V. D. Fil

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.

Elastic constants of borocarbides. New approach to acoustic measurement technique

A new version of the phase method of determining the sound velocity is proposed and implemented. It utilizes the “Nonius” measurement technique and can give acceptable accuracy (⩽1%) in samples of submillimeter size. Measurements of the sound velocity are made in single-crystal samples of the borocarbides RNi2B2C (R=Y, Lu, Ho). The elastic constants and the Debye temperature are calculated.

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.

Calculation of the electromagnetic field radiated by an elastic wave in a type-II superconductor

Formulas describing the conversion of an elastic wave into an electromagnetic field at the boundary between a type-II superconductor and the vacuum are obtained which admit passing to both the limiting cases of the normal and Meissner states. A strategy that would permit experimental measurement of the dynamical parameters of the vortex lattice is discussed. It is pointed out that the study of the inertial (Stewart–Tolman) component of the radiated field is a promising area of research.

Magnus force and acoustic stewart-tolman effect in type-II superconductors

We present an acoustic method for the study of the effective Magnus force in type-II superconductors. The method is based on the measurement of the amplitude and the phase of the electromagnetic field radiated by a superconductor due to vortex oscillations excited by a transverse elastic wave. The method does not require the flux flow regime and allows us to measure the Magnus force almost everywhere over the range of the existence of the mixed state. We have measured the field dependence of the Magnus force in nonmagnetic borocarbides (electron-type conductors) and in Nb (hole-type conductor). It is found that in borocarbides the sign of the Magnus force in the mixed state has the same sign as the Lorentz force acting on charge carriers in the normal state, and its value (counted per one vortex of unit length) has only a weak dependence on the magnetic field. In Nb the gyroscopic force changes its sign under transition from the normal to the mixed state. The essential point of the method is the registration of the electromagnetic radiation from the superconductor at zero magnetic field caused by the acoustic Stewart-Tolman effect.

Elastic and piezoelectric moduli of Nd and Sm ferroborates

The sound speeds are measured and the elastic and piezoelectric moduli are calculated for single crystal NdFe3(BO3)4 and SmFe3(BO3)4. These compounds are characterized by enhanced rigidity in the base plane with respect to stress-strain deformations and by a rather strong piezoelectric effect.

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.

Zero sound in normal and superconducting molybdenum

The authors discuss the physical nature of electron sound signals excited in molybdenum by an acoustic wave and propagating at the Fermi velocity. The experimental temperature dependences of the amplitude and the phase velocity of these signals have been studied in the normal and superconducting state. They interpret this effect observed earlier in Ga by Burma et al. as the excitation of a weakly damped zero-sound wave caused by the Fermi-liquid interaction between charge carriers. A dominating role of the electron-electron collisions in the zero-sound damping in Mo was established, and the corresponding relaxation time was estimated. Theoretical calculations of the expected zero-sound behaviour in a superconductor are in good agreement with the experimental data and enable them to determine the intensity of the Fermi-liquid interaction.

Elastic anomalies in HoNi2B2C single crystals

We have measured temperature and magnetic field dependencies of the sound velocities and the sound attenuation in HoNi2B2C single crystals. The main result is a huge softening the velocity of C66 mode due to a cooperative Jahn-Teller effect, resulting in a tetragonal-orthorhombic structural phase transition. Anomalies in the behavior of the C66 mode through various magnetic phase transitions permit us to revise the low temperature H-T phase diagrams 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.