The temperature dependences of the absorption coefficient and phase velocity of 52-MHz ultrasonic waves in iron-doped mercury selenide crystals are studied. The presence of impurities in concentrations of about 10(19) cm(-3) is found to initiate the appearance of a resonance peak in the absorption coefficient at a temperature of about 5 K and the corresponding anomaly in the velocity of the slow transverse wave propagating in the direction. It is shown that the observed effects can be accounted for by the interaction of ultrasound with electrons in the states created in hybridization of the iron impurity donor states with the conduction band states of the crystal. A straightforward theoretical description and quantitative interpretation of these effects are proposed and used to derive data on the hybridized states, which conform with the earlier treatment of the temperature and concentration anomalies of conductivity in the crystals under study.
Nickel-impurity-induced transverse displacements of ions in a Zn1−x NixSe lattice (x = 0.0025) were detected. This type of displacement correlates with macroscopic distortions of a crystal associated with transverse ultrasonic waves that are propagated along the 〈110〉 direction. The shear instability is assumed to be due to the hybridization of the sp 3 bonds with the 3d states of the impurity centers.
The lattice dynamics of rare-gas crystals is investigated in the framework of the ab initio approach with the inclusion of the nonadiabatic effects over a wide range of pressures. The frequencies of phonons in argon, krypton, and xenon crystals are calculated at pressures p ≠ 0. Analysis of the contributions from different interactions to the lattice dynamics of the crystals demonstrates that the difference between the phonon frequencies calculated within several models is most pronounced at the boundary of the Brillouin zone. Under strong compressions, the phonon spectrum along the Δ direction is distorted and the longitudinal mode is softened as a result of the electron-phonon interaction, with the relative contribution decreasing in the series Ar, Kr, and Xe. The calculated phonon frequencies are in good agreement with the experimental data available in the literature for argon crystals at a pressure p = 3.1 GPa.
The thermal conductivity of solid parahydrogen is investigated using the stationary method with a plane sample in the temperature range 1.5–6.0 K in order to reveal a Poiseuille flow in solid hydrogen. It is established that the thermal conductivity at temperatures below the low-temperature maximum decreases very rapidly in accordance with the law K ∼ T n (3 < n < 8). This finding is a direct indication that the possibility exists of observing a Poiseuille flow in solid hydrogen. The results obtained are compared with those for solid helium, in which the Poiseuille flow was observed for the first time in dielectric solids. According to the estimates, the mean free path of phonons at a temperature of approximately 3 K exceeds the radius of a cylindrical sample (3 mm). The thermal conductivity in the vicinity of the low-temperature maximum is found to be two times higher than the value available in the literature.
Electron-ion contributions to the energy of rare-gas crystals are discussed from first principles in the framework of the Tolpygo model and its variants. The frequencies of phonons in a neon crystal at pressures p ≠ 0 are calculated in terms of models that go beyond the scope of the adiabatic approximation. Analysis of the contributions from different interactions to the lattice dynamics of the crystals demonstrates that the phonon frequencies calculated in the framework of the simplest model (allowing only for the nearest neighbors) and the most complex model (with the inclusion of the nearest neighbors, next-nearest neighbors, nonadiabatic effects, etc.) for small wave vectors are close to each other. The difference between the phonon frequencies calculated within the above models is most pronounced at the Brillouin zone boundary. Under strong compression, the phonon spectrum along the Δ direction is distorted and the longitudinal mode is softened as a result of the electron-phonon interaction. The contribution from terms of higher orders in the overlap integral S at p ≠ 0 to the phonon frequencies is more significant than that obtained in the band-structure calculations of the neon crystal.
This paper reports on the results of temperature investigations into the absorption and velocity of ultrasound in ZnSe: Ni and ZnSe: Cr crystals in the frequency range 33–268 MHz. The frequency dependence of the absorption at the maximum is analyzed, and the energy of the excited state of the Ni2+ ions is calculated. The dynamic contribution to the effective elastic modulus is determined, and the results obtained are used to construct the temperature dependences of the relaxed and unrelaxed elastic moduli.
Room-temperature Raman spectra were obtained for powder samples of Zn1−x NixSe and Zn1−y CrySe compounds and for a single-crystal Zn1−x NixSe (x = 0.0025) sample in the temperature range 5–140 K. The results obtained are interpreted in terms of large-scale lattice shear strains induced by 3d elements in these solid solutions.
The electrical resistivity of single crystal α-MnS in crystallographic directions  and  was found to be anisotropic in the temperature interval 77–300 K. The change in activation energy below the Néel temperature was determined. Magnetoresistance was revealed, and reversal of its sign in the (111) plane above the Néel point was found. The experimental data are analyzed in terms of the s-d model, with the manganese ion holes interacting with localized spins assumed to be free carriers.
This paper reports on the results of investigations into the magnetic ordering of magnets with integer spins of ions and sufficiently strong easy-plane single-ion anisotropy. It is demonstrated that magnetic ordering in the studied systems occurs through a displacive magnetic phase transition. The magnetic polarization of ionic states, which spontaneously arises at the phase transition point, acts as an order parameter in displacive magnetic phase transitions. The magnetically ordered state in the magnets under consideration is formed as a result of competition between the exchange interaction and single-ion anisotropy.
Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.
The transverse dynamic susceptibility of the 2D two-sublattice Hubbard model is calculated in the static-fluctuation approximation. The static magnetic susceptibility is studied as a function of various parameters of the system. The results for the special case of the one-dimensional Hubbard model are compared to the exact solution.
A comparative analysis is performed for three optical and electrical methods of exciting space-charge waves in photosemiconductors: (i) excitation by an external ac electric field combined with a static interference pattern, (ii) excitation by a moving interference grating, and (iii) excitation by an oscillating interference grating. It is shown that, in the case when space-charge waves are excited using a combination of all three methods, the dependence of the direct current passing through a sample on the excitation frequency exhibits two peaks that correspond to the resonant excitation of two modes of space-charge oscillations, namely, drift waves and trap recharging waves. It is noted that experimental observation of the peak attributed to the excitation of trap recharging waves should not pose any problems, whereas observation of the second peak associated with the excitation of drift waves is significantly complicated because of the small magnitude of the effect, especially for materials with a low electrical conductivity (or a long Maxwell relaxation time).
The behavior of the energy bands and the band gap width of a compressed insulator crystal is studied. The conduction band energy at the center of a face of the Brillouin zone first increases and then abruptly decreases upon an increase in compression, resulting in a collapse of the forbidden gap and in an insulator-metal (IM) transition. A model proposed for the mechanism of this transition interprets it to be a phase transition of order two and a half. The compression ratio and pressure at which an IM transition occurs in neon under pressure are predicted on the basis of nonempirical calculations of the valence and conduction bands. A simplified model suitable for calculating the metallization effect in more complex crystals is proposed.
Bulk samples of oriented carbon nanotubes were prepared by electric arc evaporation of graphite in a helium environment. The temperature dependence of the conductivity σ(T), as well as the temperature and field dependences of the magnetic susceptibility χ(T, B) and magnetoresistance ρ(B, T), was measured for both the pristine and brominated samples. The pristine samples exhibit an anisotropy in the conductivity σ∥(T)/σ⊥>50, which disappears in the brominated samples. The χ(T, B) data were used to estimate the carrier concentration n 0 in the samples: n 0ini ∼3×1010 cm−2 for the pristine sample, and n 0Br∼1011 cm\t—2 for the brominated sample. Estimation of the total carrier concentration n=n e+n p from the data on ρ(B, T) yields n ini=4×1017 cm−3 (or 1.3×1010 cm−2) and n Br=2×1018 cm−3 (or 6.7×1010 cm−2). These estimates are in good agreement with one another and indicate an approximately fourfold increase in carrier concentration in samples after bromination.
The mixed state of thin narrow superconducting films with an edge barrier placed in a transverse magnetic field is considered. The boundaries of the region for the existence of metastable mixed states with an assigned number of vortices N [H min(N)⩽H⩽H max(N)] are found. The magnetic-field dependence of the critical field is found for the films. The transition from the Meissner state to the static mixed state is discussed.