Thin film quantum materials

Abstract:

We present an experimental method for the high-resolution imaging of the excitation and propagation of surface acoustic waves (SAWs) on anisotropic piezoelectric substrates. By employing a scanning acoustic force microscope (SAFM), we are able to image acoustic waves that are excitable by a single circular electrode pair source through the mixing with well-defined reference plane waves. We show amplitude and phase images of the point-source wave field, containing the angular dependence of the phase velocity of these modes, as well as their electromechanical coupling strength. The SAFM allows easy access to acoustic material properties, which are important for the design of commercial SAW devices. © 2001 American Institute of Physics.

Abstract:

We used multimode scanning acoustic force microscopy (SAFM) for studying noncollinearly propagating Rayleigh and Love wave fields. By analyzing torsion and bending movement of SAFM cantilever, normal and in-plane wave oscillation components are accessible. The SAFM principle is the down-conversion of surface oscillations into cantilever vibrations caused by the nonlinearity of the tip-sample interaction. Through mixing of complementary oscillation components, phase velocities of crossed Rayleigh waves on GaAs and crossed Rayleigh and Love waves on the layered system SiO2/ST-cut quartz were obtained simultaneously. Now, it is possible to investigate elastic properties of submicron areas through multimode SAFM measurements. Finally, we present mixing experiments of four SAWs on GaAs and discuss the various influences on the measured SAFM amplitude and phase contrast.

Abstract:

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever's torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, whereas lateral (torsional) cantilever oscillations are no longer excited. Additionally, at very high Rayleigh wave amplitudes an effect of lateral force rectification was observed. It results in a scan direction-independent appearance of the LFM traces. © 2001 American Institute of Physics.