L.E.C. Van De Leemput

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.

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.

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.

High-field measurements on the high-Tc superconductors La1.85Sr0.15CuO4-δ and YBa2Cu3O7-δ

We have measured the superconductive properties of the high-T/sub c/ superconductors La/sub 1.85/Sr/sub 0.15/CuO/sub 4-//sub delta/ and YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ in magnetic fields up to 25 tesla. Conservative estimates of the critical field at T = 0 yield respectively 45 +- 5 and 200 +- 25 tesla. The slope (dB/sub c//sub 2//dT) near T/sub c/ for the Y-Ba-Cu-O compound is about twice as large as in La-Sr-Cu-O. For both compounds the magnetoresistivity indicates a large anisotropy of the critical field. The Sommerfeld parameter ..gamma.. is estimated to be respectively 7 and 16 mJ(mole Cu)/sup -1/K/sup -2/, which indicates that the higher T/sub c/ of Y-Ba-Cu-O, when compared with La-Sr-Cu-O, might be primarily due to an increased density of states at the Fermi level.

Topographic study of the (001) surface of YBa2Cu3O7 single crystals with a scanning tunneling microscope

Abstract We report the first measurements obtained with a scanning tunneling microscope in air, on the (001) surface of YBa 2 Cu 3 O 7 single crystals. On this surface we find evidence for the fact that the bulk orthorhombic crystal structure extends to the surface. Growth steps are observed with a height corresponding with the unit c -axis. We find no clear evidence for a semiconducting behaviour of the surface. Occasional indications for the surface structures, connected with twin-boundaries, and of ordering of the oxygen vacancies at the surface are discussed.

Scanning tunneling microscopy observations of metallic clusters Pd561 and Au55 and the implications of their use as a well defined tip

We investigated ligand stabilized clusters Au55(PPh3)12Cl6 and Pd561(phen)38±2O≊200 on gold and graphite substrates with scanning tunneling microscopy (STM) under atmospheric conditions. We were able to image dense layers of clusters as well as free‐lying clusters. Clusters are frequently picked up by the tip. This results in a tip with a well defined shape and electronic structure. Analysis of images of clusters scanned with another cluster attached to the tip can yield direct information on the tunnel distance. We found a tunnel distance of 24±5 A.

Composition and racemization of amino acids in mammoth collagen determined by gas and liquid chromatography.

Abstract 1. 1. Amino acid components of mammoth bone occur exclusively as collagen which undergoes gelatin formation. 2. 2. Thermal stabilities of threonine and serine are less than those reported for older collagens. 3. 3. Racemization obeys a first order rate law. Arrhenius parameters are compared with previously reported results. 4. 4. The order of the rates of racemization of amino acid residues is identical with data from modern bones.

Determination of the energy gap in YBa2Cu3O7−δ by tunneling, far infrared reflection and andreev reflection

Abstract The energy gap of bulk and thin film YBa 2 Cu 3 O 7−δ has been studied with three independent techniques: Quasiparticle Tunneling in a low temperature scanning tunneling microscope, Andreev Reflection at a normal/superconductor interface and Far Infrared Reflection measurements. The tunneling results are consistent with a BCS density of states for the quasiparticles. In addition, the observation of Andreev Reflection is a clear indication for zero-momentum ( k,-k ) pairing. The energy gap Δ equals 14±2 meV, both for superconductor-normal and superconductor-superconductor tunneling. In the far infrared reflection we find an energy gap Δ = 14.5±1 meV. The Andreev reflection experiment yields as a lower limit for the gap Δ = 12.5±2 meV. These results yield an average gap-T c ratio2Δ/k B T c = 3.6±0.6, close to the weak-coupling BCS prediction. We discuss the observation of single electron tunneling in small capacitance tunnel junctions on YBa 2 Cu 3 O 7−δ and of the incremental charging effect on isolated small particles on these materials.

The superconductive groundstate of YBa2Cu3O7−δ

Abstract We present the results of energy resolved measurements of the reflection of charge carriers at a silver-YBa 2 Cu 3 O 7−δ interface, and of quasiparticle tunneling into thin film and sintered bulk samples of the same high T c superconductor. The observation of Andreev reflection indicates that the superconducting groundstate of YBa 2 Cu 3 O 7−δ consists of a zero-momentum paired state, commensurate with a ( k, − k ) BCS pairing. The width of the energy gap, derived from the energy dependent Andreev reflection probability, and from tunneling results on the same material is also in reasonable agreement with the weak coupling BCS prediction. We will discuss the relevance of these experiments for some of the theoretical models that are proposed to explain the high critical temperature of these materials.