Imaging of surface atoms revolving on elliptical trajectories

Abstract

Achieving atomic resolution with an STM demands a noise-free environment, where mechanical vibrations especially must be damped out. Introducing such vibrations in the form of defined ultrasound consequently leads to image distortion. In particular, the topography is smeared out. By employing surface acoustic waves, which lead to an oscillation of surface atoms on elliptically polarized trajectories, this smearing-out is directed, thereby giving a projection of the ellipse on the sample plane. However, by employing a stroboscopic heterodyne technique (mixing the highfrequency tunneling current with a slightly detuned electrical signal which is applied across the tunneling gap) a snapshot of the surface oscillation is seen. We present phase and amplitude images exhibiting atomic resolution. The atomic contrast of phase and amplitude is explained by the superposition of the surface topography and the oscillation trajectory, which can be obtained from a continuum theory model.Since its invention, scanning tunneling microscopy (STM) has been widely used to image surfaces in real space, achieving atomic resolution. However, one of the key technical problems is the damping of mechanical waves traveling through the setup to the microscope's tip area. The introduction of periodic surface oscillations by means of acoustic waves reduces the STM's contrast significantly, although it provides information about the wave's amplitude [1].In order to measure periodic high-frequency phenomena in the range from MHz to GHz, we have developed a heterodyne-type mixing STM utilizing the nonlinear dependence of the tunneling current on the tip-sample distance [2]. Here, the decisive point is the application of a high-frequency electrical signal to the tip. For the investigation of acoustic wave fields using a scanning acoustic tunneling microscope (SATM) [3] the conventional STM electronics can still be employed, since full information on the local phase and amplitude of the wave field can be transferred to a mixing frequency that can be chosen as low as a few Hz.As the heterodyne detection principle is applicable to all systems showing a nonlinear detector response, the scanning force microscope was also used for acoustic wave detection.In the scanning acoustic force microscope, acoustic waves with different amplitudes lead to different static cantilever positions allowing, for example, the mapping of surface acoustic wave (SAW) fields on conducting and nonconducting samples [4] or the qualitative investigation of elasticity [5].In this paper we present SATM measurements on Au(111) surface atoms revolving on elliptical trajectories as they are excited by propagating SAWs. We show that even though the atomic topography contrast vanishes, the phase and amplitude contrast show atomic resolution. Finally, it is demonstrated that these contrasts can be modeled, including the real surface topography and the atomic oscillation trajectory. Experimental detailsThe object under investigation was a 100 nm thick Au layer that was deposited on a Y...