I. G. Kolobov

Electronic structure and bulk properties of MB6 and MB12 borides

Ab initio band structure calculations are carried out for the higher borides MB6 and MB12. High-precision measurements of the elastic constants are performed for the compounds ZrB12, HoB12, ErB12, TmB12, LuB12, YB6 and LaB6 at low temperatures. The bulk properties of the borides are analyzed on the basis of the calculated equations of states and balanced crystal orbital overlap populations. Our calculations indicate that hexaborides with divalent metals, CaB6, SrB6, BaB6, and YbB6, are semiconductors with small energy gaps. The metallic MB6 hexaborides with trivalent M atoms are found to possess larger bulk moduli values. For dodecaborides the bulk moduli are found to be higher for MB12 with increased filling of the conduction band (ZrB12, HfB12, UB12) in comparison with M3+B12 compounds. The total energy calculations for different magnetic configurations in YbB12 point to the possibility of antiferromagnetic coupling between Yb3+ ions.

MgB2: Synthesis, sound velocity, and dynamics of the vortex phase

The sound velocity is measured in polycrystalline MgB2 synthesized from the elements, and the bulk modulus and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state.The dynamic parameters of the vortex lattice are estimated. This paper was published in Low Temp. Rhys., 2002, v.28, N3, p.270-274The sound velocity is measured in polycrystalline MgB2 synthesized from the constituent elements, and the bulk modulus in compression and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state. The dynamic parameters of the vortex lattice are estimated.

Structure, sound velocity, and thermal conductivity of the perovskite NdGaO3

X-ray (300 K) and ultrasonic (77–270 K) studies and measurements of the thermal conductivity (30–300 K) are carried out on single-crystal samples of NdGaO3 in different crystallographic directions. The values of the lattice parameters of NdGaO3 are refined. The sound velocities in the principal crystallographic directions are measured, and the elastic constants and Debye temperature are calculated. The observed anisotropy of the thermal conductivity is described in the framework of a gaskinetic model and is linked to the anisotropy of the interaction parameters of the acoustical and optical phonons.

Elastic properties and phonon spectra of quasi-two-dimensional VSe2

A recently developed method is used to make acoustic measurements of VSe2 single crystals. The components of the elastic constant tensor are determined from the data of these measurements. The experimental data are used to calculate the total and partial phonon densities of VSe2, which permit an explanation of the vibrational spectra of this compound.

Characteristics of the electric field accompanying a longitudinal acoustic wave in a metal. Anomaly in the superconducting phase

The temperature dependence of the amplitude and phase of the electric potential arising at a plane boundary of a conductor when a longitudinal acoustic wave is incident normally on it is investigated theoretically and experimentally. The surface potential is formed by two contributions, one of which is spatially periodic inside the sample, with the period of the acoustic field; the second is aperiodic and arises as a result of an additional nonuniformity of the electron distribution in a surface layer of the metal. In the nonlocal region the second contribution is dominant. The phases of these contributions are shifted by approximately \pi /2. For metals in the normal state the experiment is in qualitative agreement with the theory. The superconducting transition is accompanied by catastrophically rapid vanishing of the electric potential, in sharp contrast to the theoretical estimates, which predict behavior similar to the BCS dependence of the attenuation coefficient for a longitudinal sound.

Magnetoelastic effects inNd 2 CuO 4 single crystals

We have measured the anisotropy of the spin reorientation fields inNd2CuO4 single crystals from the magnetoelastic effects. ForT>1.5K our experimental data can be understood it terms of the four-sublattice antiferromagnet model and allow to estimate the relation between the phenomenological constants of the model. ForT<1.5K we have found a manifestation of new spin-reorientation field stimulated phase transition. These transitions exist only in narrow range of the angles between the magnetic field and [100] directions and are connected most likely with theNd spins.

Indirect observation of a soundlike collective mode in electronic cuprates

We report the results of sound velocity measurements inNd2CuO4 andPr2CuO4 single crystals. An unexpected effect is found, which consists in unusual behavior of the velocity of the quasitransverse modevt at out-of-CuO2 plane direction of the wave vectorq. Rotatingq from the [100] or [110] to the [001] direction we registered, first, a strong lowering, and, then, a steplike increase ofvt. The observed features can be described by a phenomenological model, based on the suggestion about the existence of a sound like collective mode coupled touzz lattice deformation. All components of the elastic moduli tensor, bulk moduli and Debye temperatures are obtained.