Self-modulation processes in a low-current rare-gas discharge at low pressures

Experimental investigations were made of self-modulation processes in a striated rare-gas discharge at low pressures and currents. It was found that interrelated self-modulation oscillations of the discharge current and of the voltage across the tube can develop in the discharge circuit. Depending on the resistance in the external circuit, either current or voltage self-modulation occurred. An important factor is that the transition from one form of self-modulation to the other does not produce quantitative changes in the spatial modulation of the time-averaged plasma luminescence on the discharge axis. It is shown that a qualitative interpretation of the observed phenomena can be given in terms of an equivalent striation voltage source in the anode region which exists because of interaction between traveling kinetic ionization waves and the discharge region near the anode, and also in terms of processes of a capacitative nature near the electrodes. State University, St. Petersburg. Ukhtinskii Industrial Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 103–108, January, 1997.     

Phonons and photons in crystals

The simultaneous propagation of phonons and photons in an insulator is discussed from both phenomenological and quantum mechanical points of view. A phenomenological form of the energy of an insulator is first supposed from which is obtained the equation of propagation of modes involving nuclei displacements and an electric field. This equation is then studied, mainly in the vicinity of q=0, in order to show how various limits lead to different types of propagation. The phenomenological equations are then justified from a microscopic point of view. The proof goes in two steps. A linear screened response function of the electrons in a solid is first assumed; one then proves that all the coefficients entering into the phenomenological equations may be obtained from the sole knowledge of this response function and of the charge of the nuclei. The existence of the response function is then justified from a many-body point of view. Finally, the necessary relations between the phenomenological coefficients are proved. Some other possible applications of the microscopic equations are also discussed at the end of the paper.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Phonons in mixed crystals

Dispersion relations for the optic and acoustic phonons in mixed crystals are derived as a function of concentration. The dispersion curves show localized as well as band modes. It is shown that our theory leads to the MREI model of Chang and Mitra when the wave vector q = 0.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Controlling electron-phonon interactions in graphene at ultrahigh carrier densities

We report on the temperature dependent electron transport in graphene at different carrier densities n. Employing an electrolytic gate, we demonstrate that n can be adjusted up to 4 × 10(14) cm(-2) for both electrons and holes. The measured sample resistivity ρ increases linearly with temperature T in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As T decreases, the resistivity decreases more rapidly following ρ(T) ～ T(4). This low temperature behavior can be described by a Bloch-Grüneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature Θ(BG) defining the crossover between the two distinct regimes, and show that, for all n, ρ(T) scales as a universal function of the normalized temperature T/Θ(BG).     

Role of the third-order elasticity modulus in stabilizing Na crystals at high pressures

Born’s criterion of crystal stability with respect to small variations of homogeneous deformations is formulated in a quasi-harmonic approximation. It is shown that the third-order Landau potential with respect to a tensor’s components of Lagrangian deformation is sufficient for predicting the critical pressure at which the cubic structure becomes unstable. The accuracy of prediction is no worse than that of the available experimental data.     

Suppression of electron-phonon interaction in narrow-band crystals

Taking the inelastic nature of the interaction of electrons with acoustic phonons into account is crucial in crystals having a conduction band width Δɛ comparable to the maximum acoustic phonon energy . In view of this, the laws of conservation of energy and wave vector impose stringent constraints on possible electron scattering processes, and single-phonon scattering becomes impossible for . The phonon contribution to the resistance may be negligible, therefore, in narrow-band conductors. Novosibirsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, p. 78–82, August, 1997.     

Kohn anomalies and electron-phonon interactions in graphite

We demonstrate that graphite phonon dispersions have two Kohn anomalies at the Gamma-E(2g) and K-A'1 modes. The anomalies are revealed by two sharp kinks. By an exact analytic derivation, we show that the slope of these kinks is proportional to the square of the electron-phonon coupling (EPC). Thus, we can directly measure the EPC from the experimental dispersions. The Gamma-E(2g) and K-A'1 EPCs are particularly large, while they are negligible for all the other modes at Gamma and K.     

Lattice dynamics in heavy rare-gas crystals under pressure

The lattice dynamics of compressed rare-gas crystals is theoretically investigated within the ab initio approach in the framework of the Tolpygo model, which explicitly includes the deformation of electron shells in the dipole approximation. The phonon frequencies of compressed rare-gas crystals are calculated with allowance made for the electron-phonon interaction at the mean-value points with the use of the dynamic matrix constructed with the ab initio short-range repulsive potential. The energy of zero-point vibrations and the heat capacity of compressed krypton and xenon face-centered cubic crystals are calculated in the harmonic approximation. The calculated temperature dependences of the specific heat capacity and the Debye temperature are in good agreement with the data available in the literature on the experiment at zero pressure and the results of the calculations within the density-functional theory for all pressures. The quantum effects, in particular, the energies E _zp of zero-point vibrations for krypton and xenon crystals, are investigated at different pressures.     

Effect of electron-phonon interactions on Raman line at ferromagnetic ordering

The theory of Raman scattering in half-metals by optical phonons interacting with conduction electrons is developed. We evaluate the effect of electron-phonon interactions at ferromagnetic ordering in terms of the Boltzmann equation for carriers. The chemical potential is found to decrease as the temperature decreases. Both the linewidth and frequency shift exhibit a dependence on temperature.     

Iron at high negative pressures

With the object of verifying the presence of a region of anomalous iron compressibility at negative pressures, as predicted by the ab initio calculations, the reflection of compression pulses from the surfaces of iron single crystals was detected. No evidence of the expected formation of rarefaction shock waves was observed in the range of attained tensile stresses up to 7.6 GPa. The breaking stresses achieved 25-50% of the theoretical iron ultimate strength for a load duration of ～10^?8 s. The dependence of breaking strength on the extension rate did not reveal any singularities in the region of assumed anomaly in iron compressibility.     

Lattice dynamics in light rare-gas crystals under pressure

The lattice dynamics of compressed neon and argon crystals has been theoretically investigated within the ab initio approach in the framework of the Tolpygo model, which explicitly includes the deformation of electron shells. The energy of zero-point vibrations, mean-square displacements, and specific heat capacities of compressed neon and argon face-centered cubic crystals have been calculated in the harmonic approximation using the dynamic matrix constructed with the ab initio short-range repulsive potential and integration over the mean-value points in the Brillouin zone. The calculated temperature dependences of the specific heat capacity and the Debye temperature are in good agreement with the data available in the literature on the experiment at zero pressure. The role of zero-point vibrations in the thermodynamics of the entire series of rare-gas crystals and, in particular, in the validity of the Lindemann melting criterion has been analyzed.     

Hypersonic-absorption and sound-speed in water at high pressures

Speed and attenuation of hypersound (frequency from 5.5 to 6.7 GHz) in water of 25°C have been measured by Brillouin scattering at pressures ranging up to 1.75 kb. The results of the experiment are in good agreement with ultrasonic data.