Hanno H. Weitering

Theory of the {open_quotes}Honeycomb Chain-Channel{close_quotes} Reconstruction of {ital M}/Si( 111) -(3{times}1)

First-principles electronic-structure methods are used to study a structural model for Ag/Si(111) -(3{times}1) , recently proposed on the basis of transmission electron diffraction data. The fully relaxed geometry for this model is far more energetically favorable than any previously proposed, partly due to the unusual formation of a Si double bond in the surface layer. The calculated electronic properties of this model are in complete agreement with data from angle-resolved photoemission and scanning tunneling microscopy. {copyright} {ital 1998} {ital The American Physical Society}

Hard Superconductivity of a Soft Metal in the Quantum Regime

Superconductivity is inevitably suppressed in reduced dimensionality1,2,3,4,5,6,7,8,9. Questions of how thin superconducting wires or films can be before they lose their superconducting properties have important technological ramifications and go to the heart of understanding coherence and robustness of the superconducting state in quantum-confined geometries1,2,3,4,5,6,7,8,9. Here, we exploit quantum confinement of itinerant electrons in a soft metal, Pb, to stabilize superconductors with lateral dimensions of the order of a few millimetres and vertical dimensions of only a few atomic layers10. These extremely thin superconductors show no indication of defect- or fluctuation-driven suppression of superconductivity, and sustain supercurrents of up to 10% of the depairing current density. Their magnetic hardness implies a Bean-like critical state with strong vortex pinning that is attributed to quantum trapping of vortices. This study paints a conceptually appealing, elegant picture of a model nanoscale superconductor with calculable critical-state properties and surprisingly strong phase coherence. It indicates the intriguing possibility of exploiting robust superconductivity at the nanoscale.

Linear Magnetization Dependence of the Intrinsic Anomalous Hall Effect

The anomalous Hall effect is investigated experimentally and theoretically for ferromagnetic thin films of Mn5Ge3. We have separated the intrinsic and extrinsic contributions to the experimental anomalous Hall effect and calculated the intrinsic anomalous Hall conductivity from the Berry curvature of the Bloch states using first-principles methods. The intrinsic anomalous Hall conductivity depends linearly on the magnetization, which can be understood from the long-wavelength fluctuations of the spin orientation at finite temperatures. The quantitative agreement between theory and experiment is remarkably good, not only near 0 K but also at finite temperatures, up to about approximately 240 K (0.8TC).

Charge-order fluctuations in one-dimensional silicides

Metallic nanowires are of great interest as interconnects in nanoelectronic devices. They also represent important systems for understanding the complexity of electronic interactions and conductivity in one dimension. We have fabricated exceptionally long and uniform YSi(2) nanowires through self-assembly of yttrium atoms on Si(001). The wire widths are quantized in odd multiples of the Si substrate lattice constant. The thinnest wires represent one of the closest realizations of the isolated Peierls chain, exhibiting van Hove type singularities in the one-dimensional density of states and charge-order fluctuations below 150 K. The structure of the wire was determined through a detailed comparison of scanning tunnelling microscopy data and first-principles calculations. Quantized width variations along the thinnest wires produce built-in Schottky junctions, the electronic properties of which are governed by the finite size and temperature scaling of the charge-ordering correlation. This illustrates how a collective phenomenon such as charge ordering might be exploited in nanoelectronic devices.

A cryogenic Quadraprobe scanning tunneling microscope system with fabrication capability for nanotransport research.

We describe the development and the capabilities of an advanced system for nanoscale electrical transport studies. This system consists of a low temperature four-probe scanning tunneling microscope (STM) and a high-resolution scanning electron microscope coupled to a molecular-beam epitaxy sample preparation chamber. The four STM probes can be manipulated independently with subnanometer precision, enabling atomic resolution STM imaging and four-point electrical transport study of surface electronic systems and nanostructured materials at temperatures down to 10 K. Additionally, an integrated energy analyzer allows for scanning Auger microscopy to probe chemical species of nanostructures. Some testing results are presented.

Strain tuning of topological band order in cubic semiconductors

We theoretically explore the possibility of tuning the topological order of cubic diamond/zinc-blende semiconductors with external strain. Based on the tight-binding model, we analyze the evolution of the cubic semiconductor band structure under hydrostatic or biaxial lattice expansion, by which a generic guiding principle is established that lattice expansion can induce a topological phase transition of small band-gap cubic semiconductors via a band inversion, and further breaking of the cubic symmetry leads to a topological insulating phase. Using density functional theory calculations, we demonstrate that a prototype topological trivial semiconductor, InSb, is converted to a nontrivial topological semiconductor with a 2%-3% biaxial lattice expansion.

Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

Surface characterization of silicon on insulator material

The surface of ultrathin silicon on insulator (SOI) material has been characterized with surface science analysis techniques including atomic-resolution scanning tunneling microscopy. It is shown that the (100) SOI surface can be fabricated with a comparable degree of structural perfection as the (100) surface of bulk Si. Fermi level pinning by “type C” dimer defects results in a fully depleted and thus effectively insulating Si film.

Structure and Growth of Quasi One-Dimensional YSi2 Nanophases on Si(100)

Quasi-one-dimensional YSi(2) nanostructures are formed via self-assembly on the Si(100) surface. These epitaxial nanowires are metastable and their formation strongly depends on the growth parameters. Here, we explore the various stages of yttrium silicide formation over a range of metal coverages and growth temperatures, and establish a rudimentary phase diagram for these novel and often coexisting nanophases. We identify, in addition to previously identified stoichiometric wires, several new nanowire systems. These nanowires exhibit a variety of surface reconstructions, which sometimes coexist on a single wire. From a comparison of scanning tunneling microscopy images, tunneling spectra, and first-principles density functional theory calculations, we determine that these surface reconstructions arise from local orderings of yttrium vacancies. Nanowires often agglomerate into nanowire bundles, the thinnest of which are formed from single wire pairs. The calculations show that such bundles are energetically favored compared to well-separated single wires. Thicker bundles are formed at slightly higher temperature. They extend over several microns, forming a robust network of conducting wires that could possibly be employed in nanodevice applications.

Formation of uni-directional ultrathin metallic YSi2 nanowires on Si(110)

Ultrathin YSi2 nanowires were grown epitaxially on the Si(110) surface. High-aspect-ratio nanowire growth is induced by the strongly anisotropic lattice-match between the silicide crystal lattice and the Si(110) surface, similar to the established formation of rare-earth silicide nanowires on Si(100). In contrast to the Si(100) case, however, YSi2 nanowires on Si(110) grow in a single orientation along the [11¯0] direction and exhibit a clear preference of nucleating at step edges when these edges are aligned along the [11¯0] growth direction. This suggests a promising avenue for the fabrication of regular nanowire arrays with controlled wire separation, by varying the miscut angle of the Si wafer. The nanowires are metallic and are embedded in a reconstructed Si(110)-(23×3)R54.7°-Y semiconducting surface layer.