Observation of electronic and atomic shell effects in gold nanowires

The formation of gold nanowires in vacuum at room temperature reveals a periodic spectrum of exceptionally stable diameters. This is identified as shell structure similar to that which was recently discovered for alkali metals at low temperatures. The gold nanowires present two competing `magic' series of stable diameters, one governed by electronic structure and the other by the atomic packing.Comment: 4 pages, 4 figure

On maintaining standards in chemistry

Quantum Matter and Optic

Landau diamagnetism and magnetization of interacting diffusive conductors

We show how the orbital magnetization of an interacting disordered diffusive electron gas can be simply related to the magnetization of the non-interacting system having the same geometry. This result is applied to the persistent current of a mesoscopic ring and to the relation between Landau diamagnetism and the interaction correction to the magnetization of diffusive systems. The field dependence of this interaction contribution can be deduced directly from the de Haas-van Alphen oscillations of the free electron gas. Known results for the free orbital magnetism of finite systems can be used to derive the interaction contribution in the diffusive regime in various geometries.Comment: 4 pages, 2 figure

Electronic and optical properties of electromigrated molecular junctions

Electromigrated nanoscale junctions have proven very useful for studying electronic transport at the single-molecule scale. However, confirming that conduction is through precisely the molecule of interest and not some contaminant or metal nanoparticle has remained a persistent challenge, typically requiring a statistical analysis of many devices. We review how transport mechanisms in both purely electronic and optical measurements can be used to infer information about the nanoscale junction configuration. The electronic response to optical excitation is particularly revealing. We briefly discuss surface-enhanced Raman spectroscopy on such junctions, and present new results showing that currents due to optical rectification can provide a means of estimating the local electric field at the junction due to illumination.Comment: 19 pages, 8 figures, invited paper for forthcoming special issue of Journal of Physics: Condensed Matter. For other related papers, see http://www.ruf.rice.edu/~natelson/publications.htm

Method to determine defect positions below a metal surface by STM

The oscillatory voltage dependence of the conductance of a quantum point contact in the presence of a single point-like defect has been analyzed theoretically. Such signals are detectable and may be exploited to obtain information on defect positions below a metal surface. Both tunnel junctions and ballistic contacts of adiabatic shape have been considered. The effect of quantum interference has been taking into account between the principal wave that is directly transmitted through the contact and the partial wave that is scattered by the contact and the defect. This effect leads to oscillations of the conductance as a function of applied voltage. We obtain the dependence of the period and amplitude of the conductance oscillations on the position of the defect inside the metal.Comment: 16 pages, 7 figure

Large tunable image-charge effects in single-molecule junctions

The characteristics of molecular electronic devices are critically determined by metal-organic interfaces, which influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers (SAMs) show (molecule-dependent) level shifts as well as transport-gap renormalization, suggesting that polarization effects in the metal substrate play a key role in the level alignment with respect to the metal's Fermi energy. Here, we provide direct evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport in single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied levels can be followed in situ and with simultaneous mechanical control. When increasing the electrode separation, we observe a substantial increase in the transport gap with level shifts as high as several hundreds of meV for displacements of a few \aa ngstroms. Analysis of this large and tunable gap renormalization with image-charge calculations based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions

Theory of oscillations in the STM conductance resulting from subsurface defects (Review Article)

In this review we present recent theoretical results concerning investigations of single subsurface defects by means of a scanning tunneling microscope (STM). These investigations are based on the effect of quantum interference between the electron partial waves that are directly transmitted through the contact and the partial waves scattered by the defect. In particular, we have shown the possibility imaging the defect position below a metal surface by means of STM. Different types of subsurface defects have been discussed: point-like magnetic and non-magnetic defects, magnetic clusters in a nonmagnetic host metal, and non-magnetic defects in a s-wave superconductor. The effect of Fermi surface anisotropy has been analyzed. Also, results of investigations of the effect of a strong magnetic field to the STM conductance of a tunnel point contact in the presence of a single defect has been presented.Comment: 31 pages, 10 figuers Submitted to Low. Temp. Phy

Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential

The spatial distribution of electric current under magnetic field and the resultant orbital magnetism have been studied for two-dimensional electrons under a harmonic confining potential V(\vecvar{r})=m \omega_0^2 r^2/2 in various regimes of temperature and magnetic field, and the microscopic conditions for the validity of Landau diamagnetism are clarified. Under a weak magnetic field (\omega_c\lsim\omega_0, \omega_c being a cyclotron frequency) and at low temperature (T\lsim\hbar\omega_0), where the orbital magnetic moment fluctuates as a function of the field, the currents are irregularly distributed paramagnetically or diamagnetically inside the bulk region. As the temperature is raised under such a weak field, however, the currents in the bulk region are immediately reduced and finally there only remains the diamagnetic current flowing along the edge. At the same time, the usual Landau diamagnetism results for the total magnetic moment. The origin of this dramatic temperature dependence is seen to be in the multiple reflection of electron waves by the boundary confining potential, which becomes important once the coherence length of electrons gets longer than the system length. Under a stronger field (\omega_c\gsim\omega_0), on the other hand, the currents in the bulk region cause de Haas-van Alphen effect at low temperature as T\lsim\hbar\omega_c. As the temperature gets higher (T\gsim\hbar\omega_c) under such a strong field, the bulk currents are reduced and the Landau diamagnetism by the edge current is recovered.Comment: 15 pages, 11 figure