Professor Thorsten Hesjedal FInstP

Abstract:

When Ti is deposited on Si in the 600-700 °C temperature range, the lattice mismatch between the Ti-containing deposit and the Si substrate causes TiSix nanoislands to form. The nanoislands grow when annealed at temperatures above 800 °C. When the nanoislands (either unannealed or annealed) are exposed to a Si-containing precursor gas, the Ti catalyzes the decomposition of the gas, allowing one-dimensional nanowires to grow. If oxide-patterned Si substrates are used, the Ti islands form selectively on the exposed Si and are preferentially positioned near the pattern edges. The subsequently grown Si nanowires are, therefore, positioned with respect to the larger lithographically formed pattern. Exposing the wires to an ion beam after deposition promotes the parallel alignment of nanowires. © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We present a study of the origin of ultrasound-induced friction reduction in microscopic mechanical contacts. The effect of friction reduction caused by Rayleigh-type surface acoustic waves (SAWs) is demonstrated for propagating and two-dimensional, standing wave fields using lateral force microscopy (LFM). It is shown that with increasing wave amplitude, friction is completely suppressed. To detect and distinguish between the effect of lateral and vertical surface oscillation components on the cantilever movement, we employed multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Because this effect does not appear for purely in-plane polarized Love waves, we concluded that the mechanical diode effect is most probably responsible for the SAW-induced lubrication. This explanation is also supported by vertical and longitudinal SAFM measurements, which show that, in areas where friction is completely suppressed, low frequency vertical cantilever oscillations can still be observed, whereas lateral or torsional oscillations are no longer excited.

Abstract:

© 2002 IEEE. Epitaxial MnAs films on GaAs are of interest as ferromagnetic layers suitable for spin injection into semiconductors. Recent studies revealed that the phase transition near room temperature from the paramagnetic orthorhombic β phase above 40°C to the ferromagnetic hexagonal α phase below 40°C is of crucial importance for the structural and magnetic properties of the films. In this paper we report about ordered structures of the coexisting phases as a result of epitaxial strain and demonstrate the strong coupling between the magnetic order and the two-phase system.

Abstract:

We present measurements of surface acoustic wave (SAW) scattering from single dots, periodic and locally disturbed two-dimensional dot arrays. By using the scanning acoustic force microscope (SAFM), SAW fields of arbitrary polarization can be imaged with submicron spatial resolution and sub-Å wave amplitude sensitivity. The influence of a wavelength-sized single dot on SAW diffraction is studied. Forward- and back-scattered wave components can be imaged by insonifying the dot with a pump and a probe beam under different angles. SAW diffraction images of a regular dot array reveal a wavefield that is localized around the dots. In case of a disturbed scattering array, the localized SAW pattern vanishes in the vicinity of the distortion.

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.