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We study the external vibrations of the A₃C₆₀ fullerides with F_m3m structure, where the A are alkali-metal elements. It is argued that strong screening occurs in these crystals and the force constants extending to a few shells of neighbours are needed in numerical simulations. By means of the force-constant matrix approach, we have simulated the dispersion curves along three principal symmetry directions.

Upon the application of an electric field in a superconducting film device, the drain source resistivity increases and when the polarity of the electric field is reversed, it decreases; the oxygen ions move from the occupied to the empty sites thereby creating a finite dipole moment owing to pinning which contributes to the carrier concentration and hence to the conductivity of the system. The resistivity is quantized and depends inversely on the dipole moment. The calculated results are in good agreement with the experimental measurements in a YBa₂Cu₃O_{7- delta} film device.

The permittivity tensor of low-symmetry crystals could be essentially anisotropic in the soft-X-ray region if the wavelength is close to some atomic core level absorption edge. Simple Fresnel's formulae are generally invalid in this case for calculations of the X-ray reflectivity, and a number of effects well known in crystal optics occur. The method of permittivity tensor calculations near the K absorption edge is proposed in the present paper. The method is used in order to study the dependence of the X-ray reflectivity fine structure on the polarization and orientation of the incident wave in the case of a uniaxial crystal and small glancing angles. It is shown that, using such dependences, one could obtain complete information about the atomic scattering factor tensor in the crystal in the anomalous dispersion region. The X-ray reflectivity fine structure for different faces of a hexagonal BN crystal has been calculated and compared with experiment. The results obtained have shown evidence of strong crystal optic effects in the case treated.

Lithium-borate glasses containing 0.1-10 mol% Gd₂O₃ have been studied by ¹⁵⁵Gd Mossbauer spectroscopy. Isomer shifts near 0.6 mm s^-1, independent of the Gd concentration, are in the range typical of ionic Gd compounds. The spectra are very well described by a distribution of quadrupole splittings corresponding to a random distribution of ligand coordinations. Additional temperature-independent broadening of the resonance lines is ascribed to effects of spin-spin relaxation of Gd moments.

It has been shown that insulating LiH and LiD transform gradually to a semiconducting state at about 30 GPa and room temperature. It was also found that these compounds undergo a semiconductor-to-semimetal transition at about 43 GPa. The observed phenomena can probably be explained by a first-order phase B1-to-B2 structure transition.

We propose a new method to describe the interacting Bose gas at zero temperature. For the three-dimensional system the correction to the ground-state energy in density is reproduced. For a two-dimensional dilute Bose gas the ground-state energy in the leading order in the parameter mod ln p alpha ² mod ^-1 where alpha is the scattering length is obtained.

We have investigated the electronic and atomistic structure of stoichiometric and As-rich GaAs by treating this material as a ternary alloy of fourfold-coordinated As⁴⁺, Ga³⁺ and threefold-coordinated As³⁺. Using the configurational averaging technique of Verges we have calculated the density of states (DOS) of this material, not only in terms of the stoichiometric index x and proportion of As³⁺ atoms alpha , but also in terms of the short-range order (SRO) parameters. By correlating with experiment we obtain valuable information about the SRO. In particular, we have shown that compensation occurs at a value of alpha that is a function of x only. Using this value of alpha and assuming that less than 1% of As⁴⁺-As⁴⁺ bonds occur (all other As-As bonds being allowed) we were able to (i) reproduce the EXAFS data and the variation of the optical gap with x and (ii) account for the observed shoulder in the lowest peak of the valence band and the subsidiary shell of As neighbours of Ga. Finally, at other values of the SRO parameters, we find features in the DOS that could explain some experiments on other amorphous III-V semiconductors.

We study the dynamics of non-adiabatic Holstein-type impurities embedded in an infinite linear chain. The impurities are modelled as Einstein oscillators coupled to specific sites of an infinite one-dimensional tight-binding host. We present numerical evidence providing bounds for the onset of self-trapping that depend critically on the initial conditions of the oscillators. We show that, in general, small but finite oscillator masses do not substantially change the self-trapped character of the states. For intermediate as well as large oscillator masses self-trapping can still occur for some initial oscillator preparations.

The high-momentum components of the electron and positron Bloch waves in the three alkali metals Li, Na, and K have been studied by comparing calculations employing the linear muffin tin orbital method and the independent particle model with accurate measurements of the two-dimensional angular correlation of annihilation radiation. It is found that such an analysis provides a sensitive test of electronic structure, and that electron-positron correlation effects do not distort the high-momentum components in this example in any significant way. It is shown that thermal effects due to Debye-Waller damping are important.

We compare the anticrossing of the spin and cyclotron resonances in spin-polarized and non-polarized phases of a two-dimensional electron gas subjected to a strong tilted magnetic field. The spin-orbit coupling splits these resonances into three lines with the gaps between them exactly equal to delta _2n+1=V_so rho _F and delta _2n=v_so rho _F/ square root 2 at odd- and even-integer factors, respectively. The 1/ square root 2 factor difference between the gaps delta _2n and delta _2n+1 arises from the existence of two collective spin-wave-density modes in the polarized phase of interacting electrons in the system with two filled Landau levels. This allows us to predict a new type of spectral Shubnikov-de Haas oscillations, which indicates the re-entrance of the system into the spin-polarized state.

The interaction of two radio waves with noticeably different frequencies omega and omega ₁ ( omega >> omega ₁) inside a tungsten single-crystal plate at liquid-helium temperature have been studied. The magnetodynamic mechanism of non-linearity owing to the existence of the high-frequency wave with a large amplitude H has been realized. The low-frequency surface impedance Z was analysed as a function of H and DC magnetic field H. The hysteresis of Z(H) was found and studied. The origin of this non-linear phenomenon which has a threshold character is connected with the generation of the current states.

The electrical conductivity of semiconducting vanadium tellurite glasses containing bismuth oxide is reported in the temperature range 80-450 K. The experimental results are interpreted in terms of polaron-hopping theories. The results suggest that the addition of bismuth oxide to the binary vanadium tellurite glasses changes the nature of the hopping mechanism from non-adiabatic to adiabatic.

We report magnetic susceptibility measurements on single crystals of very-high-purity VSi₂, NbSi₂ and TaSi₂ from 4 K to room temperature. VSi₂ is paramagnetic while NbSi₂ and TaSi₂ are diamagnetic. A systematic anisotropy of chi is observed for the three compounds. The results are in good agreement with previous investigations of the electronic properties of these materials. The different contributions of chi which account for these data are discussed.

A new model of strong itinerant ferromagnetism, taking into account the presence of a narrow density-of-states peak at E_F, which is due to merging of van Hove singularities, is proposed. The states of the peak (e_g-type states for Fe) are treated within the framework of the Hubbard model with strong correlations. The role of the non-quasi-particle states, which lie near E_F and are not described by the theory of the ferromagnetic Fermi liquid, is investigated. In particular, experimental data on the spin polarization of conduction electrons and T-dependence of resistivity are discussed.

An ideal superparaelectric is treated as an ensemble of independent, coherently polarizing regions, of linear dimension lambda , each of which behaves as a 'Devonshire ferroelectric', with transition temperature T₀, and local polarization, P_s approximately (T-T₀)¹2/. For sufficiently small lambda , the direction of the local polarization vector can fluctuate with thermal energies, giving rise to a static permittivity, epsilon _s, which follows a modified Curie law: E_s approximately (T-T₀) lambda ³/T. In addition, the peak in permittivity at T₀ is suppressed due to spatially uniform thermal fluctuations in the magnitude of the local polarization. The activation energy for the directional fluctuations increases with decreasing temperature: E_a approximately (T-T₀)² lambda ³, giving rise to Debye-type relaxation and peaks in the real and imaginary permittivity around T_m, where T₀-T_m increases with decreasing lambda . For a fictitious superparaelectric, based on Pb(Zr_0.7Ti_0.3)O₃, the effects described above become important for lambda <or=g 15 nm. Relaxor-like frequency dependence of the imaginary part of the permittivity, epsilon '', is only observed when distributions of lambda , width Delta lambda , are introduced. The best qualitative match to typical relaxor behaviour is seen when both lambda and Delta lambda diverge at some non-zero temperature. The introduction of dipolar coupling, in the form of a mean field, produces a transition to a macroscopic ferroelectric state in the static properties, at a temperature which increases with increasing coupling strength. For sufficiently strong coupling, a spontaneous transition may be apparent in observable time scales; otherwise the system remains glassy.