H. van Kempen

Scanning tunnelling microscopy

The scanning tunnelling microscope, or STM, has emerged over the last few years as a fascinating new technique for examining conducting solid surfaces with high resolution [10.1–5]. A sharpened metal wire is brought close enough to the surface so that the electrons “tunnel” across the narrow gap (0.5–1.5 nm). A small bias potential (2 mV-2 V) provides the necessary potential difference for tunnelling to occur. As a result of the exponential dependence of tunnelling current on separation, the tip height above the surface can be kept constant by using a feedback controller. The tunnel current is monitored and applied to a “PI” controller which in turn drives a piezoelectric arm attached to the tip. By scanning another set of piezoelectric arms, the tip can be rastered in an XY plane whilst simultaneously following the surface corrugations. In this way, a three-dimensional image of the surface can be formed by plotting z(x, y). The imaging is non-destructive since the tip does not normally touch the surface during the scans.

Conductance of Pd-H Nanojunctions

Results of an experimental study of palladium nanojunctions in a hydrogen environment are presented. Two new hydrogen-related atomic configurations are found, which have conductances of similar to0.5 and similar to1 quantum unit (2e(2)/h). Phonon spectrum measurements demonstrate that these configurations are situated between electrodes containing dissolved hydrogen. The crucial differences compared to the previously studied Pt-H-2 junctions and the possible microscopic realizations of the new configurations in palladium-hydrogen atomic-sized contacts are discussed.

One-Dimensional Reconstruction Observed on Fe3o4(110) by Scanning-Tunneling-Microscopy

The (110) surface of magnetite Fe3O4 single crystals was studied by scanning tunneling microscopy (STM). A clean and regular surface was obtained after sputtering and annealing at 1200 K. This preparation procedure resulted in a one-dimensional reconstruction consisting of rows running in the (110) direction. The row spacing was found to vary on different terraces, where the most frequent row separation was determined to be 25 A. Current-versus-voltage curves display a transition from semiconducting to metallic character when the tip–sample distance is reduced. These results, together with complementary low energy electron diffraction (LEED) measurements, are compared with a bulk termination of Fe3O4 and the other known iron-oxide phases.

The capacitive origin of the picosecond electrical transients detected by a photoconductively gated scanning tunneling microscope

We present experimental results and numerical calculations on the detection of picosecond electrical transients by a photoconductively gated scanning tunneling microscope. We show that the transient signal detected in the tunneling regime is coupled by the 5 fF geometric capacitance between tip and sample leading to a correlation current that is linearly proportional to the tunnel conductance.

Calibration and characterization of piezoelectric elements as used in scanning tunneling microscopy

We investigated the behavior of piezoelectric elements used in an STM for fine positioning of the tip. Special attention was paid to the influence of hysteresis. We found that the expansion coefficient of the elements depends on the magnitude of the displacements they produce. If the range is limited to several nanometers, the sensitivity can be taken constant and hysteresis can be neglected. However,if displacements of about a micron are involved, the sensitivity is about 40% higher and the hysteresis will significantly disturb the image. We also compared different calibration methods. An inductive displacement transducer yields results which correspond to those obtained with the known height of steps on a flat gold surface. A Michelson interferometer yields results which are valid for large displacements which cannot be used if small‐scale structures are imaged.

High‐stability scanning tunneling microscope

We have constructed a scanning tunneling microscope for operation under UHV conditions (10−8 Pa). With this instrument topographic measurements can be made on metal and semiconductor surfaces by means of a scanning tip electrode, driven by piezoelectric ceramic elements. The maximum area covered ranges up to 4000×4000 A2 with a resolution better than 10 A laterally and 0.15 A rms perpendicular to the plane. Because of its compact design, the scan unit is very insensitive to vibrations and has a response time down to 0.3 ms. This allows a high scan rate to be used. In order to minimize temperature effects, special attention is paid to the geometry of the construction and the materials used, resulting in a drift ≤4 A/min along the surface and 0.5 A/min perpendicular.

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/ tau -MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of tau -MnAl along the tunnelling direction; the latter is corroborated by band structure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening

Nanowires of different natures have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for Au wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials, and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.

Determination of the energy gap in a thin YBa2Cu3O7−x film by Andreev reflection and by tunneling

Abstract We have observed for the first time Andreev reflection at the normal metal-superconductor interface in a thin film Ag-YBa 2 Cu 3 O 7− x sample, which indicates a zero-momentum paired state in this high- T c superconductor. The energy-dependence of the reflection probability indicates a lower limit of the energy gap in the YBa 2 Cu 3 O 7− x equal to Δ = 12.5±2 meV. Tunneling measurements on the same thin-film samples are in reasonable agreement with this value and yield Δ = 14±2 meV. Both results are not incompatible with the weak-coupling BCS prediction 2 Δ / k B T c = 3.5.

Morphology and surface topology of YBa2Cu3O7-x crystals; theory and STM observations

A network analysis according to the periodic bond chain (PBC) theory of Hartman and Perdok, which predicts the macroscopical morphology and some aspects of the microscopic surface topography, is presented for the high Tc superconductor YBa2Cu3O7-x. We find that growth forms will be characterised by four possible crystal face with a morphological importance (MI) given by MI{001} ≈ 8 MI{010} ≫ MI{103} > MI{110}. Scanning tunneling microscopy (STM) measurements of the microscopica surface structures on the {001} crystal surface support the network analysis and yield further information of the growth process. Among the results are the first observations of small {103} facets occuring on the {001} surface.