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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. [ABSTRACT FROM AUTHOR]

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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The conductance-voltage G(V) characteristics for contacts between electrodes of highly oriented pyrolytic graphite were studied in a wide range of G(0), bias voltages, and temperatures using the mechanically controllable break junction technique. Our results show a striking resemblance to corresponding data for conducting multiwall carbon nanotubes. For high-ohmic contacts between a single or a few graphene sheets, zero bias anomalies related to strong e–e Coulomb interactions and quantum interference of electrons is observed. [ABSTRACT FROM AUTHOR]

Relatively large pyramid structures with heights upto 15 μm were grown selectively by MOCVD. Lineair growth rates for the pyramid facets, independent of the size of the mask openings, were obtained by paying special attention to the mask design. Three low-index facets, (0 0 1), {0 1 1} and {1 1 1}B, bound the pyramid and all of them are able to define the pyramid tip geometry. From facet growth rates and pyramid geometry the combination of growth time and size of the mask opening for obtaining a pyramid with a sharp tip can be accurately calculated. [Copyright &y& Elsevier]

A scanning tunneling microscope (STM) has been developed, dedicated for use on artificially fabricated nanostructures at low temperatures. With this STM mesoscopic phenomena can be studied, combining the unique possibilities of scanning tunneling microscopy and artificially fabricated nanostructures for investigating the physics in this regime. The STM is vibration insensitive, has reliable coarse approach mechanisms in three dimensions, and operates at low temperatures. In order to be able to position the tip above the structure of interest, also a searching strategy has been developed. In this article we describe design and operation of the STM and demonstrate searching. [ABSTRACT FROM AUTHOR]

We have constructed a double point-contact system for use at liquid-helium temperatures which is adjustable from the outside. Both point-contact wires can be independently displaced vertically with a travel of 5 mm and horizontally with a travel of 4 mm. Displacements of 5 μm can be achieved. The sample table is rotatable over 360° and can be translated over 2 mm, which implies that point-contact wires can be placed at any desired position of a sample. Moreover, the point-contact wires and the sample can be viewed from the outside to check positions and a magnetic field up to 1.2 T can be applied. The point contacts obtained with this system are both mechanically and electrically stable, also in a magnetic field. The instrument is of great use for transverse electron focusing studies of transport properties of metals, surfaces, and interfaces. Because of versatility, the instrument is also suitable for single point-contact experiments to study interfaces of normal metals and superconductors or to perform point-contact spectroscopy experiments. [ABSTRACT FROM AUTHOR]

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 Å2 with a resolution better than 10 Å laterally and 0.15 Å 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 Å/min along the surface and 0.5 Å/min perpendicular. [ABSTRACT FROM AUTHOR]

We deposit a laser-collimated chromium beam onto a substrate through a laser standing-wave (SW) tuned above the atomic resonance at either of the two [sup 52]Cr transitions [sup 7]S[sub 3]→[sup 7]P[sup o,sub 3] at 427.600 nm or [sup 7]S[sub 3]→[sup 7]P[sup o,sub 4] at 425.553 nm. In both of these cases, the resulting pattern on the surface consists of nanolines with a period of that of the SW. We extend the range of periods accessible to laser-focused atom deposition by superimposing the structures grown at both these resonances. The resulting beating pattern exhibits a period of 44.46±0.04 µm as determined with a polarizing optical microscope. This structure provides a link between nanoscopic and macroscopic worlds and could potentially become a calibration standard for length metrology. [ABSTRACT FROM AUTHOR]

We used the mechanically controllable break junction technique to discriminate between the electric field and temperature effects in the process of surface modification. The electric field strength at the surface of electrodes was accurately determined using the field emission resonance spectra and was gradually raised to the point where surface modification starts. We found that only a limited number of metals with large values of the work function, >=4.5–5 eV, and evaporation fields, <=1.5–2 V/Å, can be modified in this way. Adsorption of He on the surface drastically increases the local work function of material and enables the field induced nanostructuring of the electrode surface practically for all metals. [ABSTRACT FROM AUTHOR]

High magnetic fields were used for the alignment of J-aggregates of cyanine dyes in solution leading to strongly polarized optical properties, yielding a maximum dichroic ratio of 13. The aligned aggregates were fixed by gelation resulting in samples that are stable at room temperature and exhibit strongly polarized absorption and emission spectra. [ABSTRACT FROM AUTHOR]

Presents information on a study conducted which investigated the growth temperature and magnetic magnetic properties of epitaxial Fe layers on MgO(001). Characteristics of MgO(001); Examination of the low-energy electron diffraction of the Fe layers; How the Fe layers were grown; Assessment of the magnetic properties of the films; Methods of the study; Results of the study.

Examines the edge effects in the magnetoconductance of macroscopic samples of near-intrinsic germanium near room temperature as a model system for an ambipolar conductor. Cause of the spatial variation of the density of charge carriers; Estimates for the surface recombination velocity and the bulk recombination rate; Relation between the ratio of the two four-terminal resistance and proportionality factors.

Abstract: Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (001) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2×2)-MnAu(001) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5×23)-like Au reconstruction appears instead of the c(2×2) structure and local density of states peaks obtained on the c(2×2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer. [Copyright &y& Elsevier]

To elucidate the magnetic properties of a magnetic multilayer film, an understanding of the local geometry, electronic structure, and possible intermixing at the interfaces is of utmost importance. By gradually increasing the Fe coverage on the Mn(0 0 1) film, we obtained this information on the interface region by means of Auger spectroscopy and scanning tunneling microscopy/spectroscopy. Intermixed atoms and various geometries and density of states are found to exist in this system. [Copyright &y& Elsevier]

The growth of ultrathin Mn films on an Fe(0 0 1) whisker at 370 K is studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy at room temperature in ultrahigh vacuum. Atomically and chemically resolved STM images show that the Mn film grows with the same in-plane lattice constant as Fe(0 0 1) and that Fe atoms intermix with the first (14%), the second (4%), and the third Mn layer (2%), while a negligible amount of Fe atoms is found above the third layer. The growth mode changes from layer-by-layer to layer-plus-island at a coverage of 3 ML Mn. dI/dV curves which are normalized by voltage-dependent tunneling probability functions show clear peaks on each Mn layer. These peaks are tentatively ascribed to surface states. On the first Mn layer, peaks are found at +0.35 eV on pure Mn areas and at +0.28 eV on mixed MnFe areas. The second and the third Mn layer show peaks at +0.20 and +0.8 eV, respectively. Mn films thicker than three layers reveal besides a strong peak at +0.8 eV a weaker peak at −0.6 eV. Our apparent step height measurements show that the Mn film relaxes at the third layer: the interlayer spacing is ∼0.16 nm for the first two layers, and it increases to ∼0.18 nm at the third layer. Starting from the fourth layer the interlayer spacings are geometrically equivalent (∼0.165 nm). [Copyright &y& Elsevier]

The submonolayer growth of Mn on Fe(0 0 1) in the temperature range between 50 and 200 °C is studied by scanning tunneling microscopy and scanning tunneling spectroscopy. For growth temperatures above 100 °C, atomically resolved STM images with chemical contrast clearly reveal the incorporation of Mn atoms in the Fe(0 0 1) substrate. The fraction of place exchanged Mn atoms is observed to increase with growth temperature. Although on islands a c(2×2) structure forms locally which is attributed to an ordered MnFe surface alloy, long range order could not be obtained for the growth temperatures and coverages studied. Spectroscopy results are presented for clean Fe(0 0 1), pure Mn ad-islands, single incorporated Mn atoms and c(2×2)-ordered MnFe areas. In contrast to embedded Cr atoms in the Fe(0 0 1) surface [Phys. Rev. Lett. 76 (1996) 4175], isolated embedded Mn atoms do not lead to double peak structures in dI/dV curves. Nevertheless, on the ordered c(2×2) MnFe structure and the pure Mn monoatomic islands surface states are detected as peaks in the dI/dV and (dI/dV)/(I/V) curves. However, due to the strong influence of a tip-dependent background on these peaks, the corresponding surface state energies cannot be found directly from those curves. The real surface state energies were recovered by normalizing the measured dI/dV curves by fitted quadratic backgrounds. Thus, surface state energies of about +0.35 and +0.25 eV are estimated for pure Mn islands and alloyed MnFe areas, respectively. [Copyright &y& Elsevier]

Abstract. We present a novel spectroscopic technique that is based on the ballistic injection of minority carriers from the tip of a scanning-tunneling microscope into a semiconductor heterostructure. By analyzing the resulting electroluminescence spectrum as a function of tip-sample bias, both the injection barrier height and the carrier relaxation rate GAMMA[sub s] alter injection can be determined. At 4.2 K we found GAMMA[sub s] =5 x 10[sup 13] s[sup -1] and at 77 K we found GAMMA[sub s] = 8 x 10[sup 13] s[sup -1]. From current-dependent measurements we find that, at room temperature, a large fraction of the carriers is trapped prior to radiative recombination. At high currents or low temperatures the traps become saturated. We tentatively identify the Be acceptors in the structure as trapping centers. [ABSTRACT FROM AUTHOR]

Abstract. We have performed optical spectroscopy measurements on a STM-tip-induced quantum dot in a GaAs layer. The dominant confinement in the (hole) quantum dot is found to be in the direction parallel to the tip axis. Electron confinement is achieved by a sub-surface AlGaAs barrier. Currentdependent measurements indicate that many-body interactions can cause spectral blueshifts up to 75 meV for a tunneling current of 10 nA. Consequently, a full treatment of the many-body problem is required for an accurate description of this type of system. [ABSTRACT FROM AUTHOR]

We have designed and built a low-temperature (1.3-4.2 K) scanning-tunneling microscope which is capable of collecting light that is generated in the tunneling region. Light collection is done by means of two fibers whose cleaved front is in close proximity (approx. 1 mm) to the tunneling region. The whole system can be operated in high magnetic fields (11 T) without loss of optical signal strength. As a demonstration, we measured the electroluminescence spectra of an InGaAs quantum well at various temperatures. At 4.2 K, we found an electron-to-photon conversion factor that is three orders of magnitude higher than at room temperature. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents an accurate method for calibration of the relative displacement of mechanically controllable break junction electrodes and determination of the metal work function using oscillating behavior of tunnel conductance in a field emission region. Junction conductivity in the field emission region; Accuracy of the method.

Reports on an adaptation of the mechanically controllable break junction technique using thin metallic films to create point contacts and tunnel junctions. Presentation of point-contact spectra of some materials and tunneling data to illustrate the possibilities offered by the method.

Presents a sensitive capacitance thermometer at low temperature for use in magnetic fields. Temperature range; Sensitivity of the thermometer at low temperatures; Reproducibility of the thermometer.

Describes the design and performance of a low temperature scanning tunneling microscope (STM) setup that can be used in magnetic fields. Sensitivity of the instrument to external vibrations; Influence of external magnetic fields on the performance of the STM; Spectroscopic capability.

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. [ABSTRACT FROM AUTHOR]

Holographic transmission electron microscopy showed that magnetic flux does not diverge from the top of an Fe-coated W tip indicating the in-plane magnetization at the apex of successful spin-polarized tips. A dependence of differential conductivities of GaAs with Fe-coated W tips on the helicity of illuminating laser light was confirmed at negative sample bias where partially spin-polarized electrons in the conduction band of p-GaAs tunnel into spin-dependent Fe empty states while no dependence of those with nonmagnetic W tips. [ABSTRACT FROM AUTHOR]

The present letter describes a method to make a spin-polarized scanning tunneling microscopy tip by applying voltage pulses between a W tip and a magnetic sample. This spin-polarized tip has the similar characteristics as an Fe-coated W tip, which was confirmed by observations of antiferromagnetically coupled ferromagnetic Mn(001) layers (> 3 ML) grown on an Fe(001) whisker at 370 K. Furthermore, we demonstrate that these voltage pulses can vary the tip magnetization direction. [ABSTRACT FROM AUTHOR]

Investigates the electronic structure with atomic spatial resolution using scanning tunnelling microscopy. Accounts on the intrinsic property of Cr(001) surface; Types of impurities on the cleaved surface; Visualization of superconducting gap near a zinc atom.

The thermal expansion dynamics of W, Pt–Ir, and Au mechanically controllable break junction electrodes was studied in the millisecond range. By measuring a transient tunnel current as a function of time, we found that, at low temperatures, the electrode elongation Δs∼t[sup ½] due to the large values of thermal diffusivity of metals. The magnitude of Δs varies in direct proportion to the power P dissipated in the electrodes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the sensitivity of the semiconductor scanning tunneling microscope (STM) tips to an incident optical field. Characteristics of the semiconductor STM tip; Representation of the surface photovoltage under steady-state photoexcitation; Adjustment of the STM junction to obtain a photovoltaic or photoamperic behavior.

Examines the (Pb[sub 0.45]Sn[sub 0.55])[sub 2]P[sub 2]Se[sub 6] solid solution for low temperature thermometry. Occurrence of two phase transitions; Temperature dependence of the dielectric constant; Comparison of the thermal sensitivity of the crystalline material with glass-ceramic thermometers.

A bromide-modified silver (111) surface, covered by Jelly-type aggregates of a cyanine dye, is investigated by scanning tunneling microscopy (STM) in air. We show that, under appropriate conditions, light is emitted out of the junction by inelastic electron tunneling. Study of the relation between the light intensity and the tunnel bias clearly reveals a molecular influence on the light emission. Furthermore, through a spectral analysis, we show that sharp resonances appear for dye-covered surfaces by coupling of tip-induced plasmons with molecular excitations. This demonstrates that through STM-induced luminescence local spectroscopic information can be obtained on molecules. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A magneto-optical near-field scanning tunneling microscope is used to image the prewritten magnetic domain structure of a Pt/Co multilayer. A semiconducting tip acts as a local photodetector to measure the magnetic circular dichroism signal coming from the magnetic sample. The resolution of the magnetic imaging is given by the photoelectrically active volume of the tip. Reduction of the laser light wavelength resulted in a factor of 4 improvement of the magnetic resolution. Based on a sound and applicable definition we estimate the resolution to be (60±35) nm for a wavelength of 532 nm. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]