Low-temperature Phonon Research Articles

Low-temperature phonon, magnon and exciton effects on the polarization of BaXF4

Studies of the polarization deviation ΔP and of the dynamic pyroelectric response (I/cρ) (dP/dT) at low temperatures of the family of barium-transition metal-fluorides BaXF4, for X = Mg, Zn, Mn, Ni and Co are reported. The various effects of acoustic and optic phonon excitation, 2D and 3D antiferromagnetic ordering in the Ni and Co salts. Frenkel excitons in the Mn salt and the subtle structural transition in the Mn salt are demonstrated.

Hot electron diffusivity in silicon

Hot electron diffusion coefficients in silicon at room temperature are theoretically studied by incorporating the band non-parabolicity and the effect of the diffusion current on the distribution function. Two models of intervalley scattering are considered: in one model, coupling with high-temperature intervalley phonons only is assumed; in the other, low-temperature intervalley phonons are also included. Both the models give practically identical results and the calculated values are found to agree closely with the experiment.

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

The infrared emission of CO trapped in solid Ne and Ar is observed at low temperature. The first vibrational level of 12C 16O is excited by a Q-switched frequency doubled CO 2 laser. The emission spectrum consists of several lines arising from upper vibrational levels of 12C 16O and also of 13C 16O and 12C 18O which are present in natural abundance. An interpretation is proposed which is based on the assumption that long range dipole—dipole interaction is the main physical process involved in these experiments. Resonance energy transfer produces an energy migration among 12C 16O molecules without any change in vibrational populations. Phonon assisted energy transfer takes place between vibrational levels of the various isotopic species present in the solution. In order to satisfy the resonance condition a phonon is emitted or absorbed whose energy compensates for the energy mismatch between the transitions in each interacting molecule due to isotopic effect and or vibrational anharmonicity. The range of this process is greatly extended by energy migration. At the low phonon bath temperature phonon emission is much more probable than phonon absorption. So a strong excitation of upper vibrational levels with in some cases population inversions is observed. Molecular impurities act as efficient quenching centers even at very low concentration. When highly purified samples are used, the fluorescence decay time is found to be 20.6 ms in Ne and 14.5 ms in Ar and does not significantly depend upon concentration and temperature. It is concluded that radiationless relaxation is unimportant.

Low temperature specific heat and phonon anomalies in transition metal compounds

The specific heat of the following pairs of transition metal compounds (TMC’s) has been measured in the temperature range 1.8<T<100 K : TiN/TiC, NbC/ZrC, and TaC/HfC. Theoretical results for the lattice specific heat, obtained from phonon densities calculated on the basis of appropriate shell models agree well with the experimental data. Values for Debye temperatures and Sommerfeld coefficients are given and compared with previously published data. We show that from the difference in the lattice specific heat between the superconducting and the normal compound of each pair, it is possible to determine the average frequency of the lowest-lying phonon anomalies occurring in the superconducting TMC. For TiN, NbC, and TaC, this frequency was determined to lie at 5.8, 4.8, and 3.1 THz, respectively. These values compare favorably with results from neutron scattering experiments. Thus, specific heat measurements can serve as a valuable tool to determine the mean frequency of the phonon anomalies in the superconducting compounds of this class of materials.

The effect of electrical field of impurity centres and phonon-electron screening in phonon conductivity of III-V semiconductors at low temperatures

It is established in the present work that the decrease in low-temperature phonon conductivity of III-V semiconductors, caused by an introduction of impurities with concentration >or approximately=1017cm-3, may be explained by Kosarev's mechanism for the scattering of phonons by charge carriers in the effective electric field of the impurity centres for q>2kF, and a more consistent consideration of the screening of the phonon-electron interaction in the region of the long-wavelength phonons for Ziman scattering for q>2kF, as suggested by Crosby and Grenier (1971).

The thermopower of pure tungsten below 9 K

The thermopower of a pure crystal of tungsten (RRR 16300) has been measured at temperatures between 2 K and 9 K. The thermopower was positive over the entire range with a 0.07 mu V K-1 peak near 6.5 K. The temperature dependence may be due to a secondary low temperature phonon drag peak or to the combination of an electron-electron scattering thermopower and the tail of the primary (80 K) phonon drag peak.

Longitudinal Magnetoresistance of Pure Single-Crystal Copper

The longitudinal magnetoresistance was measured along the [100] direction of a pure single crystal of copper. The temperature dependence of the saturation ratio, $\frac{\ensuremath{\rho}(B=\mathrm{sat}.)}{\ensuremath{\rho}(B=0)}$, is given between 4 and 35\ifmmode^\circ\else\textdegree\fi{}K. A limit is placed on the diffusion approximation for small-angle electron scattering and it is suggested that the assumption of a relaxation time or of a mean free path parallel to the velocity is not valid for some types of impurity or low-temperature phonon scattering.

The influence of small-angle scattering on metallic conduction

Certain transport effects are known to be sensitive to the type of scattering responsible for resistance, so that the relaxation-time approximation becomes inadequate. It is argued that small-angle scattering, such as is caused by low-temperature phonons and extended defects, may be as effective as isotropic scattering if the disturbance of the distribution function by the applied field varies abruptly over the Fermi surface. Any scattering process between points differing greatly in their disturbed distribution, however close together in k -space, creates entropy and is an effective collision. A number of examples are discussed in qualitative terms from this viewpoint, especially the size-effect and various magnetoresistance phenomena in which the Fermi surface topology and the magnetic field together create the required conditions for abrupt variations. In the second section, several special models are analysed in detail: (1) The free-electron gas acted on by a spatially periodic electric field; the conductivity is found to differ by at most 20% between conditions of isotropic and small-angle scattering, in spite of considerable effects of the scattering function on the form of the disturbed distribution. (2) Transverse magnetoresistance in a model metal whose Fermi surface resembles that of copper, etc., in generating alternate belts of electron and hole orbits. Scattering between the belts delays saturation of the resistance, which in very strong fields may rise to a much higher value than for isotropic scattering. If open orbits are present, however, the non-saturating quadratic increase with field is diminished by small-angle scattering. (3) Transverse magnetoresistance in a model metal with a nearly free electron Fermi surface just overlapping the first Brillouin zone (e. g. Al, Mg, Zn). Small-angle scattering between zones produces effects similar to those in (2), though the analysis is quite different. In the third section, some of the scanty published data on Cu, In and Al are discussed briefly to indicate that the puzzles they present may be resolved if small-angle scattering (or equivalent effects due to mosaic structure) dominates the behaviour of ultra-pure samples. But there is need for more experiment before anything can be said for certain.

Thermal Conductivity, Second Sound, and Phonon Hydrodynamic Phenomena in Nonmetallic Crystals

A variety of phonon-gas phenomena in nonmetals are discussed in a unified manner using a set of macroscopic equations developed from the solution of the linearized phonon Boltzmann equation. This set of macroscopic equations, appropriate for the description of a low-temperature phonon gas, is solved for a cylindrical sample in the limit ${\ensuremath{\lambda}}_{N}\ensuremath{\ll}R$; ${\ensuremath{\lambda}}_{N}\ensuremath{\lambda}_{R}^{}{}_{}{}^{z}\ensuremath{\gg}{R}^{2}$. Here ${\ensuremath{\lambda}}_{N}$ is the normal-process mean free path, $\ensuremath{\lambda}_{R}^{}{}_{}{}^{z}$ is the mean free path for momentum-loss scattering calculated in the Ziman limit, and $R$ is the radius of the sample. The solution in this limit exhibits Poiseuille flow of the phonon gas as first discussed by Sussmann and Thellung. An equation for the thermal conductivity which correctly includes this phenomenon is found. Using this equation, the possible outcomes of steady-state thermal-conductivity measurements are discussed in terms of the microscopic scattering rates. Heat-pulse propagation is discussed from a similar point of view. The existence of Poiseuille flow in steady-state thermal-conductivity measurements bears directly on the possibility of observing second sound in solids. A quantitative analysis of available data on LiF suggests that the chemical purity of these samples sets very stringent limits on the observation of either of these effects. The observation of Poiseuille flow in solid ${\mathrm{He}}^{4}$ samples by Mezov-Deglin strongly suggests that this material is a prime subject for investigations of second-sound propagation.

Thermal Conductivity, Thermoelectric Power, and the Electrical Resistivity of Stoichiometric TiNi in the 3° to 300°K Temperature Range

Stoichiometric TiNi and its alloys containing Cu or excess Ni were measured to study the lattice component of the thermal conductivity. The low-temperature phonon mean-free-path is constant, depends upon solute species, and increases with solute concentration. In addition, the ideal component of the electrical resistivity shows the T2 dependence below 100°K found for the element and alloys with this same electron number. Values of the Fermi energy, effective mass, and the relative valence of the solutes are derived. Finally, one of the alloyed samples exhibits phase changes which are believed related to those seen in the pure material at temperatures above 300°K.