N. G. Burma

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.

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.

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.

Experimental observation of electromagnetic beam waves

The electromagnetic transparency of high-purity gallium single crystals at zero magnetic field is investigated at temperatures of 1.7–4.2K. Under conditions of strong temporal dispersion beam electromagnetic waves are observed which are due to electrons from the flattenings of the Fermi surface. The velocities, damping, moduli, and initial phases of the excitation coefficients are measured and the frequency dependence of the damping and the dependence of the phase velocity on the relaxation time are obtained. Beam waves, unlike ballistic (quasiwave) transport of electromagnetic field, are characterized by the presence of Landau damping and significant decrease of the phase velocity with decreasing relaxation time.

Mass of an Abrikosov vortex

In the excitation of a vortex lattice in the mixed phase of Yb6 single crystals by an elastic wave, the dynamic response is found to have a negative component quadratic in the frequency; we associate this component with the vortex mass. The value of the effect is in catastrophic contradiction with the existing theoretical estimates.

Anomalous acoustic transparency under conditions of the tilt effect

An anomalous acoustic transparency phenomenon is discovered and investigated in high-purity Ga samples under conditions of the tilt effect. This phenomenon is due to resonance coupling of the high-frequency elastic waves with weakly damped electromagnetic eigenmodes of the metal.

Effect of resonance electron-phonon interactions on the electronic spectra of tungsten and gallium

Features of the angular dependences of the Fermi velocity of charge carriers in tungsten and gallium associated with resonance electron-phonon interactions are studied. These features show up as a substantial reduction, by at least a factor of 1.5, in the velocity in the direction of the normals to the flattened parts of the Fermi surfaces of these metals and as a much smaller, on the order of 5%, reduction in the velocity on a cylindrical Fermi surface.

Electric potential of the electron sound wave: Sharp disappearance in the superconducting state

(Received 6 August 2011; revised manuscript received 11 November 2011; published 2 December 2011)Westudytheacelectricpotentialinducedbytheelectronsoundwave(aperturbationoftheelectrondistributionfunctionpropagatingwiththeFermivelocity)insinglecrystalsofhigh-puritygallium.Thepotentialandtheelasticcomponents of the electron sound demonstrate qualitatively different dependencies on the electron relaxationrate: while the phase of the potential increases with temperature, the phase of elastic displacement decreases.Thiseffectisexplainedwithinthemultibandmodel,inwhichthepotentialisattributedtotheballisticquasiwave,while the elastic component is associated with the zero-sound wave. We observed a mysterious property of thesuperconductingstate:allmanifestationsofthepotentialaccompanyingthelatticedeformations,includingusualsound wave, disappear below

Beam waves in tungsten

The electromagnetic transparency of samples of single-crystal, high purity tungsten is studied at liquid helium temperatures in zero magnetic field. The velocity, damping, moduli, and initial phases of the excitation coefficients are measured for electromagnetic signals consisting of long wavelength beam waves and quasiwaves passing through the test samples. An anomaly is observed in the excitation of charge-carrier beam waves in flat segments of the Fermi surface.

Zero sound in metals

Abstract The excitation of the zero sound (ZS) by means of an elastic wave in pure gallium has been realized. The influence of temperature alternating, superconducting transition and magnetic fields on ZS are studied. The results are discussed in terms of a simple model of a compensated metal with two spherical Fermi surfaces.