P. Wyder

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.

Scanning tunneling microscopy on photoconductive semi‐insulating GaAs

The increase in surface conductivity upon illumination of semi‐insulating GaAs was used to enable surface‐topography measurements with a scanning tunneling microscope (STM). Images were recorded in ultrahigh vacuum and in air using a HeNe laser and a halogen lamp for the generation of a photoconductive top layer. From the current‐voltage characteristics of the tip‐sample contact the maximum feedback‐controlled tunnel current at a given voltage can be deduced. A calculation of the increase in carrier density and conductance upon illumination is given, which confirms the possibility of using STM on highly resistive photoconductive materials. Further implications and applications are discussed.

Cyclotron resonance in an InAs-GaSb superlattice

Abstract Optical transmission in the far infrared region in an InAs-GaSb superlattice is studied as a function of magnetic field and frequency. Cyclotron resonance is observed for electrons confined in the InAs conduction band whose ground state is shifted to higher energy due to the periodic superlattice potential. These measurements confirm the calculated quantisation of the energy levels in a superlattice by ascertaining the effective mass directly.

Particle aggregation in colloids in high magnetic fields

Abstract The influence of high magnetic fields on the stability of colloidal dispersions of paramagnetic and diamagnetic particles has been investigated. The theory of Svoboda is extended to allow examination of the dynamics of particle aggregation in colloids. Particle trajectories are calculated leading to binary pair formation. Experimental results are presented on particle aggregation in high magnetic fields (flocculation). Here binary pairs seem to form and to be stable at enhanced rates of settling.

Resonant polaron coupling of the n = 1 Landau level and the 2p+ donor state in GaAs

Abstract The n = 0 → > n = 1 Landau level and 1 s −2 p + impurity transitions in GaAs were investigated up to energies above the optical phonon energy ħΩ LO and d.c. magnetic fields up to 25 T. Pinning of both transitions to an energy slightly above and below ħΩ LO was observed. At an energy very close to ħΩ LO two additional impurity transitions are found. These features are attributed to the resonant polaron effect which leads to hybridization and dipole selection rule breakdown. Also the spin doublet splitting of both transitions were resolved showing a strong magnetic field dependence which can not be explained by nonparabolicity of the conduction band alone.

Effective mass determination of a highly doped InAs-GaSb superlattice using helicon wave propagation

Abstract The far infrared radiation transmission of a highly doped InAs-GaSb superlattice as a function of the magnetic field, shows helicon wave propagation. The effective mass and the carrier density are determined from an analysis of the results as a function of frequency to be 0.082 ± 0.005 m 0 and 3.4 × 10 18 cm −3 . The carrier density is equal to that obtained from Hall measurements. The effective mass is significantly higher than the value expected from the InAs conduction band nonparabolicity (0.063 m 0 ).

Tip structure determination by scanning tunneling microscopy

Abstract A real-space image of the geometry of a tungsten tip by scanning tunneling microscopy is presented. The micrographs show the existence of distinctly faceted mini tips, which are approximately 1000 A wide and have a radius of the order of 10 A and top angles ≤50° with respect to the axis of the tungsten wire. SEM and STEM pictures show a very rough tip structure on a micron scale and confirm the existence and dimensions of the mini tips, which are situated on top of a drop-like structure probably formed after in-situ melting of the top of the tip.

SCANNING TUNNELING MICROSCOPY AND (SCANNING) TUNNELING SPECTROSCOPY ON STEPPED NI(111)/H

Topographic and electronic properties of both the (111) flat surface and atomic steps of nickel have been measured with scanning tunneling microscopy (STM) and (scanning) tunneling spectroscopy in UHV. Micrographs of the (111) plane show a hexagonal pattern with a dimension of two times the unit structure and a corrugation of 1 A which increases towards a step. This suggests a p(2 × 2)2H monolayer, induced by the high partial pressure of hydrogen. Micrographs of atomic steps show protrusions of 1–3 A at the corners of the steps. Several explanations are discussed. Spectroscopic images of these areas show the same hexagonal pattern on the (111) surface and indicate an increase in the density of states at the adsorption sites and at the step edges. dIdV versus V spectra were also recorded. Various peaks in the density of states, at different energies below 1 eV above EF showed up, their intensity strongly dependent on the lateral position of the tip. These local changes in the density of states might indicate the presence of adsorbate-induced surface states.

Temperature dependence of the static dielectric constant of Rb2ZnBr4: Solitons in a modulated structure?

Abstract The static dielectric constant of Rb2ZnBr4 was measured as a function of temperature for a number of different single crystals. In a part of the samples a Curie-Weiss behaviour was observed at the lock-in transition from the incommensurate to the commensurate phase. Besides, in a few samples, a deviation of this behaviour was observed which can be ascribed to the appearance of solitons yielding a soliton density ns proportional to (T−T c ) 1 2 . At temperatures below Tc two new peaks are observed in the direction of the a - axis .

Application of point-contact spectroscopy in metals to the Kondo problem

A general relationship between the characteristic structure in the electrical resistance of a point contact between two metals and the dynamics of the scatterers is used to discuss new experimental results on Kondo systems. The method allows one to measure the energy dependence of the relaxation time of the conduction electrons directly.