Scanning tunneling microscopy of longitudinally polarized surface acoustic waves ͑SAWs͒ yields amplitude and phase images with nanometer resolution. The eccentricity of the surface oscillation calculated from the experimental data by the model of Chilla et al. is 88°Ϯ5°in good agreement with the macroscopic value for high velocity pseudo SAWs. Our study reveals that reliable amplitude and phase information can be deduced even from atomic scale features; however, the local surface geometry and

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... p shape need to be treated in more detail. Surface acoustic waves ͑SAWs͒ are of great interest in many respects: ͑i͒ SAWs play a key role for frequency filtering in mobile phone and satellite telecommunication; ͑ii͒ SAWs are a well-established probe in materials science for material characterization 1 and quantitative determination of the elastic constants of bulk and thin films; 2 and ͑iii͒ recently SAWs have also been employed in more basic studies, e.g., to modify the electronic properties and to induce spin transport in semiconductors. [3] [4] [5] Thus, understanding the nature of acoustic wave fields, particularly of their interaction with structural features and local elasticity, is not only important for the advancement of SAW devices but also of scientific interest. Chilla and co-workers demonstrated that scanning probe techniques-despite their large response times-are sensitive to high frequency ͑HF͒ surface acoustic wave fields (ϳ10 MHz→ϳGHz). By utilizing the unique spatial resolution of the scanning tunneling microscope ͑STM͒, amplitude and phase of Rayleigh waves were detected with nanoscale resolution, 6,7 i.e., far beyond the diffraction limit of optical detection systems. 8 Behme et al. 9 recently determined the phase velocity of Love waves by atomic force microscope; these SAWs exhibit a purely in-plane polarization and therefore are difficult to detect with standard techniques. In this study we employed our UHV SAW-STM 10 to investigate high-velocity-pseudo surface acoustic waves ͑HVPSAWs͒ on Y -cut LiNbO 3 . HVPSAWs are special types of PSAWs which are known for leaking energy into the bulk during propagation. Since HVPSAWs exhibit higher phase velocities than normal SAW modes, the operating frequency of SAW devices can be increased without necessarily decreasing the lithographical feature sizes. Compared with the well known Rayleigh waves, HVSAWs have a predominant longitudinal oscillation behavior. On Y -cut LiNbO 3 , for instance, the component perpendicular to the surface (u 3 ) amounts only 3% of the in-plane component (u 1 ); for the correponding Rayleigh mode u 3 Ϸ1.3u 1 . Although HVP-SAWs have been extensively studied theoretically in the past, very little experimental work has been devoted to them because of the difficulties involved in detecting their pre-dominant in-plane polarization. We will show here that also the SAW-STM is sensitive to the in-plane components of the surface oscillation and that amplitude and phase information can be deduced even from atomic scale features of the surface. The experiments presented here were performed in an UHV system consisting of separate chambers for sample preparation and SAW-STM investigation (base pressureϽ2 ϫ10 Ϫ10 hPa). The UHV SAW-STM-developed recently in our group 10 -is based on a commercial Omicron STM-1 and has been modified by adding a UHV-compatible highfrequency wiring system for SAW excitation and signal detection up to frequencies of 1 GHz. The substrate is a Y -cut LiNbO 3 piezoelectric single crystal, carrying a lithographically fabricated interdigital transducer ͑IDT͒ for exciting the HVPSAW. As confirmed by a frequency analyzer, its frequency is 210.06 MHz, i.e., well separated from the Rayleigh wave at 120 MHz. In addition, a 100-nm-thick gold film-forming the conducting layer for the SAW-STM experiments-was deposited in situ into the acoustic beam path of the IDT. To favor the formation of extended flat terraces we choose a substrate temperature of 400°C and a rate of 0.1 nm/s. 11 The experimental setup of a SAW-STM is schematically illustrated in Fig. 1 . The STM tip is positioned above the conducting film on the piezoelectric sample, which is located in the propagation region of the SAW excited by the IDT on the left. The SAW-induced surface oscillations at the frequency f SAW give rise to a HF contribution to the tunneling current between tip and conducting layer. The HF component is mixed at the nonlinear current-distance characteristics of the tunneling gap with a HF voltage V mod at the frequency f SAW ϩ⌬ f , which is added to the common dc tunneling voltage V 0 . The mixing signal at the difference frequency ⌬ f is chosen to be in the kilohertz range, where it can be easily analyzed by conventional STM electronics and lock-in technique. The mixing signal, which is recorded simultaneously with the topography, exhibits the amplitude and phase information of the SAW. Particularly the latter can be used to quantitatively determine the eccentricity of the SAW oscillation ellipse on areas as small as 5ϫ5 nm ͑Ref. 12͒. The Au film-used as a conducting layer in our tunnel-a͒ Electronic