Friction is caused by dissipative lateral forces that act between macroscopic objects. An improved understanding of friction is therefore expected from measurements of dissipative lateral forces acting between individual atoms. Here we establish atomic resolution of both conservative and dissipative forces by lateral force microscopy, presenting the resolution of atomic defects. The interaction between a single-tip atom that is oscillated parallel to an Si (111)

The structure of single atoms in real space is investigated by scanning tunneling microscopy. Very high resolution is possible by a dramatic reduction of the tip-sample distance. The instabilities which are normally encountered when using small tip-sample distances are avoided by oscillating the tip of the scanning tunneling microscope vertically with respect to the sample. The surface atoms of Si(111)-(7 x 7) with their well-known electronic configuration are used to image individual samarium, cobalt, iron and silicon atoms. The resulting images resemble the charge density corresponding to 4f, 3d and 3p atomic orbitals.Comment: Submitted to Phys. Rev. B, 17 pages, 7 figure

We report measurements of the shot noise on single-molecule Au-1,4-benzenedithiol-Au junctions, fabricated with the mechanically controllable break junction (MCBJ) technique at 4.2 K in a wide range of conductance values from 10(-2) to 0.24 conductance quanta. We introduce a simple measurement scheme using a current amplifier and a spectrum analyzer and that does not imply special requirements regarding the electrical leads. The experimental findings provide evidence that the current is carried by a single conduction channel throughout the whole conductance range. This observation suggests that the number of channels is limited by the Au-thiol bonds and that contributions due to direct tunneling from the Au to the π-system of the aromatic ring are negligible also for high conductance. The results are supported by quantum transport calculations using density functional theory.

In frequency modulation atomic force microscopy, the stiffness, quality factor and oscillation amplitude of the cantilever are important parameters. While the first atomic resolution results were obtained with amplitudes of a few hundred ångstrom, it has subsequently been shown that smaller amplitudes should result in a better signal-to-noise ratio and an increased sensitivity to the short-range components of the tip-sample interaction. Stable oscillation at small amplitudes is possible if the product of stiffness and amplitude and the energy stored in the oscillating cantilever are large enough. For small amplitudes, stability can be achieved by using stiff cantilevers. Here, we discuss the physical requirements for small amplitude operation and present design criteria and technical details of the qPlus sensor, a self-sensing cantilever with large stiffness that allows small amplitude operation.

Abstract. Laser frequency stabilization is notably one of the major challenges on the way to a space-borne gravitational wave observatory. The proposed Laser Interferometer Space Antenna ͑LISA͒ is presently under development in an ESA, NASA collaboration. We present a novel method for active laser stabilization and phase noise suppression in such a gravitational wave detector. The proposed approach is a further evolution of the "arm-locking" method, which in essence consists of using an interferometer arm as an optical cavity, exploiting the extreme long-run stability of the cavity size in the frequency band of interest. We extend this method by using the natural interferometer arm length differences and existing interferometer signals as additional information sources for the reconstruction and active suppression of the quasi-periodic laser frequency noise, enhancing the resolution power of space-borne gravitational wave detectors. © For the test of general relativity and astrophysical research, measurements of gravitational waves in space are in preparation. Gravitational waves could be detected by observing the strain of space over time. One possible instrument for such measurements is an optical interferometer, which enables comparison of the time-dependent strain in different directions of space. If for such interferometers, the wavelength of the gravitational wave is much bigger than the arm length of the interferometer, then the detected signal can be linearly increased by increasing the interferometer arm length.1,2 For the LISA mission, the interferometer components are housed in identical satellites 5 ϫ 10 9 m apart, forming a triangle in space and providing maximum sensitivity for wave periods between 10 −4 and 10 −1 Hz. The corresponding optical interferometers are of Michelson type with an angle of Ϸ60 deg between the interferometer arms.1,2 Figure 1 shows an outline of an example geometry and a nomenclature of the different interferometer arms.The dimension of such an interferometer implies an increased sensitivity at frequencies below Ϸ30 mHz.1,2 The interferometer reference mirrors are quasi-free falling test masses shielded by the spacecraft from non-gravitational disturbances. The spacecraft follow the test mass motion via an electro-mechanical drag-free control system. Since the motion of the test masses is also subject to interplanetary forces, causing deviations from ideal orbits, the differences in the interferometer arm lengths cannot be kept below a certain limit. In case of LISA, the arm lengths are supposed to breath relatively with an amplitude of about 1% of the mean arm length in the course of a year, causing a slow variation of the corresponding Doppler shifts at a frequency of Ϸ3 ϫ 10 −8 Hz 1,2 and of the relative angle of 60± 1 deg.These unavoidable arm-length differences in spaceborne gravitational wave detectors are the source of the detected laser frequency noise in such interferometers at the expected sensitivity maximum at a frequency of Ϸ10 −3 Hz. Due to the unequal arm le...

We present a measurement scheme that enables quantitative detection of the shot noise in a scanning tunnelling microscope while scanning the sample. As test objects we study defect structures produced on an iridium single crystal at low temperatures. The defect structures appear in the constant current images as protrusions with curvature radii well below the atomic diameter. The measured power spectral density of the noise is very near to the quantum limit with Fano factor F = 1. While the constant current images show detailed structures expected for tunnelling involving d-atomic orbitals of Ir, we find the current noise to be without pronounced spatial variation as expected for shot noise arising from statistically independent events.

The local work function of a surface determines the spatial decay of the charge density at the Fermi level normal to the surface. Here, we present a method that enables simultaneous measurements of local work function and tip-sample forces. A combined dynamic scanning tunneling microscope and atomic force microscope is used to measure the tunneling current between an oscillating tip and the sample in real time as a function of the cantilever's deflection. Atomically resolved work function measurements on a silicon ( 111)-(7 × 7) surface are presented and related to concurrently recorded tunneling current-and force-measurements.

We present a versatile design of freely suspended quantum point contacts with particular large one-dimensional subband quantization energies of up to Narrow constrictions define quantum point contacts that are capacitively controlled via local in-plane side gates. Employing transport spectroscopy, we investigate the transition from electrostatic subbands to Landau-quantization in a perpendicular magnetic field.The large subband quantization energies allow us to utilize a wide magnetic field range and thereby observe a large exchange splitted spin-gap of the two lowest Landau-levels.