Symmetry and Electronic Structure of Noble Metal Nanoparticles and the Role of Relativity

High resolution photoelectron spectra of cold mass selected Cu_n-, Ag_n- and Au_n- with n =53-58 have been measured at a photon energy of 6.42 eV. The observed electron density of states is not the expected simple electron shell structure, but seems to be strongly influenced by electron-lattice interactions. Only Cu55- and Ag55- exhibit highly degenerate states. This is a direct consequence of their icosahedral symmetry, as is confirmed by density functional theory calculations. Neighboring sizes exhibit perturbed electronic structures, as they are formed by removal or addition of atoms to the icosahedron and therefore have lower symmetries. Gold clusters in the same size range show completely different spectra with almost no degeneracy, which indicates that they have structures of much lower symmetry. This behaviour is related to strong relativistic bonding effects in gold, as demonstrated by ab initio calculations for Au55-.Comment: 10 pages, 3 figure

Electronic-structure-induced deformations of liquid metal clusters

Ab initio molecular dynamics is used to study deformations of sodium clusters at temperatures $500\cdots 1100$ K. Open-shell Na$_{14}$ cluster has two shape isomers, prolate and oblate, in the liquid state. The deformation is stabilized by opening a gap at the Fermi level. The closed-shell Na$_8$ remains magic also at the liquid state.Comment: REVTex, 11 pages, no figures, figures (2) available upon request (e-mail to hakkinen at jyfl.jyu.fi), submitted to Phys. Rev.

Structure and Magnetism of Neutral and Anionic Palladium Clusters

The properties of neutral and anionic Pd_N clusters were investigated with spin-density-functional calculations. The ground state structures are three-dimensional for N>3 and they are magnetic with a spin-triplet for 2<=N<=7 and a spin nonet for N=13 neutral clusters. Structural- and spin-isomers were determined and an anomalous increase of the magnetic moment with temperature is predicted for a Pd_7 ensemble. Vertical electron detachment and ionization energies were calculated and the former agree well with measured values for anionic Pd_N clusters.Comment: 5 pages, 3 figures, fig. 2 in color, accepted to Phys. Rev. Lett. (2001

Close-Packing of Clusters: Application to Al_100

The lowest energy configurations of close-packed clusters up to N=110 atoms with stacking faults are studied using the Monte Carlo method with Metropolis algorithm. Two types of contact interactions, a pair-potential and a many-atom interaction, are used. Enhanced stability is shown for N=12, 26, 38, 50, 59, 61, 68, 75, 79, 86, 100 and 102, of which only the sizes 38, 75, 79, 86, and 102 are pure FCC clusters, the others having stacking faults. A connection between the model potential and density functional calculations is studied in the case of Al_100. The density functional calculations are consistent with the experimental fact that there exist epitaxially grown FCC clusters starting from relatively small cluster sizes. Calculations also show that several other close-packed motifs existwith comparable total energies.Comment: 9 pages, 7 figure

Common Origin for Surface Reconstruction and the Formation of Chains of Metal Atoms

During the fracture of nanocontacts gold spontaneously forms freely suspended chains of atoms, which is not observed for the iso-electronic noble metals Ag and Cu. Au also differs from Ag and Cu in forming reconstructions at its low-index surfaces. Using mechanically controllable break junctions we show that all the 5d metals that show similar reconstructions (Ir, Pt and Au) also form chains of atoms, while both properties are absent in the 4d neighbor elements (Rh, Pd, Ag), indicating a common origin for these two phenomena. A competition between s and d bonding is proposed as an explanation

The role of structural evolution on the quantum conductance behavior of gold nanowires during stretching

Gold nanowires generated by mechanical stretching have been shown to adopt only three kinds of configurations where their atomic arrangements adjust such that either the [100], [111] or [110] zone axes lie parallel to the elongation direction. We have analyzed the relationship between structural rearrangements and electronic transport behavior during the elongation of Au nanowires for each of the three possibilities. We have used two independent experiments to tackle this problem, high resolution transmission high resolution electron microscopy to observe the atomic structure and a mechanically controlled break junction to measure the transport properties. We have estimated the conductance of nanowires using a theoretical method based on the extended H\"uckel theory that takes into account the atom species and their positions. Aided by these calculations, we have consistently connected both sets of experimental results and modeled the evolution process of gold nanowires whose conductance lies within the first and third conductance quanta. We have also presented evidence that carbon acts as a contaminant, lowering the conductance of one-atom-thick wires.Comment: 10 page

Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations

Photoelectron (PES) spectra from aluminum cluster anions (from 12 to 15 atoms) at various temperature regimes, were studied using ab-initio molecular dynamics simulations and experimentally. The calculated PES spectra, obtained via shifting of the simulated electronic densities of states by the self-consistently determined values of the asymptotic exchange-correlation potential, agree well with the measured ones, allowing reliable structural assignments and theoretical estimation of the clusters' temperatures.Comment: RevTex, 3 gif figures. Scheduled for Oct 15, 1999, issue of Phys. Rev. B as Rapid Communicatio

Spectroscopy with random and displaced random ensembles

Due to the time reversal invariance of the angular momentum operator J^2, the average energies and variances at fixed J for random two-body Hamiltonians exhibit odd-even-J staggering, that may be especially strong for J=0. It is shown that upon ensemble averaging over random runs, this behaviour is reflected in the yrast states. Displaced (attractive) random ensembles lead to rotational spectra with strongly enhanced BE2 transitions for a certain class of model spaces. It is explained how to generalize these results to other forms of collectivity.Comment: 4 pages, 4 figure

Reoperations after first lumbar disc herniation surgery; a special interest on residives during a 5-year follow-up

BACKGROUND: The overall rate of operations after recurrent lumbar disc herniation has been shown to be 3–11%. However, little is known about the rate of residives. Thus the aim of this study was to explore the cumulative rates of re-operations and especially residive disc herniations at the same side and level as the primary disc herniation after first lumbar disc herniation surgery and the factors that influence the risk of re-operations over a five year follow-up study. METHODS: 166 virgin lumbar disc herniation patients (mean age 42 years, 57% males) were studied. Data on patients' initial disc operations and type and timing of re-operations during the follow-up were collected from patient files. Back and leg pain on visual analog scale and employment status were collected by questionnaires. RESULTS: The cumulative rate of re-operations for lumbar disc herniation was 10.2% (95% Cl 6.0 to 15.1). The rate of residives at initial site was 7.4% (95% Cl 3.7 to 11.3) and rate of lumbar disc herniations at other sites was 3.1% (95% Cl 0.6 to 6.2). The occurrence of residive lumbar disc herniations was evenly distributed across the 5 years. Neither age, gender, preoperative symptoms, physical activity nor employment had effect on the probability of re-operation. CONCLUSION: Seven percent of the lumbar disc patients had a residive lumbar disc operation within five years of their first operation. No specific factors influencing the risk for re-operation were found

Stability of gold nanowires at large Au-Au separations

The unusual structural stability of gold nanowires at large separations of gold atoms is explained from first-principles quantum mechanical calculations. We show that undetected light atoms, in particular hydrogen, stabilize the experimentally observed structures, which would be unstable in pure gold wires. The enhanced cohesion is due to the partial charge transfer from gold to the light atoms. This finding should resolve a long-standing controversy between theoretical predictions and experimental observations.Comment: 7 pages, 3 figure