Session A15: Metals: 1D, 2D, 3D

Ultrasmall metal clusters and their interaction with a molecular matrix -- the fullerene-Au model system

Metal clusters with less than 50 atoms, are of great interest in the development of true nanoscale electronics. The electronic structure is strongly size dependent and future applications rely on a narrow size distribution. Our goal was to investigate the possibility to use fullerene surfaces as templates in the formation of cluster arrays, to analyze the cluster-fullerene interface and the electronic structure of the components. This study was performed by using photoelectron spectroscopy, and scanning tunneling microscopy and spectroscopy (UHV-conditions). The Au-cluster size is controlled by the coverage and the fullerene lattice limits the cluster surface mobility, making them accessible to STM analysis. The fullerene matrix enhances the cluster stability and ripening occurs above 500 K. A comprehensive description of the Au-cluster-fullerene system has been achieved and includes the interface characteristics, local and global electronic structure, and the spatial distribution of Au-clusters with respect to the fullerene matrix. Our understanding is currently most advanced for the Au-C$_{60}$ and Si- C$_{60 }$system, where in the latter, a preferential nucleation at the interstitial lattice sites of the C$_{60}$ was confirmed.   

Simulation of tunneling I-V curves from gold nanocrystals

Tunneling current versus voltage (I-V) spectra were simulated for model double junction systems where the central electrode has a discrete energy spectrum, chosen to approximately model gold nanocrystals in the diameter range of 1-3 nm. The interplay between Coulomb charging and the discrete energy states introduces interesting features in the I-V curves. Electron-hole excited configurations of the nanocrystal, with up to two e-h excitations, have been included in the calculations. As compared with the orthodox theory, features in the dI/dV spectra can be modified substantially as a consequence of the nanocrystal excited states, depending on the energy spacing between states relative to both the HOMO-LUMO gap and the charging energy.   

Modeling of UV laser-induced patterning of ultrathin Co films on bulk SiO2: verification of short- and long-range ordering mechanisms

Irradiating ultrathin Co films (1 to 10 nm) by a short-pulsed UV laser leads to pattern formation with both short- and long-range order (SRO, LRO). Single beam irradiation produces SRO, while two-beam interference irradiation produces a quasi-2D arrangement of nanoparticles with LRO and SRO. The pattern formation primarily occurs in the molten phase. An estimate of the thermal behavior of the film/substrate composite following a laser pulse is presented. The thermal behavior includes the lifetime of the liquid phase and the thermal gradient during interference heating. Based on this evidence, the SRO is attributed to spinodal dewetting of the film while surface tension gradients induced by the laser interference pattern appear to influence LRO [1]. [1] C.Favazza, J.Trice, H.Krishna, R.Sureshkumar, and R.Kalyanaraman, unpublished.   

X-Ray Study of the Epitaxial Growth of Magnetron Sputtered Ni-Al Thin Films

Epitaxial growth of metals on semiconductors is of significant interest for both fundamental and technological reasons. Nickel-rich NiAl alloys are model materials for high-temperature shape-memory alloys. The sputter deposition growth of NiAl thin films and the X-ray study of the different phases grown on those films are the main purpose of this work. NiAl films were grown on different substrates. The Ni63Al37/Si(001) system is investigated and showed a fiber textured growth along the [111] direction of the low temperature phase L1o. On the other hand, the system NiAl/Cu(001)/Si(001) with different compositions ranging from 36{\%} to 50{\%} Aluminium showed epitaxial BCC films with 2 different domains. The two domains grow in the [110] direction and are rotated 90 degrees with respect to each other. The FCC phase L12 (Ni3Al) was seen to grow first in sample Ni63Al37/Cu(001)/Si(001).   

Two-dimensional magic Ag nanopucks on Pb quantum islands

Periodic patterns of electronic origin found on Pb quantum islands can be employed as the templates to grow self-organized nanopucks of various materials. Owing to a strong interaction existing between Ag and the template, nearly perfect two-dimensional arrays of Ag nanopucks can be grown from 70 K to 150K. Not only is the notable site-selected feature of Ag nanopucks observed on Pb islands but their size distribution also registers a remarkable abundance variation. Those nanopucks with significantly enhanced intensity are denoted as two dimensional magic Ag nanoclusters. Detailed calculations based on \textit{ab initio} density functional theory have been made to illuminate how the size and shape effects related to electronic confinement influence the formation of two-dimensional metal nanostructures. Furthermore, when the Ag nanopuck grows to a certain size, the geometrical effect takes hold from the electronic effect as the major attribute, which drive the Ag nanopucks towards well defined hexagonal crystalline structures.   

Geometric and Electronic Structure of Self-Assembled Monolayers Grown on Noble Metal Substrates: Dodecanethiol on Au, Ag, Cu, and Pt

The geometric and electronic structure of dodecanethiol (C$_{12}$H$_{25}$SH) SAMs on Au(111), Ag(111), Cu(111), and Pt(111)) substrates has been studied using angle-resolved ultra-violet photoelectron spectroscopy and low energy electron diffraction. The SAMs were grown both by vapor deposition in UHV and in solution. The electronic structure of the fully saturated SAM is similar on all of these substrates, with peaks observed at binding energies of 6.5, 10, 14, and 20 eV. The geometric structure of the molecular films at intermediate coverages is different for each substrate. Growth on Au proceeds through a well-ordered lying-down phase followed by a disordered phase and a well-ordered $\surd $3 standing-up phase at saturation. Initial growth on Pt(111) shows first a p(2x2)symmetry followed by a $\surd $3 symmetry, which indicates that the initial growth is via standing-up phases on Pt. This is followed by a disordered phase at saturation. Films on Ag show a great deal of disorder at all stages of growth.   

Scanning tunneling microscopy study of the charge density wave in rare-earth tritellurides

A number of correlated electron oxides exhibit surprisingly intricate ordered density modulations which underlie their novel properties. These modulations arise from incommensurate charge density waves (IC-CDWs). We have applied scanning tunneling microscopy (STM) to study the nature of IC-CDWs in the rare- earth tritellurides -- simple, cleavable, layered materials. In particular, for CeTe3 we have observed both the atomic lattice of surface Te atoms and the CDW modulations oriented at 45 degrees with respect to the Te net. Two-dimensional Fourier transforms of the STM images were obtained to search for discommensurations. These are essentially domain walls whose presence has been strongly suggested by atomic pair distribution function studies. We observe satellite peaks in addition to the principal CDW peaks. These represent strong evidence for the presence of discommensurations with a characteristic length scale of about 38 angstroms.   

Electronic Structure of Ultra-thin Graphite

We report a high resolution angle resolved photoemission spectroscopy (ARPES) study on ultra-thin graphite samples, grown and characterized successfully in ultra-high-vacuum environment by a thermalization of SiC. We discuss similarities to and differences from the data obtained on bulk graphite. Notable differences include doping, electronic structure as a function of k$_{z}$ (momentum component perpendicular to graphene layer), line width, and substrate-induced features in ultra-thin graphite samples. We discuss the effects of grain boundaries, disorder, and symmetry breaking, as possible explanations of these differences.   

Transverse Plasmon Wakes in the Electron Gas

Relativistic electrons have transverse electric fields comparable in magnitude to the longitudinal fields. We determine the relative effects of transverse and longitudinal fields of a moving point charge on the dielectric response of a uniform electron gas, using Lindhard's longitudinal and transverse dielectric functions and, separately, the Drude dielectric function. In the direction of motion, the transverse wake fields are approximately $-v^2/c^2$ times the longitudinal wake fields. The stopping power, as determined by applying Poynting's theorem, is reduced. Perpendicular to the direction of motion, the transverse wake fields are small compared to the longitudinal fields. Electromagnetic fields of a moving point charge are given in Fourier space for the Lorentz, Hamiltonian, and Coulomb gauges.   

Optical conductivity of MnN: a combined experimental and theoretical study.

A comparison between measured and calculated optical conductivity is presented for MnN films prepared by ion assisted deposition. X-ray diffraction and extended x-ray absorption fine structure show the films to be nanocrystalline but phase pure. X-ray emission spectroscopy of the N K-edge and X-ray absorption near edge spectroscopy of the N K- and Mn L-edges are used to probe the occupied and empty densities of states, which compare well with the N-2p and Mn-3d partial densities of states calculated using the linearized muffin-tin orbital method. The optical conductivity was measured by spectroscopic ellipsometry/reflectivity in the infared to UV range. The major differences between calculated and measured spectra can be understood on the basis of a limited electron mean-free-path in these nanocrystalline films, which relaxes the momentum conservation requirement. The calculated optical functions are analyzed in terms of their dominant band-to-band contributions including the polarization dependence. The temperature dependent conductivity shows a clear metallic behaviour and a weak Kondo-like low temperature anomaly.   

Europium $L_{2,3}$ and iron $K$-edge x-ray magnetic circular dichroism investigation of ferromagnetic ordering in EuFe$_{4}$Sb$_{12}$

The magnetic behavior of Eu and Fe in the filled skutterudite ferromagnet EuFe$_{4}$Sb$_{12}$ has been investigated using Eu $L_{2,3}$ edge and Fe $K$ edge x-ray magnetic circular dichroism (XMCD) spectroscopy. Eu $L_3$ edge x-ray absorption spectra (XAS) in EuFe$_{4}$Sb$_{12}$ clearly show that Eu is in a mixed valence state with about 15$\%$ non-magnetic Eu$^{3+}$ states at 5 K. By comparing the XMCD spectra measured at the Eu $L_{2,3}$ edges in the ferromagnetic state at 5 K in EuFe$_{4}$Sb$_{12}$ and in the clathrate Eu$_8$Ga$_{16}$Ge$_{30}$, in which the {\it 4f} magnetic moment of Eu is known to be 7 $\mu_B$, and by accounting for the mixed valence of Eu in the XAS, we show that Eu$^{2+}$ has the free ion like magnetic moment of $\sim$7 $\mu_B$ in EuFe$_{4}$Sb$_{12}$. XMCD observed at the Fe $K$ edge in EuFe$_{4}$Sb$_{12}$ at 5 K indicates magnetic short range order and a small orbital magnetic moment for the {\it 4p} states of Fe. The relative signs of XMCD at Eu $L_3$ edge and Fe $K$ edge indicate that the $5d$ spin moment of Eu and the $4p$ spin moment of Fe are ferromagnetically coupled in EuFe$_{4}$Sb$_{12}$.   

Modeling of LMM-MVV Auger-Auger Coincidence Spectra From Solids

Atoms that are highly excited due to the presence of a hole in an inner shell often relax via an Auger transition. This auto-ionizing process results in a final state with two or more holes from an Auger cascade. We present results of the direct measurements of the second and third Auger decays in this sequence. We have measured the Mn MVV Auger spectra from a single-crystal sample of MnO in time coincidence with Auger electrons emitted from prior Mn LMM Auger decays and find these to be much wider than the MVV spectrum measured in time coincidence with M core photoelectron emission. We present a model which attributes the increased energy width of the MVV transitions that follow LMM decays to the rearrangement of ``not so innocent'' bystander hole(s) in the valence band. The energetics of the Auger cascade process are modeled mathematically in terms of correlation integral(s) and convolution integral(s) over the valence band density of states. Comparisons with recent Auger-Auger coincidence studies of Ag and Pd will be made. Acknowledgements: Welch Foundation, NSF DMR98-12628, NSF DMR98-01681, and DOE DE-AC02-98CH10886.   

EuB$_6$ and the Low-Density Double Exchange Model

The low electronic density regime of the double exchange model is explored in the presence of electron-electron interactions. The single particle problem and its extension to low densities, when a Wigner crystal of magnetic polarons is generated due to unscreened Coulomb interactions, is studied. It is argued that the Wigner crystal is the natural alternative to phase separation when the Coulomb interaction is taken into account. We address the thermal and quantum stability of the crystalline phase towards a polaronic Fermi liquid and a homogeneous, metallic, ferromagnetic phase. These results are relevant in the context of the polaronic physics recentely observed in EuB$_6$, and provide an important consistency-check for the double-exchange description of these magnetic hexaborides. \newline {\bf References:} \newline V.~M.~Pereira \emph{et~al.}, \texttt{ cond-mat/0505741}; \newline V.~M. Pereira \emph{et~al.}, Phys. Rev. Lett. \textbf{93}, 147202 (2004).   

Magneto-optical evidence of double exchange in a percolating lattice

Substituting $Eu$ by $Ca$ in ferromagnetic $EuB_6$ leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the $Eu_{1- x}Ca_{x}B_6$ series, based on measurements of the reflectivity $R(\omega)$ from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of $R(\omega)$ we extract the complete absorption spectra of samples with different values of $x$. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing $Ca$-content at low temperatures.   

Constructing the effective spin model of Mn$_{6}$R$_{6}$ from ab initio calculations

Determination of the effective spin model is an important issue for molecular magnetism. Generally, the effective spin model is determined so that the model reproduces the structure of the excitation spectrum. Only this strategy sometimes could not determine which model is the most suitable among several candidates. An example showing this difficulty is seen in [Mn(hfac)$_{2}$NITPh]$_{6}$ abbreviated as Mn$_{6}$R$_{6}$. Two different models are proposed for Mn$_{6}$R$_{6}$ by reproducing the magnetization process. One is the Heisenberg model with three-spin interactions. The other is the Heisenberg model with frustration. In this work, in order to examine the validity of each model, we calculate the spin density by the \textit{ab- initio} calculation based on the unrestricted Hartree-Fock method and the density functional theory. We also calculate the spin density of the ground state in each model by numerical diagonalization. Each model shows the different spin density. The spin density of the ground state should give essential knowledge so as to constructing the effective spin model. We discuss the method to connect the effective spin model with the first principle calculation by calculating the spin density.