Ultrasonic Phonons Research Articles

Neutron Back- and Front-Face Bragg Diffraction on a Thin Si Single Crystal Excited by Ultrasound

In this research project, we measured and analyzed the spatial distribution of neutron beam Bragg diffracted from the front- and back-faces of thin Si single crystal undergoing on ultrasound excitation. For the perfect crystal, it is shown that when the acoustic wave amplitude is increased, the front-face peak position remains unchanged and its value grows linearly. The values of ultrasound wave amplitude were determined. The back-face peak becomes asymmetric and tends to disappear. New types of Pendellosung fringes in the center of the diffraction profiles were observed at the first time. It is supposed that in the perfect crystal this effect may be due to the appearance of the new “sonic” extinction length, depending on the amplitude of the ultrasonic wave. Thus, it leads to the new interference interactions between neutron wave and ultrasonic phonons. It is established that within the framework of the dynamical theory of the neutron scattering, some asymptotic models valid for the case of Laue geometry as well as Kato’s quasi-classical approximation can be applied in the case of Bragg geometry also. Good agreement between experimental data and the theory has been obtained.

"Monochromatic" ultrasonic phonon studies of condensed matter: An apparent challenge to the uncertainty principle.

"Monochromatic" ultrasonic phonon studies of condensed matter: An apparent challenge to the uncertainty principle.

Surface induced phonon decay rates in thin film nano-structures

Nano-scale structure significantly impacts phonon transport and related phonon relaxation rates, with order of magnitude effects on the thermal conductivity of dielectric thin films and quantum wires, and even larger effects on the lifetimes of ultrasonic phonons of micro- (nano-) oscillators. In both cases, efforts to explain the data have been hampered by our lack of knowledge of the effects of confined dimensionality on phonon-phonon scattering rates. Using a phonon Boltzmann equation with appropriate boundary conditions on the free surfaces to take surface roughness into account, we have obtained an expression yielding phonon lifetimes in 2-D dielectric nanostructures(thin films) resulting from phonon-phonon scattering in conjunction with phonon-surface scattering. We present these theoretical results and, in the limit in which surface induced losses dominate, obtain explicit predictions for the phonon lifetimes. The predicted temperature dependence of the ultrason! ic loss does not explain the observed saturation of the loss at low temperatures(τ(T) → const), but does give results of the order of magnitude of measured ultrasonic lifetimes.

Acoustoelectric current through a quantum wire containing a point impurity

A dc electric current due to a flow of ultrasonic phonons through a quantum wire containing an isolated point impurity is analyzed theoretically. An explicit expression for the acoustoelectric (AE) current is obtained and its dependence on the chemical potential of the electron gas and the induction of the external magnetic field is investigated. It is shown that a point impurity may lead to a partial or complete destruction of the steplike structure of the AE current as a function of the chemical potential.

Application of ultrasound for the simultaneous improvement intensity and resolution of the Bonse–Hart diffractometer

It is calculated that simultaneous ultrasonic excitation of the Bonse–Hart monochromator and analyzer leads to the improvement of resolution and increasing of the intensity. It is argued that incoherent ultrasonic field is applicable in similar experiments. Ultrasonic phonon satellites were studied by means of Bonse–Hart channel cut crystal. Strong improvement of contrast with moderate loss of intensity was observed in this case. Double Crystal diffractometer with ultrasonically excited monochromator and analyzer was applied for the ultrasonic phonon satellite research. It was observed for the first time that this “double sound” technique leads to the strong improvement of contrast and simultaneously 60% increasing of the intensity.

Quantization of acoustoelectric current in a ballistic channel

The electric current induced by an ultrasonic phonon flux in a ballistic quasi-two-dimensional quantum channel is investigated theoretically. Two types of confining potential are considered. An analytic expression for the acoustoelectric current is derived, and its dependence on the chemical potential and on the magnitude of the longitudinal magnetic field is investigated. It is shown that the dependence of the acoustoelectric current on the chemical potential may be of the experimentally observed step type. The oscillatory dependence of the acoustoelectric current on the magnetic field is considered for the cases of weak and strong magnetic quantization.

Evidence for a quasi-two-dimensional proton glass state in Cs5H3(SO4)(4); xH(2)O Crystals.

We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs 5H (3)(SO (4))(4);xH 2O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (T(g) approximately 260 K). Near T(g) the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons. The damping of LA phonons propagating perpendicular to the basal plane can be fit by the Debye model and is due to the interaction between protons and LA phonons. This suggests that the proton glass state that is realized at T<T(g) has a quasi-two-dimensional nature.

Mössbauer diffraction in perfect crystals undergoing ultrasonic excitation

The influence of the ultrasonic wave on the dynamic diffraction of the Mossbauer γ quanta in the perfect Si crystals has been studied. Both the scattering intensities and the diffractional Mossbauer spectra for different reflections and ultrasonic frequencesνs have been measured. Some unusual results such as the enhancement of the elastic scattering at highνs with ultrasound amplitude growing and non-exponential decrease of the Debye-Waller factor (connected with ultrasonic phonons) at lowνs have been obtained. The experimental data are in good agreement with the theoretical calculations.

Dielectric rotary echoes in vitreous silica.

Coherent experiments at very low temperatures have proven to be very useful tools in the study of the interaction between the two-level tunneling states in glasses /1/ and external fields. In particular, spontaneous and stimulated echo experiments /2/ have made it possible to directly investigate the resonant coupling of these states to RF electric fields and to ultrasonic phonons. Also the transverse lifetime T2 and -with limited accuracy- the longitudinal lifetime T1 of the tunneling states could be determined from these experiments.

Nonlinear Optical Properties of Rigid Rod Polymers and Model Compounds

ABSTRACTPicosecond and femtosecond degenerate four-wave mixing have been used to measure third-order nonlinear optical susceptibility of several rigid-rod polymers and model compounds. A significant X(3) with femtosecond response was observed. The effects of processing conditions and structural variations on X(3) were studied. Four-wave mixing studies can also be conveniently used to generate ultrasonic phonons of well defined wave length and propagation direction. From this result one can investigate mechanical properties of polymers. Using this technique both the longitudinal and shear components of the elastic moduli for a uniaxially oriented rigid-rod polymer, poly-p-phenylene bis benzooxazole (PBO), were determined.

Interaction of ultrasonic phonons with defects in hydrogenated amorphous germanium

The attenuation of acoustic surface waves of 300 MHz has been measured in sputtered films of amorphous Ge and a-GeHx (0≦x≦0.25) at temperatures between 0.5 K and 475 K. A strong absorption maximum due to structural relaxation is found at 135 K in pure a-Ge. This absorption maximum is shifted by annealing and is suppressed by hydrogen incorporation. Below 20 K the attentuation varies linearly with temperatures in all films investigated so far.

Ultrasonic phonon behavior near the pressure-induced cubic-to-orthorhombic transformation in PbF 2

Measurements have been made of the transit times τ of pulses of 30 MHz longitudinal and transverse ultrasonic waves in PbF 2 at room temperature using hydrostatic pressures up to where the irreversible cubic-to-orthorthombic transformation occurs (near 4 kbar). Above 3 kbar 1/τ no longer increased linearly with pressure and exhibited some time dependence at fixed pressures before echo disappearance. The elastic stiffness moduli showed no significant softening before the transformation began. Length, mass density, and X-ray measurements on samples after they had been returned to 1 bar confirmed that milky white regions in them were orthorhombic but filled with defects.

The breakdown of the electromagnetic screening of transverse ultrasonic phonons in cadmium

Measurements of the absorption of fast shear waves in cadmium along [112̄0] in the frequency range 10–760 MHz have shown that the electromagnetic screening begins to break down at 200 MHz.

Ultrasonic Attenuation Measurements Near the Upper Critical Field in Clean Niobium

Ultrasonic attenuation measurements are reported for the superconducting transition in clean niobium. Linear relationships are found between measured attenuation and magnetic field as well as between attenuation and temperature. Data taken in the two experimental situations q⊥H and q∥H are compared with an effort to measure the anistropy of the flux line lattice. Here q = 2π/λ, the ultrasonic phonon wave vector for phonons of wavelength λ and H is the external field. [Work supported by Air Force Office of Scientific Research, Grant AFOSR 71-2079.]

Deformation Potential and Piezoelectric Ultrasonic Phonon-Charge Carrier Coupling in Indium Antimonide

The interaction of ultrasonic phonons with charge carriers in nearly intrinsic InSb via deformation potential and piezoelectric coupling mechanisms has been investigated at 296 K in magnetic fields of 0 to 11 kOe and over a frequency range of 10 to 200 MHz. The single crystal InSb was p type with an average acceptor concentration of (1.45±0.05)×1016 cm−3. The experiment permitted an ultrasonic determination of the electron and hole mobilities. The values obtained were μn0 = (58 000±4000) cm2 V−1 sec−1 and μp0 = (720±80) cm2 V−1 sec−1. Values of the deformation potential and piezoelectric coupling constants obtained were | Cn+Cp | = (5.8±0.4) eV and e14 = | 0.076±0.010 | C/m2.

Spin-Lattice Relaxation near the Critical Point: RbMnF3, MnF2, EuO

Electronic spin-lattice relaxation in disordered ferro- and antiferromagnets is discussed with particular emphasis on the region near the critical point. A general expression for the relaxation time is derived. The time it takes for the spins to come to thermal equilibrium with the lattice is directly proportional to the magnetic specific heat, as anticipated from thermo-dynamic arguments. It is shown that systems whose ordering temperature is at least comparable to the peak in the density of states of the acoustic-phonon modes can have very short relaxation times. This happens because the spins can interact with essentially all the acoustic modes. Relaxation times are estimated for three systems: RbMn${\mathrm{F}}_{3}$, Mn${\mathrm{F}}_{2}$, and EuO. The implication of these results for the interpretation of ultrasonic attenuation measurements is discussed. It is found that the calculated values for the relaxation times agree qualitatively with the values inferred from ultrasonic data for RbMn${\mathrm{F}}_{3}$. The behavior of EuO is analyzed. We find that our values of ${T}_{1}$ are compatible with the dominance of the spin-lattice mechanism in the decay of the energy-density fluctuations with wavelengths comparable to the wavelengths of the ultrasonic phonons.

Some ultrasonic attenuation measurements on indium antimonide and gallium arsenide

Measurements have been made of the temperature dependence at 1 GHz of the ultrasonic attenuation of various modes in single crystals of n -type indium antimonide and gallium arsenide. The observed attenuation has been compared with predictions based on the work of Simons and Maris which takes into account the finite lifetime of the thermal phonons with which the ultrasonic phonons interact. The agreement obtained is in general acceptable, considering that in the treatment given, there are no adjustable parameters. Further experiments are suggested to identify the individual processes which are responsible for the attenuation. It is suggested that information concerning the lifetime of specific thermal modes may soon be available from such attenuation measurements.

Frequency Modulation of ResonantγRays in Iron Foils: Influence of the State of Magnetization

The properties of sidebands in M\"ossbauer spectra in the presence of a transversal radio-frequency field have been investigated in iron slabs. The relative intensity of the sidebands is analyzed in terms of fast phonon relaxation, because the single-modulation-index model does not account for the observed sideband intensities. The results obtained in the presence of the external magnetic field ${H}_{0}$ show that the generation of sidebands is caused by the oscillatory motion of the bulk magnetization rather than the domain-wall motion. The mean-square value of the atomic displacement, ${x}^{2}$, as a function of ${H}_{0}$ shows an abrupt decrease near the anisotropy field ${H}_{A}$ in the hard direction (375 Oe) and drops to zero very rapidly. We point out that, concerning magnetostriction as a possible mechanism, the application of the formula $\ensuremath{\Delta}l={l}_{0}{\ensuremath{\Lambda}}_{s}({cos}^{2}\ensuremath{\theta}\ensuremath{-}\frac{1}{3})$ is correct only for powdered samples where the displacement of atoms is expected to be a quasistatic coherent phenomenon. However, in the case of thin slabs, where the wavelength of ultrasonic phonons at ${\ensuremath{\omega}}_{\mathrm{rf}}$ is much smaller than the sample size, the application of the above formula does not account for the observed modulation. We have calculated ${x}^{2}$ by evaluating the average elastic energy density of the sample under rf irradiation. The strong modulation observed implies that the magneto-elastic coupling coefficient is much larger in the dynamic condition than in the static one. From our discussion it follows that magnetostriction could also be responsible for the published results on a low-magnetostriction material such as supermalloy.

Electromagnetic attenuation of shear waves in superconductors

A detailed analysis of the electromagnetic attenuation in superconductors has been carried out, with specific reference to type I materials. The rapid rise atTc predicted by Cullen and Ferrell is found to be most pronounced in superconductors with low acoustic velocities. Also, the effect occurs at lowest frequencies in these materials. It is demonstrated how the shape of the transition curve reflects anisotropies in electronic and elastic properties. Very close toTc the attenuation at low frequencies is inversely proportional to the real conductivity function, while at high frequencies it is proportional to that conductivity. Numerical results for Al, In, Pb, and Sn are presented. Although these materials are similar with regard to electronic structure, they are characteristically different with regard to electrodynamic properties as probed by ultrasonic phonons. From the present analysis it is concluded that ultrasonic probing of the electrodynamics of superconductors offers a sensitive and, at the present, unique way of determining the response of a superconductor to a single variable wave number. In addition the wave numbers available in this way are in a range where it has been most difficult to obtain such information by penetration depth studies, namely, in the transition region between pure local and nonlocal behavior.

Saturation of Paramagnetic Spins by 13-Mc/sec Ultrasonic Phonons

Saturation of Paramagnetic Spins by 13-Mc/sec Ultrasonic Phonons