Thin film quantum materials

Abstract:

We present scanning acoustic force microscopy (SAFM) mixing experiments of differently polarized surface acoustic waves (SAW) with noncollinear propagation directions. The phase velocities of the SAWs are measured at a submicron lateral scale, employing a multimode SAFM that is capable of detecting the wave's normal and in-plane oscillation components. Hereby, the down conversion of the surface oscillations into cantilever vibrations due to the nonlinearity of the tip-sample interaction is utilized. The simultaneous determination of the phase velocities within a microscopic sample area is demonstrated for the mixing of Rayleigh and Love waves on the layered system SiO2/ST-cut quartz. © 2000 American Institute of Physics.

Abstract:

We studied dynamic friction phenomena introduced by ultrasonic surface acoustic waves using a scanning force microscope in the lateral force mode and a scanning acoustic force microscope. An effect of friction reduction was found when applying surface acoustic waves to the micromechanical tip-sample contact. Employing standing acoustic wave fields, the wave amplitude dependent friction variation can be visualized within a microscopic area. At higher wave amplitudes, a regime was found where friction vanishes completely. This behavior is explained by the mechanical diode effect, where the tip's rest position is shifted away from the surface in response to ultrasonic waves.

Abstract:

The 3D islands of the Stranski-Krastanow system Ge/Si(001) that form either during the annealing of previously flat and nearly strain-relieved Ge films at 1020 K or directly at the Ge deposition at 1020 K are found to be composed of a mixture of Ge and Si, thus pointing to considerable interdiffusion at 1020 K. Direct measurement of the elastic energy unambiguously reveals that neither the 3D islanding nor the Si in-diffusion are driven by the reduction of misfit strain; this strain being the result of increasing configurational entropy.

Abstract:

This paper reports new developments in the field of spatially resolved surface acoustic wave (SAW) analysis. With scanning acoustic force microscopy (SAFM) the investigation of SAW phenomena with lateral resolution of the scanning force microscope became possible. This technique was limited to SAW modes with an out-of-plane oscillation component. Recently, we demonstrated that purely in-plane polarized SAWs can also be investigated by using a non-linear coupling to the cantilever's torsional movement. Now it is possible to measure the SAW phase velocity dispersion for any given SAW polarization. We used SAFM for investigation of the layered system SiO2 on ST-cut quartz.

Abstract:

With the increasing interest in nanostructures and thin films, the need for a quantitative measuring method for elastic constants on the nanometer scale has become more evident. The fundamental physical quantity characterizing the elastic constants is the acoustic phase velocity. Due to the strong localization of surface acoustic waves (SAWs) in the near-surface region, SAWs are particularly favored for such investigations. The velocity measurement is commonly performed by time delay and acoustic far-field methods. Therefore the lateral resolution of the velocity measurement is restricted by the wavelength involved to some tens of microns. Recently, we introduced the scanning acoustic force microscope (SAFM) for the measurement of SAW amplitude distributions with nanometer lateral resolution. The key to detecting high-frequency surface oscillations by the slowly responding force microscope cantilever is the nonlinear force curve. This nonlinearity can be exploited in a heterodynetype setup for high-frequency wave mixing of a probe and a reference wave, revealing the phase of the probe wave. The difference frequency can be chosen to be as low as 1 kHz. We present measurements of the phase velocity over a lateral distance of 19:9 nm. The phase velocity dispersion due to Au layers on a quartz substrate was measured over distances as small as 200 nm and compared with calculations. © 1998 Springer-Verlag.