Ultrasound and vortex oscillations in high-temperature superconductors

An HTSC powder sample with grain (particle) diameter of 20–50 μm placed in a dc magnetic field B ₀ and cooled to a temperature below the superconducting transition temperature was exposed to the radiofrequency (rf) pulsed magnetic field B _∼ (B _∼ ⊥ B ₀) at a carrier frequency of 30.7 MHz. Stable echo signals were recorded which followed different rf-pulse trains. This phenomenon has the following mechanism. The rf magnetic field stimulates fluxoid oscillations on the HTSC grain surface, which are transformed into lattice oscillations through the pinning centers and induce a propagating sound wave. The second-order nonlinearity with respect to the gradient of the crystal lattice deviation from the equilibrium position taken into account in the sound wave equation yields the dependence of the crystal lattice natural frequency on the amplitude and length of the pulses which excite these oscillations. This dependence is responsible for the emergence of echo signals.     

RF field dependence of memory echoes in LiNbO3 and GaAs powder

Measurements of the rf electric field dependence of the amplitude of the “memory” echo in LiNbO₃ and GaAs powders are reported. Consistent with the torque-rotation model, the dependence on each of the three applied rf pulses is linear in the low power regime. At high power, more complex behavior is found including an irreversible decrease in the echo amplitude when the third applied pulse is larger than the preceeding two pulses.     

High-frequency magnetic modulation of recoilless gamma radiation of57Fe

Radio-frequency (rf) magnetic modulation has been used to generate sidebands in⁵⁷Fe Mössbauer spectra of Fe_0.18Ni_0.82 Permalloy foils which have the smallest constant of magnetostriction among Fe?Ni alloys. Sidebands in Mössbauer spectra were observed at 30 MHz and 55 MHz. In addition to the generation of sidebands, the external rf magnetic field was found to alter the line positions of the original six line spectrum. An attempt was made to study acoustic vibrations in the foil by means of X-ray diffraction. The rf magnetic field caused changes in diffraction peak intensities and positions. It was found that X-ray diffraction can be used to study the amplitude of acoustic vibrations in Permalloy foils.     

Manifestation of basal-plane anisotropy and mechanical boundary conditions in magnetic birefringence of sound in hematite

The angular dependence of the magnetic birefringence of sound in hematite is experimentally investigated as a function of the direction of a magnetic field applied in the basal plane of the hematite crystal. It is found that, at room temperature, the curve of magnetoacoustic oscillations in the magnetic field, i.e., the oscillatory dependence of the amplitude of an acoustic wave transmitted through the crystal on the magnetic field strength, is characterized by hexagonal and uniaxial anisotropy. It is shown that the hexagonal anisotropy is governed by the basal-plane anisotropy of higher orders. The appearance of the uniaxial magnetic anisotropy in the basal plane of the crystal is explained by the mechanical stresses arising in the sample when piezoelectric transducers are glued to the sample ends. This assumption is confirmed by the observed change in the direction of the uniaxial anisotropy axis under variations in the boundary conditions.     

Nonlinear Spherical Standing Waves in an Acoustically Excited Liquid Drop

Nonlinear evolution of a standing acoustic wave in a spherical resonator with a perfectly soft surface is analyzed. Quadratic approximation of nonlinear acoustics is used to analyze oscillations in the resonator by the slowly varying amplitude method for the standing wave harmonics and slowly varying profile method for the standing wave profile. It is demonstrated that nonlinear effects may lead to considerable increase in peak pressure at the center of the resonator. The proposed theoretical model is used to analyze the acoustic field in liquid drops of an acoustic fountain. It is shown that, as a result of nonlinear evolution, the peak negative pressure may exceed the mechanical strength of the liquid, which may account for the explosive instability of drops observed in experiments.     

Frequency dependence of the amplitude of domain-wall oscillations in an acoustic wave field

The velocity of oscillatory motion of domain walls is investigated as a function of the parameters of a magnetic material and an external acoustic field. The dependence of the amplitude of domain-wall oscillations on the frequency of an external acoustic wave is determined. It is found that this dependence exhibits a resonant behavior.     

Analysis of macroparticle charging in the near-electrode layer of a high-frequency capacitive discharge

Spatial variation of dust particle charges are estimated numerically for typical laboratory experiment conditions in a radio-frequency (rf) capacitive discharge. The surface potentials of macroparticles levitating in the upper part of the near-electrode layer of the rf discharge are measured. A model is proposed for the formation of irregular dust oscillations due to stochastic motion of dust in the bulk of a spatially inhomogeneous plasma (in the presence of a dust charge gradient). This mechanism is used for analyzing the results of measurements of the amplitude of vertical vibrations of dust particles in the near-electrode layer of the rf discharge. It is found that the dust charge gradient may be responsible for the development of such vibrations.     

Second harmonic wave generation from Joule heating in layered organic conductors

The amplitude of a second harmonic wave (SHW) generated from Joule heating as a heat source in organic conductor β-(BEDT-TTF)₂IBr₂ is analyzed as a function of the magnetic field strength and its orientation with respect to the plane of the layers. Angular oscillations of the SHW amplitude are correlated with the angular changes of in-plane conductivity that arise from the periodic dependence of charge carriers velocity on the field orientation. It was found that the nonlinear effect of wave generation leads to a shift between the position of the peaks of the wave amplitude and in-plane conductivity. This allows an important information on the parameter values of organic conductors as well as wave velocity to be obtained. Magnetic field dependence shows that the wave is not strongly attenuated with increasing field and might give insights on the interactions between the electromagnetic, temperature and acoustic oscillations. We found that these observations are completely different compared to those of linear acoustic wave generation. It has been shown that the necessary conditions for observing the nonlinear acoustic wave generation are fulfilled in a wide range of fields and angles that allow the acoustic properties of organic conductors to be studied in detail.     

Magnetic resonance spin echo amplitudes at low rf fields

It is shown that for magnetic resonance the variation of spin echo amplitude with radio frequency (rf) field exhibits a fourth power law at the low rf limit. Depending on the line width and the pulse width, a cube law dependence may also be obtained, as has been reported for experimental measurements of ferromagnets.     

Inhomogeneous electronic structure probed by spin-echo experiments in the electron doped high-Tc superconductor Pr1.85Ce0.15CuO4-y

63Cu nuclear magnetic resonance spin-echo decay rate (T-12) measurements are reported for the normal and superconducting states of a single crystal of Pr(1.85)Ce(0.15)CuO(4-y) in a magnetic field B(0)=9 T over the temperature range 2相似文献   

Behavior of higher harmonics of the granular HTSC response to a low-frequency magnetic field

A theoretical explanation is proposed for the experimental results on the behavior of higher harmonics of the granular HTSC response to a variable magnetic field of frequencies ～10² Hz. The theory explains the periodicity in the dependence of the harmonics amplitudes on a static external magnetic field; the dependence of the period on the harmonics number and on the amplitude of the variable magnetic field; the existence of the threshold in the dependence of the odd-harmonics amplitudes on the amplitude of the variable magnetic field; and the possible formation of pairs for these dependences for the neighboring odd harmonics. It is shown that the experimental dependences can be explained without a detailed analysis of microprocesses in individual Josephson loops only by treating HTSC as a macroscopic medium characterized by the dependence Φ(H) of the magnetic flux on the external magnetic field, which is typical of type II superconductors.     

Dynamic and stimulated rf echo in high-Tc superconducting powders

Two-pulse and three-pulse echoes in powdered yttrium and bismuth high-T _c superconductors are investigated to determine the dependence of the signal amplitude on the magnetic field, the temperature, and the gas pressure. The temperature is measured as a function of the relaxation time of the echo signal. The properties of the long-lived rf echo are studied in detail; it exhibits a persistent (lasting more than several hours) memory of a time series of write pulses and a cumulative storage effect. The experimental results can be explained qualitatively within the framework of the theory proposed by Asadullin [Sverkhprovodimost’ 6, 545 (1993)] to account for the nonlinear motion of vortices associated with sample defects. Zh. éksp. Teor. Fiz. 111, 1032–1046 (March 1997)     

Primary acoustic echo during excitation of a paramagnetic crystal by picosecond elastic video pulses

The primary acoustic echo formed during excitation of a paramagnetic crystal with effective spin S=1 by two transverse picosecond elastic video pulses is investigated theoretically. Both exciting video pulses are applied perpendicular to the external magnetic field. It is shown that the primary acoustic echo in the general case consists of six longitudinal and transverse signals at the frequencies of the transitions within a Zeeman triplet. The optimal parameters of the exciting video pulses for the appearance of different echo signals are determined. Fiz. Tverd. Tela (St. Petersburg) 41, 623–628 (April 1999)     

Linear and nonlinear vibrations and waves in optical or acoustic superlattices (photonic or phonon crystals)

A model of a dielectric or an elastic superlattice is proposed which describes quite simply the frequency spectrum of electromagnetic or acoustic waves. The frequency band spectrum of a one-dimensional lattice consists of minibands, which narrow down with increasing frequency (so that the forbidden bands in the spectrum broaden with increasing frequency). An elementary analysis of the spectrum of a one-dimensional lattice reveals the presence of many forbidden frequency bands in this case as well. It is shown that dynamic equations for superlattices can be generalized to the nonlinear case, leading to equations of the type of the nonlinear Schrödinger equation for the lattice. Soliton excitations are described and the particle-like dynamics of solitons is demonstrated. Local vibrations near point defects of different complexity in superlattices are studied and graphically illustrated. The existence of Bloch oscillations of a wave packet in a superlattice in a homogeneous external field is discussed.     

Laser-excited dust lattice waves in plasma crystals

Laser-excited waves in a two-dimensional dust plasma crystal have been observed experimentally in a parallel plate rf discharge. The measured dispersion relation is compared with theoretical models. Agreement is found with dust lattice waves, whereas deviations from dust acoustic waves exist. From the dispersion relation of a dust lattice wave the screening of the particles in the rf sheath is determined.     

Variation in the magnetic structure of a single-domain particle ensemble and its response to an rf pulse

A set of equations in the Gilbert form that describes the motion of the magnetization vector in an ensemble of interacting magnetic nanoparticles is numerically solved for the case of high-amplitude rf pulses. Based on the numerical solution, the magnetic structure of spherical particle ensembles showing cubic anisotropy at certain parameters of the variable field is studied. This phenomenon is shown to have a threshold. The dependence of the field threshold amplitude on the acting pulse repetition rate and amount of magnetic interaction is determined. It is demonstrated that a change in the magnetic structure of the interacting particle ensembles causes a change in the spectrum of the response. This fact can be used for pulsed rf writing and readout of information based on ferromagnetic resonance.     

X-ray diffraction analysis of the surface acoustic wave propagation in langatate crystal

X-ray diffraction on a langatate crystal (La₃Ga_5.5Ta_0.5O₁₄, LGT) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) was studied in a double axis X-ray diffractometer scheme at the BESSY synchrotron radiation source. SAW propagation in the crystal causes sinusoidal modulation of the crystal lattice and the appearance of diffraction satellites on the rocking curves, with their number, angular positions, and intensities depending on the wavelength and amplitude of acoustic vibrations of the crystal lattice. Strong absorption of X-ray radiation in LGT enables the observation of the diffraction spectra extinction at certain SAW amplitudes. X-ray diffraction spectra analysis makes it possible to determine SAW amplitudes and wavelengths, to measure the power flow angles, and investigate the diffraction divergence in acoustic beam in LGT.     

On the theory of X-ray acoustic resonance in the Bragg geometry

A theory of X-ray acoustic resonance in the Bragg geometry is developed as applied to triple-axial X-ray diffractometry. In contrast to existing approaches, transverse ultrasonic oscillations propagating in the surface region of a crystal are considered in the model of a Rayleigh surface wave. Numerical simulation of reciprocal space maps and their cross sections is carried out depending on the ultrasound amplitude. The influence of multiple scattering on diffraction profiles of various diffraction orders under X-ray acoustic resonance conditions is demonstrated.     

Atom-molecule Rabi oscillations in a Mott insulator

We observe large-amplitude Rabi oscillations between an atomic and a molecular state near a Feshbach resonance. The experiment uses 87Rb in an optical lattice and a Feshbach resonance near 414 G. The frequency and amplitude of the oscillations depend on the magnetic field in a way that is well described by a two-level model. The observed density dependence of the oscillation frequency agrees with theoretical expectations. We confirmed that the state produced after a half-cycle contains exactly one molecule at each lattice site. In addition, we show that, for energies in a gap of the lattice band structure, the molecules cannot dissociate.     

Fermi resonance of optical one-phonon and acoustic two-phonon vibrations in light metals

It is shown that the anharmonicity of crystal lattice vibrations in light metals such as beryllium, can give rise to a Fermi resonance of optical one-phonon and acoustic two-phonon vibrations. New hybridized vibrational states are formed as a result of such a resonance interaction: biphonon and quasibiphonon vibrations and renormalized optical vibrations. Depending on the wave vector, these vibrational states can be both damped and stationary. The corresponding dispersion equation is obtained, whose solution made it possible to determine the spectrum of these vibrations (dispersion curves and the wave vector dependence of the damping for damped vibrations). It is shown that ultrafast damping of optical vibrations, similar to the well-known superradiance effect for Frenkel’ and Wannier-Mott excitons, is possible. Fiz. Tverd. Tela (St. Petersburg) 39, 542–546 (March 1997)