Using molecular beam epitaxy we have fabricated a quantum wedge: a nanoscale flat-top lead island on a stepped Si(111) surface. Imaging the top surface of the wedge with a scanning tunneling microscopy reveals the phenomenon of electron interference fringes: a discrete periodic spatial variation of the tunnel current originating from the quantization of electron states in the wedge.[S0031-9007(97)02817-2] PACS numbers: 73.20.Dx, 61.16.Ch, 85.40.Ux Fizeau fringes, a well known interference phenomenon, occur when a monochromatic light illuminates a thin optical wedge [1]. Because of the wave nature of electron a similar phenomenon of electron interference fringes also exists, and has been observed by transmitting an energetic coherent electron beam through a thin film [2]. Unlike the optical interference, electron fringes can in principle appear spontaneously on a thin metal wedge, since the interfering particles, electrons, are already present in the metal and confined by the boundaries [3]. Observing these fringes, however, requires a technique capable of imaging a standing wave inside a wedge. A perfect tool for this task is a scanning tunneling microscope (STM), which probes the weak evanescent tails of electrons outside the metal without destroying the interference pattern. Indeed, the STM has been used successfully to reveal the interference phenomena due to the scattering of electrons in a vacuum gap [4,5], at surface defects [6,7], or in a quantum corral [8].To achieve sharp interference fringes, one can exploit the strong energy quantization of the electron states in a metal wedge with an average thickness of a few nanometers. We name a nanoscale wedge with its thickness varied monotonically by discrete atomic planes a quantum wedge, for a change of its thickness by a single atomic plane can lead to an appreciable shift of the electron energy spectrum. The discrete nature of a quantum wedge can give rise to a discrete contrast in the interference pattern. For example, for a metal with a Fermi electron wavelength l F 4a 0 (a 0 being the atomic plane spacing), the variation in the thickness by a 0 changes the interference condition from destructive to constructive, or vice versa, and the fringe contrast will have a square wave profile. In this Letter we demonstrate the first experimental realization of such electron interference fringes on a quantum wedge. The quantum wedge was fabricated by the epitaxial growth of Pb on a stepped surface of Si(111), while the fringes were observed in situ with a low-temperature STM.

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Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

Lead quantum wells (QW) epitaxially grown on annealed Pb/Si(111) interface form a model system for the study of interactions between quantized electrons and adiabatically modulated boundaries. Tunnel spectra of this system reveal a previously unknown adiabatic shift of QW resonances due to lateral variations of the electronic reflection phase at the buried interface. With this effect, lateral distribution of the subsurface reflection phase can be probed, using scanning tunneling microscopy.

A magnetic probe microscopy study of levitation and atomic-scale friction is reported for Fe on YBCO (Tc = 92.5 K) in the temperature range 65–293 K. Below Tc, the friction coefficient is constant and exhibits no correlation with the strength of superconducting levitation forces. Above Tc, the friction coefficient increases progressively, and nearly doubles between Tc and room temperature. The results are discussed within the context of the underlying atomic-scale electronic and phononic mechanisms that give rise to friction, and it is concluded that contact electrification and static electricity may play a significant role in the non-superconducting phase. Given that the properties of YBCO can be finely tuned, the results point the way to a variety of interesting studies of friction and superconductors.