME. Carroll MS. Wolfe JP.
"Propagation of optically generated acoustic phonons in Si." Physical Review B 47, 12510-12526 (1993).
Abstract: This paper deals with the spectral and spatial distributions of nonequilibrium acoustic phonons produced by optical excitation of a high-purity Si crystal at low temperatures (T<2 K). The authors report the observation of a quasidiffusion process in Si, resulting from anharmonic-decay and elastic-scattering processes. They show that the observation of quasidiffusive propagation was previously masked by the losses of high-frequency phonons into the helium bath. By removing the contact of the helium bath from the excitation surface, they have quantified the effects of the bath on the detected heat pulses. As the power density of the optical pulses is increased, qualitative changes occur in the shapes of the heat pulses, and at high density, the quasidiffusive propagation is bypassed by the emergence of a localized source of low-frequency phonons. The threshold density for the formation of the localized source is far below that calculated for a 'hot spot' based on phonon-phonon interactions. The authors postulate that the photoexcited carriers, largely in the form of electron-hole droplets, are playing a dominant role in determining the frequency distribution of emitted phonons. Their experiments employ a wide variety of techniques to characterize the propagation of nonequilibrium phonons in silicon: phonon imaging is used to gauge the size and lifetime of the phonon sources, as well as indicate the frequency distribution of the detected phonons. Comparison is made between direct photoexcitation of silicon and optical excitation of a metal film deposited on the silicon surface. These experiments and Monte Carlo simulations give insight into the diffusion of high-frequency phonons near the interface. The occurrence of a helium bubble at a point of high excitation is shown to have a marked influence on the detected heat pulses. (34 References).

Weilert MA. Msall ME. Wolfe JP. Anderson AC.
"Mode dependent scattering of phonons by domain walls in ferroelectric KDP." Zeitschrift fuer Physik B 91, 179-188 (1993).
Abstract: Thermal conductivity and ballistic phonon imaging measurements in KH/sub 2/PO/sub 4/ (KDP) at low temperature (T<3 K) indicate that scattering from domain walls has a large effect on phonon transport. KDP has a ferroelectric phase transition from tetragonal to orthorhombic structure at T/sub c/=122 K. Below T/sub c/ domains of opposite electric polarization and crystal orientation form unless the sample is cooled in an electric field. Thermal conductivity measured along the (100) (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. The authors attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domain wall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission. (13 References).

Msall ME. Tamura S. Esipov SE. Wolfe JP.
"Quasidiffusion and the localized phonon source in photoexcited Si." Physical Review Letters 70, 3463-3466 (1993).
Abstract: Previous observations of heat pulses produced by localized photoexcitation of silicon do not support the predictions of phonon 'quasidiffusion' via anharmonic decay and elastic scattering. The authors experiment, with controlled boundary conditions, verify that quasidiffusive theory is relevant in Si under very weak photoexcitation. Beyond this domain a transition to a localized source of low frequency phonons is attributed to excited carrier interactions. Photoluminescence experiments confirm the presence of electron-hole droplets coincident with this localized phonon source. (14 References).

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