Attenuation Of Surface Acoustic Waves Research Articles

Saw attenuation by the localised states of a 2D carrier system in a magnetic field

The surface acoustic wave attenuation and dispersion have been measured as a function of magnetic field for both a 2D electron system and a 2D hole system at frequencies between 100 and 1100 MHz. The experiments were made at 350 mK in the quantum Hall regime. In the region of filling factor one where the quasi-DC conductivity becomes very small, an attenuation and dispersion is found which we attribute to localised carrier states. We estimate that the relaxation time of the localised electrons is about 60 ps while the relaxation time for localised holes is about 20 ps.

Saw attenuation in C 60 thin films at transition temperature

Surface acoustic wave (SAW) attenuation of microcrystalline C 60 film prepared by vacuum evaporation is measured in the temperature range 150–300 K. A strong attenuation is observed near the temperature of the fcc–sc transition, at about 260 K. The attenuation may be related to a thermal activated relaxation mechanism having an activation energy of about 68 meV. No thermal hysteresis in the attenuation is observed in the temperature range.

Measurement of a Stiffening Layer Density by Acoustic Microscopy

Measurement of a Stiffening Layer Density by Acoustic Microscopy

Electromechanical coupling coefficient for surface acoustic waves in proton-exchanged 128°-rotated Y-cut lithium niobate

The attenuation of surface acoustic waves propagating along crystallographic X axis in proton-exchanged 128° rotated Y-cut lithium niobate during the evaporation of copper film on the crystal surface has been measured as a function of sheet resistivity of the film. From the magnitude of attenuation maxima, the effective electromechanical coupling coefficients for samples with different thicknesses of HxLi1−xNbO3 layers have been evaluated, and from the position of maxima in the resistivity scale, the effective dielectric permittivities of the structure have been estimated. The electromechanical coupling coefficient is found to be reduced practically to zero with increasing thickness of protonated layer and acoustic frequency, and the dielectric permittivity is not significantly affected by the proton exchange.

Anomalies of attenuation and phase velocity of surface acoustic waves in thin films in the vicinity of

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa2Cu3O YBa2Cu3O7‒δ films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa2Cu3O7‒δ thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned.

Attenuation of surface acoustic waves by quantum dots

Abstract The attenuation Γ of surface acoustic waves (SAWs) by an array of quantum dots is considered. The processes contributing to Γ, direct and intralevel (Debye) absorption and (elastic) scattering off the direction of the SAW are discussed for two limiting cases of the energy spectra of the dots: firstly a continuous spectrum, which is realized when the SAW frequency ω is much larger than the mean level spacing Δ or when the broadening of the levels is sufficiently strong, Δ < τφ -1, τφ -1 being the phase breaking rate; secondly a discrete spectrum, obtained for ω, τφ -1 < Δ. The results show that the sensitivity of Γ to weak-localization effects such as magnetic fields, spin-orbit scattering and temperature, are considerably enhanced for a discrete spectrum. It is argued that the non-invasive character of the measurement of Γ makes the SAW experiments especially suitable for the determination of the temperature dependence of τφ -1 and the equilibration rate of the electronically isolated dots.

Attenuation of surface acoustic waves by quantum dots

Attenuation of surface acoustic waves by quantum dots

Monitoring of NH3 gas by LB polypyrrole-based SAW sensor

Abstract Surface acoustic wave (SAW) devices have been fabricated and tested as sensors of NH3 in gaseous phase. Polypyrrole films, prepared by Langmuir-Blodgett (LB) technique, have been deposited onto the surface of SAW devices as gas absorbent layers. Simultaneous measurements of SAW phase velocity and attenuation have been carried out in order to investigate the sensing mechanisms. The sensor response shows high sensitivity towards NH3 gas and excellent selectivity with respect to main interfering gases (CO, CH4, H2, O2), at room temperature. The low cross-sensitivity permits the recognition of NH3 gas as one component of a mixture of interfering gases.

Suppression of the leaky SAW attenuation with heavy mechanical loading

We discuss effects on the propagation of surface acoustic waves (SAW) due to heavy mass loading on Y-cut lithium niobate and lithium tantalate substrates. An abrupt reduction in the leaky-SAW (LSAW) attenuation is observed in the measured admittance of a long resonator test structure on 64 degrees -YX-cut lithium niobate for aluminum electrodes of thickness h/lambda(0) beyond 9-10%. This experimental fact is explained theoretically as the slowing down of the leaky wave below the velocity of the slow shear surface-skimming bulk wave (SSBW), such that energy dissipation into bulk-wave emission becomes inhibited. An infinite transducer structure is modeled using the periodic Green's function and the boundary-element method (BEM); the computed theoretical properties well explain for the experimental findings. The model is further employed to quantify the leaky surface-wave attenuation characteristics as functions of the crystal-cut angle and the thickness of the electrodes. The resonance and antiresonance frequencies and the corresponding Q values are investigated to facilitate the selection of crystal cuts and electrode thicknesses. The transformation of the leaky SAW into a SAW-type nonleaky wave is also predicted to occur for gold electrodes, with considerably thinner finger structures.

A method for calibrating the line-focus-beam acoustic microscopy system

Absolute accuracy of the line-focus-beam (LFB) acoustic microscopy system is investigated for measurements of the leaky surface acoustic wave (LSAW) velocity and attenuation, and a method of system calibration is proposed. In order to discuss the accuracy, it is necessary to introduce a standard specimen whose bulk acoustic properties, (e.g., the independent elastic constants and density) are measured with high accuracy. Single crystal substrates of gadolinium gallium garnet (GGG) are taken as standard specimens. The LSAW propagation characteristics are measured and compared with the calculated results using the measured bulk acoustic properties. Calibration is demonstrated for the system using two LFB acoustic lens devices with a cylindrical concave surface of 1-mm radius in the frequency range 100 to 300 MHz.

Property Evaluation of Vapor-Deposited TiN Film by the Analysis of Elastic Waves. (1st Report, Nondestructive Evaluation of Elastic Properties by Laser Surface Acoustic Waves.)

The elastic properties of TiN films deposited on stainless steel SUS317J2 by three physical vapor deposition methods were estimated from the velocity dispersion of laser surface acoustic waves(SAWs) in the frequency range of 30 to 60 MHz. The velocity dispersion of Rayleigh waves was obtained by the scanning interference fringes(SIF) system and was used to estimate the elastic stiffness of the films. Depending on the deposition method, Young's moduli of TiN films were found to increase in the order of activation reaction method(ARE) → hollow cathode discharge(HCD) → arc ion discharge(AID) method. The attenuation of SAWs was the highest for AID film due to the poor quality of the film.

Phonon Scattering of Composite Fermions

We study the principal aspects of the interaction between acoustic phonons and two-dimensional electrons in quantizing magnetic fields corresponding to even denominator fractions. Using the composite fermion approach we derive the vertex of the electron-phonon coupling mediated by the Chern-Simons gauge field. We estimate acoustic phonon contribution to electronic mobility, phonon-drag thermopower, and hot electron energy loss rate, which all, depending on the temperature regime, are either proportional to lower powers of T than their zero field counterparts, or enhanced by the same numerical factor as the coefficient of surface acoustic wave attenuation.

Temperature-dependent attenuation of surface acoustic waves in proton-exchanged 128°-rotated Y-cut LiNbO 3

has been experimentally studied in the frequency range 100 to 460 MHz and in the temperature range 90 to 300 K. At room temperature, the proton exchange leads to the considerable enhancement of acoustic attenuation as compared to the pure samples. Annealing in air, in general, reduces the attenuation. However, an anomalous enhancement of the attenuation at several frequencies for particular conditions of annealing is observed. When the temperature is reduced, the attenuation decreases practically to zero in as-exchanged samples. In the annealed ones, the attenuation attains a minimum in the vicinity of 160 K, and begins to grow at lower temperatures. A sharp peak in the attenuation is observed at 210 K. Several different physical mechanisms including acousto-protonic interaction and structural phase transition seem to be responsible for the observed acoustic attenuation behaviour.

Surface acoustic wave scattering in quantum wire arrays: theory and experiment

We have investigated anisotropic surface acoustic wave (SAW) scattering from quantum wire arrays as a function of electron concentration and magnetic field, and we have Fermi's golden rule to model the SAW scattering. With the SAW perpendicular to the wires we observe oscillations in the transmitted intensity, reflecting the magnetic depopulation of ID subbands, and this agrees well with the theory. With the SAW parallel to the wires the theory predicts negligible attenuation, and this is confirmed experimentally at low electron concentrations. Surprisingly, at high electron concentrations the SAW attenuation increases sharply in this orientation to the point where it is three times larger than in the unstructured 2DEG. We speculate that this is due to the excitation of intraband 2D plasmon-like modes.

Surface acoustic wave measurements of YBa2Cu3O7−δ thin films and single crystals

Surface acoustic wave (SAW) velocity and attenuation measurements have been performed on high-temperature superconducting YBa2Cu3O7−δ (YBCO) thin films (deposited on nonpiezoelectric substrates), and YBCO single crystals. A “bridge” technique is employed in which piezoelectric substrates, patterned with interdigital transducers, are bonded to the sample under investigation. The temperature dependence of 168 MHz SAW attenuation in a single crystal near its superconducting transition has been measured in applied magnetic fields up to 1.6 T. Features are seen above the superconducting transition temperature which may be indicative of a transition in the electric polarization of the material. A field-dependent attenuation peak, seen in the superconducting state, is discussed in terms of the theory of thermally activated vortex motion.

Surface acoustic-wave attenuation by a two-dimensional electron gas in a strong magnetic field.

The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs heterostructures is studied in the case where the two-dimensional electron gas (2DEG) is subject to a strong magnetic field and a smooth random potential with correlation length Lambda and amplitude Delta. The electron wave functions are described in a quasiclassical picture using results of percolation theory for two-dimensional systems. In accordance with the experimental situation, Lambda is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to electrons occupying extended trajectories of fractal structure. Both piezoelectric and deformation potential interactions of surface acoustic phonons with electrons are considered and the corresponding interaction vertices are derived. These vertices are found to differ from those valid for three-dimensional bulk phonon systems with respect to the phonon wave vector dependence. We derive the appropriate dielectric function varepsilon(omega,q) to describe the effect of screening on the electron-phonon coupling. In the low temperature, high frequency regime T << Delta (omega_q*Lambda /v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q) are independent of temperature. The classical percolation indices give alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW occurs is found to be given by the scaling law |Delta \bar{\nu}| approx (omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon coupling and the screening due to the 2DEG on the filling factor leads to a double-peak structure for Gamma(\bar{\nu}).

Anisotropic surface acoustic wave scattering in quantum-wire arrays.

We have investigated anisotropic surface-acoustic-wave (SAW) scattering from quantum-wire arrays as a function of electron concentration and magnetic field. With the SAW's propagating perpendicular to the wires, when one naively expects no interaction classically, we observe strong oscillations in the transmitted intensity reflecting the magnetic depopulation of one-dimensional subbands as described by a full quantum-mechanical description of the SAW-electron interaction. With the SAW parallel to the wires we observe suppression of SAW-electron scattering rates at low electron concentrations in narrow wires as predicted on the basis of phase-space arguments. Surprisingly, at high electron concentrations, the SAW attenuation increases sharply to where it is three times larger than the maximum possible in the unstructured two-dimensional electron gas. We speculate that this could possibly be due to the excitation of intraband two-dimensional plasmonlike modes.

Surface Acoustic Wave Velocity and Attenuation Dispersion Measurement by Phase Velocity Scanning of Laser Interference Fringes

We developed a noncontact, nondestructive surface acoustic waves (SAW) velocity and attenuation measurement method in the frequency range of 30–110 MHz. This method is based on the phase velocity scanning method previously proposed by the authors that uses laser interference fringes scanned at the phase velocity of SAW to generate single-mode SAW with high intensity and directivity. In order to verify the validity and accuracy of the method, we measured SAW velocity and attenuation in Al and AISI 304 steel. The error in SAW velocity measurement was less than 1% in Al. Also, the attenuation was obtained with high precision in the AISI 304 steel. Then, we applied this method to porous silicon (PS) films on Si wafers as an example of a layered medium. The SAW velocity in PS films decreased with increasing porosity. By fitting the calculated dispersion curve to the measured one, the elastic stiffness of PS films was determined with a relative accuracy of 0.2%, which is thought to be the highest using laser ultrasonic methods. The attenuation of PS films was found to be 6–80 dB/cm in the frequency range of 30–70 MHz. The frequency dependence of attenuation was obtained with high precisely.

Surface acoustic waves in high- Tc superconducting thin films

Piezoelectric PbTiO 3 films with preferred c-axis orientation were fabricated on MgO substrates by RF-sputtering method. By using a fabricated interdigital electrode, surface acoustic waves (SAWs) with a frequency of 192 MHz were propagated on PbTiO 3 film/MgO substrate and on superconducting Bi(Pb) 2Sr 2Ca 2Cu 2O y film/PbTiO 3/MgO substrate. The temperature dependence of the SAW attenuation coefficient was investigated below room temperature.

The acousto-electric effect in the 2-D hole system using SAW

Experiments have been made using surface acoustic waves (SAW) propagating on two GaAs wafers on which a 2-dimensional hole system (2DHS) had been fabricated by MBE. On a GaAs (100) surface we used SAW at a frequency of 120 MHz. Amplitude modulation and a lock-in amplifier were used to measure the longitudinal and transverse acousto-electric voltages from a Hall-bar structure. Measurements have been made in a magnetic field up to 5 T and at temperatures between 4.2 and 2 K. A large longitudinal voltage was found which increased linearly with SAW power and also increased with applied magnetic field. We have also used a GaAs (311) wafer and SAW at a frequency of 113 MHz. Using pulse modulation, measurements have been made of the SAW attenuation and dispersion as a function of magnetic field up to 13 T and at temperatures below 100 mK. Square wave amplitude modulation and a lock-in amplifier were also used to measure the longitudinal and transverse acousto-electric voltages from a Hall-bar structure. The acousto-electric signals are compared with theoretical predictions using measured values of the longitudinal and transverse resistivity.