Session H23: Low-D Metals

The growth of Manganese wires on Si(100): observation of the sub-monolayer coverage regime with STM

The study of thin film magnetic materials and the doping of semiconductors with magnetically active dopant atoms has received increased attention due their potential applications in magnetic memory devices and spintronics. We observe the deposition of Mn on the Si(100) 2x1 reconstructed surface in the sub-monolayer regime with STM. Short Mn wires with a length of 5 to about 20 atoms are formed an oriented perpendicular to the Si-dimer rows. At higher coverage some Mn wires are anchored with one end of the wire at the edge and extend onto the lower lying surface. The region in between the wires is particularly interesting: if the Mn wire distance is reduced the dimers change their orientation and are tilted, or begin to form zig-zag lines. The wire length and dimer deformation is likely governed by local strain. We will discuss the wire statistics (lengths, orientation, and position), control of their growth and present first data on the electronic structure of the wires. The growth of Si and Ge overlayers and incorporation of Mn wires in Ge-quantum dots is currently explored.   

Growth and Characterization of a Combinatorial Array of Magnesium-Aluminum Alloys

We have used combinatorial gradient controlled sputter deposition to produce a library of thin films with a wide range of compositions within the Mg-Al alloy system. We have successfully isolated the {\ss} (Mg$_{17}$Al$_{12})$ phase. The importance of understanding the physical properties of the {\ss} phase becomes apparent when one realizes the contrary effects associated with its presence in these alloys. The presence of the {\ss} phase is desirable for increasing corrosion resistance while undesirable as it generally produces reduced mechanical strength of the alloy. We present details of the growth procedure, as well as structural and compositional characterization.   

Atomic structures of 13-atom clusters by density functional theory

The 13-atom cluster structures of the alkaline metals, alkaline earth metals, boron group, 3d, 4d, and 5d transition metals in the periodic table, and Pb are investigated by density functional theory with three kinds of exchange correlation approximation: i) LDA (Local Density Approximation), ii) GGA (Generalized Gradient Approximation) [1], and iii) PBE (Perdew-Burke-Ernzerhof) [2]. The results mainly focus on five 3-D structures: icosahedral, cuboctahedral, hexagonal-closed packed, body-center cubic, decahedral, and the other two layer structures: buckled biplanar (bbp) and garrison-cap biplanar (gbp) structures. Limited by accuracy of exchange correlation approximation, two interesting results are found. The ground states of Ca$_{13}$, Sr$_{13}$, Ba$_{13}$, Sc$_{13}$, Y$_{13}$, La$_{13}$, Ti$_{13}$, Zr$_{13}$, and Hf$_{13}$ are icosahedral structures. The clusters of Ir$_{13}$, Pt$_{13}$, Cu$_{13}$, Ag$_{13}$, and Au$_{13}$ are more favorable for layer structures (i.e. bbp and gbp) than the other five 3-D structures. \newline [1] J. P. Perdew et al., Phys. Rev. B 46, 6671 (1992). \newline [2] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996).   

Length characterization of DNA-wrapped carbon nanotubes using Raman Spectroscopy

The systematic resonance Raman study has been carried out on DNA-wrapped SWNTs of different lengths using several different values of laser excitation energy. The correlation observed between the intensity ratio of the D -band and G-band features (I$_{D}$/I$_{G})$ and the average nanotube energy lengths indicates that nanotube length can be used as the dominant structural parameter in Raman characterization, and that the I$_{D}$/I$_{G}$ ratio can be used as a qualitative gauge for estimating the average nanotube length. By systematically varying the laser excitation energy, we have also found that the I$_{D}$/I$_{G}$ ratio strongly depends on whether the tubes are metallic or semiconducting, as well as on the laser excitation energy. Further directions for this research will be presented. The authors gratefully acknowledge support for this research from the National Science Foundation grant DMR-04-05538.   

Gold Cluster Formation on C$_{60}$ Surfaces: Au-Cluster Beads and Self-Organized Structures

Petra Reinke, Hui Liu, Department of Materials Science and Engineering, University of Virginia The investigation of C$_{60}$-Au interaction is central to the advancement of solar cell and nanotechnology applications of C$_{60}$. C$_{60}$ grows in a quasi-layer-by-layer mode on a pristine graphite surface and form a special surface structure (coexistence of round and fractal islands). The deposition of Au leads to the formation of a complex array of different surface structures, while the basic island structure of the C$_{60}$ is preserved. The Au-clusters nucleate preferentially at the graphite-first fullerene layer islands edge forming beadlike structures. A roughness analysis of the fullerene surface indicates the presence of Au atoms embedded in the fullerene surface, situated in the troughs in between the large molecules. The analysis of the spatial and size distributions of Au clusters provides the basis for the development of a qualitative model which describes the relevant surface processes in the Au-fullerene system. The simultaneous deposition of Au and C$_{60 }$leads to the formation of organized structures, in which Au clusters are embedded in a ring of fullerene molecules with a constant width.   

Self-Assembly of Thiol Adsorbates on the Au(111)surface

A long-standing controversy related to the dimer pattern formed by methanethiol (CH3SH) and methylthiolate (CH3S) on the Au(111) surface has been resolved using density functional theory within periodic boundary conditions. It is found that the S atoms of methanethiol adsorbates on the Au(111) surface form Van der Waals dimers. For methylthiolate, it is shown that no dimerization occurs at high coverage. At intermediate coverage, however, a Van der Waals dimer pattern emerges. The presence of defects in the Au(111) surface does not change this conclusion. Molecular dynamics simulation at high coverage demonstrates that the observed dialkyl disulfide species emerge during the desorption process, and thus are not attached to the surface. A meta-stable monomer pattern has been shown to be only marginally higher in adsorption energy than the dimer configuration which explains the observed fragility of the dimers. For the understanding of these results, it is of crucial importance that methanethiol molecules, contrary to a widely held assumption, remain stable when deposited on clean Au(111) surfaces /1, 2/. In the presence of defects, however, methanethiol adsorbates dissociate and form methylthiolate. /1/ I. Rzeznicka, J. Lee, P. Maksymovych, J. Yates, Jr., J. Phys. Chem. B109, 15992 (2005). /2/ J. Zhou, F. Hagelberg, Phys. Rev. Lett. 97, 45505 (2006).   

Chemical pressure and hidden one-dimensional behavior

We report on the first optical measurements of the rare-earth tri-telluride charge-density-wave systems. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density-wave gap excitation, respectively. The data analysis displays a diminishing impact of the charge-density- wave condensate on the electronic properties with decreasing lattice constant across the rare-earth series. We propose a possible mechanism describing this behavior and we suggest the presence of a one-dimensional character in these two-dimensional compounds. We also envisage that interactions and umklapp processes might play a relevant role in the formation of the charge-density-wave state in these compounds.   

Fermi-liquid effects in the magnetization oscillations in quasi-two-dimensional conductors

In this work we present the results of theoretical analysis of the Haas-van Alphen oscillations in quasi-two-dimensional metals. We have been studying the effect of the Fermi-liquid correlation of charge carriers on the above oscillations. It was shown that at reasonably low temperatures and weak electron scattering the Fermi-liquid interactions may cause noticeable changes in both amplitude and shape of the oscillations even at realistically small values of the Fermi-liquid parameters. Also, we show that the Fermi-liquid interactions in the system of the charge carriers may cause magnetic instability of a quasi-two-dimensional metal near the peaks of quantum oscillations in the electron density of states at the Fermi surface, indicating the possibility for the diamagnetic phase transition within the relevant ranges of the applied magnetic fields. The obtained results are applicable to strongly anisotropic organic metals, and to other quasi-two-dimensional compounds.   

Interface mobility from interface random walk

Computational studies aimed at extracting interface mobilities require driving forces orders of magnitude higher than those occurring experimentally. We present a computational methodology that extracts the absolute interface mobility in the zero driving force limit by monitoring the one-dimensional random walk of the mean interface position along the interface normal. The method exploits a fluctuation-dissipation relation similar to the Stokes-Einstein relation, which relates the diffusion coefficient of this Brownian-like random walk to the interface mobility. Atomic-scale simulations of grain boundaries in model crystalline systems validate the theoretical predictions, and also highlight the profound effect of impurities. The generality of this technique combined with its inherent spatial-temporal efficiency should allow computational studies to effectively complement experiments in understanding interface kinetics in diverse material systems.   

Ab-initio investigations on electronic and lattice dynamical properties on intercalation of Cu (copper) into hexagonal boron nitride(hBN).

Layered structure of hexagonal boron nitride(hBN) and its intercalation with transition metals have been the subject of many recent studies. In this work, we investigate the electronic structure and the lattice dynamical properties of copper intercalated hBN by using Density Functional Theory(DFT) with a plane-wave basis set for the electronic wave functions and periodic boundary conditions.The interaction between valance electrons, the nuclei and the core electrons is described by norm-conserving pseudopotentials. We report the result of calculations on lattice geometry, electronic and lattice dynamical properties of the compound. Possible effects of Cu-incorporation on the structure of hBN were determined from a consideration of minimizing the quantum mechanical total energy and forces. Intercalated Cu atom is found to prefer the position between B and N atoms of two layers. The signature of intercalated Cu states were determined from the calculated electronic local density of states. The phonon frequencies were computed at the center of the Brillouin zone and four Cu-related bands were found at 187, 560, 960, 1206 cm$^{-1}$ which can be measured by IR spectroscopy.   

Current-Driven Phase Oscillation and Domain-Wall Propagation in W$_{x}$V$_{1-x}$O$_{2}$ Nanobeams

We report the observation of a current-driven metal (M)-insulator (I) phase oscillation in two-terminal devices incorporating individual W$_{x}$V$_{1-x}$O$_{2}$ nanobeams connected in parallel with a shunt capacitor. The phase oscillation frequency reaches above 5 MHz for $\sim $1-$\mu $m-long devices. The M-I phase oscillation coincides with the charging/discharging of the capacitor and occurs through the axial drift of a single M-I domain wall driven by Joule heating and the Peltier effect.   

Coincidence Measurements of the Auger Cascade Process in MnO, Ag and Pd.

The Auger spectra associated with Auger cascade processes provides a probe of many-electron phenomena, the effects of screening and correlation in the intermediate and final many hole states. Here we present the first direct measurements of the energy spectra of electrons emitted in the later steps of Auger cascade processes in MnO, Pd and Ag performed using Auger-Auger coincidence spectroscopy (AACS). The Auger spectra resulting from the decay of core holes generated by a previous Auger cascade step (as measured by AACS) are shown to be broadened and shifted as compared to the Auger spectra resulting from the direct photo excitation of the corresponding core holes as measured by Auger photoelectron coincidence (APECS). The large differences between the Auger spectra resulting from the different origins of the core hole excitation are discussed in terms of the correlation effects in many hole excited states.   

Scandium Oxide Thin Films and Their Optical Properties in the EUV

In recent years, it was conjectured that scandium thin films could be used to produce highly reflective coatings in the Extreme Ultraviolet (EUV). However, scandium's affinity to form new compounds prevents such coatings from achieving calculated reflectivities. In this project, thin films of scandium oxide are studied to supplement the understanding and use of scandium, and possibly as a substitute for scandium in multilayer coatings. This study reports on the physical and optical characterization of scandium oxide thin films. Thin films of scandium oxide, 15-50 nanometers thick, were deposited on silicon photodiodes by reactively sputtering scandium in an oxygen environment. These samples were measured using EUV synchrotron radiation at the Lawrence Berkeley National Laboratory Advanced Light Source, Beamline 6.3.2. Reflection and transmission measurements, at variable angles, were taken simultaneously from 2.7-50 nanometers. Analysis of the data has provided experimentally determined optical constants. Additional characterization of the samples includes ellipsometry, scanning transmission electron microscopy, energy dispersive x-ray analysis, and high resolution transmission electron microscopy.   

Quantum statistics for a finite number of polarons in a neutralizing background

The ground state energy of an $N$-polaron system, confined to a spherical quantum dot with a neutralizing background charge, is investigated within an all-coupling many-body path-integral variational principle, taking into account both the Fermi statistics of the polarons and the electron-electron interaction. The treatment of the ground-state energy is performed for both closed-shell and open-shell systems. The average fermion density in the neutral spherical dot is characterized by the Wigner-Seitz parameter $r_s$. For a sufficiently large but finite number of polarons, the dependency of the ground state energy on $r_s$ is similar to that for a polaron gas in bulk. Herefrom, we can conclude that the ground state properties of a polaron gas in bulk can be qualitatively described using a model of a finite number of polarons in a confinement potential provided by a neutralizing background charge.