Nonequilibrium acoustic phonons in ZnTe at liquid-helium temperatures: Effect of impurities and structural defects on the phonon mean free path

ZnTe crystallites isolated from a druse of coarse-grained polycrystalline ZnTe, prepared by chemical vapor synthesis at a temperature of ～650°C, were studied using x-ray diffractometry, luminescence, and the heat pulse technique. The crystallites are stacked in {110}-oriented macroscopic layers with a common twin system. The {111} twin planes separated at a distance of 50–100 μm are perpendicular to the (110) growth layers. Acoustic phonon propagation was studied using the heat pulse technique. A comparison of the responses to the arrival of differently polarized phonons in a given sample with the responses obtained on high-purity coarse-grained ZnTe with randomly distributed twin systems with a separation of 5–10 μm and on twin-free single-crystal ZnTe suggests that twins radically affect the scattering of acoustic phonons. The mean free paths of LA, FTA, and STA phonons were determined by comparing the experimental responses with Monte Carlo calculations.     

Investigation of the thermophysical properties of oxide ceramic materials at liquid-helium temperatures

The main regularities in the transport of thermal phonons in oxide ceramic materials are investigated at liquid-helium temperatures. The dependences of the thermophysical characteristics of ceramic materials on their structural parameters (such as the grain size R, the grain boundary thickness d, and the structure of grain boundaries) are analyzed. It is demonstrated that, in dense coarse-grained ceramic materials with qR?1 (where q is the phonon wave vector), the grain boundaries and the grain size are the main factors responsible for the thermophysical characteristics of the material at liquid-helium temperatures. A comparative analysis of the thermophysical characteristics of optically transparent ceramic materials based on the Y₃Al₅O₁₂ (YAG) and Y₂O₃ cubic oxides synthesized under different technological conditions is performed using the proposed criterion.     

Ion-implanted buried layer in diamond as a source of ballistic phonons at liquid-helium temperatures

It is shown that an approximately 150 nm thick ion-implanted buried layer in diamond and excited by a pulsed laser at wavelength λ=337 nm is a source of nonequilibrium acoustic phonons propagating ballistically through the diamond sample at temperatures ∼2 K. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 270–272 (25 August 1996)     

X-ray induced conductivity of ZnSe sensors at high temperatures

Measurements of intrinsic conductivity and X-ray induced conductivity were performed on specially undoped ZnSe samples. The measurements demonstrated that sensors made of ZnSe have minor intrinsic conductivity when heating up to the temperature of 180 °C, and significant X-ray induced conductivity. Dose dependence “dose rate - current” is described with simple power function which considerably simplifies calibration of sensors. This results can be used during the designing of high-temperature X-ray and gamma-radiation detectors for radiation hot rolling thickness gauges which are widely used in the metallurgy.     

Phonon-dislocation scattering and phonon equilibration in metals at low temperatures

It is shown that phonon-dislocation scattering does not contribute significantly to equilibrating the phonon system for potassium. The implications of this result are discussed for the phonon-drag contribution to the low-temperature electrical resistivity.     

Kinetics of thermal phonons and the structure of nanodispersed iron-containing corundum-based cermets at liquid-helium temperatures

The kinetics of weakly nonequilibrium subterahertz thermal phonons is studied in nanodispersed iron-containing corundum-based cermets at liquid-helium temperatures. For the chosen method of fabricating cermets (which restricts grain growth), iron inclusions are shown to be described as point trapping centers of phonons. The transport of nonequilibrium phonons is analyzed in a ceramic matrix containing metallic trapping centers.     

Positron angular correlation measurements on single-crystal aluminum at various temperatures

We have tested the trapping model by comparing the temperature dependence of the peak and the tail of the angular correlation curve from single crystal aluminum. The integral over the tail decreased as the integral over the peak increased with temperature, in such a way that the same vacancy formation energy could be deduced from both integrals. Paper B6 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973). Case Western Reserve University, Cleveland, Ohio, USA     

Aharonov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge, this is the highest temperature at which the AB effect has been observed in solid-state and molecular nanostructures.     

Peripheral phonons and phonon conductivity of a metal at low temperatures

The role of the peripheral and non-peripheral phonons in the estimation of the lattice thermal conductivity of a metal has been studied at low temperatures by calculating their separate contributions towards the total lattice thermal conductivity. The study is made in the temperature range 0.4–2.5 K with the help of the Ziman expression for the scattering of phonons by the charge carriers and the Callaway expression of the phonon conductivity, and Sb is taken as an example. The separate percentage contributions due to peripheral and non-peripheral phonons have also been studied and it is found that the percentage contribution due to peripheral phonons increases with increasing temperature while the percentage contribution due to non-peripheral phonons decreases with increasing temperature. The percentage contributions of the lattice thermal resistivities due to electrons and holes towards the total lattice thermal resistivity of Sb have also been reported in the present note.     

Some properties of the renormalized harmonic phonon approximation at high temperatures

Using the Einstein and the high temperature approximation the renormalized harmonic phonon approximation yields at constant pressure a first order phase transition similar to the van der Waals phase transition. Calculation of the compressibility by different methods leads to an inconsistency of about 55%.     

Magnetization of single-crystal terbium at very low temperatures by nuclear orientation

We report an investigation of the magnetization of high-purity single crystal terbium below 50 mK, using the nuclear orientation of¹⁶⁰Tb, which substitutionally replaces naturally-occurring, monoisotopic¹⁵⁹Tb. The nuclear alignment is determined as a function of applied magnetic field, and can be related to the macroscopic magnetization of the host crystal by a suitable model. In the easy direction (b-axis), a very rapid rise is seen in the first 7 mT of applied field; this is followed by a pleateau region up to 0.1 T, then a slow saturation, completed at ca. 0.4 T. Along an a-axis, a similar rapid increase to about 50% of saturation is observed below 10 mT, followed by a slow, nearly linear increase which agrees with that calculated for domain rotation using the measured crystalline anisotropy constants.     

The paired phonon model for liquid4He at low temperatures

The paired phonon model for the one-body density matrix is studied for liquid⁴He at low temperatures within the framework of hypernetted chain theory. Analytic expressions are derived for this quantity and the condensate fraction in the uniform limit. The results reproduce correctly theT ²-dependence of the condensate fraction derived earlier by other independent methods.     

The theory of phonon relaxation in He(II) at low temperatures

We have investigated analytically relaxation processes in the phonon system in helium(II) at sufficiently low temperatures where rotons are not exited. In accordance with the recent experimental data, the phonon velocity dispersion is supposed to be positive, though small. Two different relaxation processes exist in the phonon system in this case: (i) the fast longitudinal relaxation establishing equilibrium phonon distribution along each direction in the momentum space with the temperature and the drift velocity depending on the direction; (ii) the slow transverse relaxation setting up equilibrium between different directions.Using the energy and momentum conservation and general principles of the irreversible thermodynamics we have derived the expression for the transverse relaxation operator. It appears to be a differential operator of the fourth order and depends on a function of the “phonon temperature” Θ that cannot be determined from the general consideration. We have calculated this function for the case of three-phonon collisions.Physical properties of the transverse relaxation operator are discussed and the corresponding boundary conditions are formulated. Several typical physical problems, both linear and nonlinear, which can be formulated in terms of the transverse relaxation operator, are enumerated. With the help of the diagram method the contribution of multiphonon collisions both in the longitudional and in the transverse relaxation is evaluated.     

Radiation-induced formation of ZnO nanoparticles on the ZnSe single-crystal surface

The possible formation of ZnO nanocrystals was studied as a result of radiolysis of a ZnSe crystal surface exposed to zinc vapor and irradiated with gamma rays and in producing ZnSe-ZnO heterostructures. Under ⁶⁰Co gamma radiation in air, nanocrystals ～27 nm in size are formed from nanoscopic ZnO nuclei. Under a mixed flux of gamma rays and thermal-neutron radiation, a twin structure is formed in the host ZnSe lattice and ZnO is removed. The oxide layer is also destroyed under proton irradiation in vacuum. It is found that the growth of ZnO nanocrystallites causes a manyfold increase in the luminescence intensity in the ～600-nm band and in microhardness and also a decrease in the resistance and blocking and threshold voltages irrespective of polarity. Thus, gamma irradiation brings about the formation of light-emitting ZnSe-ZnO: Zn semiconductor structures with a p-n junction.