Self-consistent charge densities at isolated planar defects in metals

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Control of polymorphism in coronene by the application of magnetic fields

Coronene, a polyaromatic hydrocarbon, has been crystallized for the first time in a different polymorph using a crystal growth method that utilizes magnetic fields to access a unit cell configuration that was hitherto unknown. Crystals grown in magnetic field of 1 T are larger, have a different appearance to those grown in zero field and retain their structure in ambient conditions. We identify the new form, beta-coronene, as the most stable at low temperatures. As a result of the new supramolecular configuration we report significantly altered electronic, optical and mechanical properties.Comment: 32 pages, 17 figure

Self-consistent charge and dipole density functional tight binding method and application to carbon-based systems

The density functional tight binding (DFTB) method is a fast, semi-empirical, total energy electronic structure method based upon and parameterized to density functional theory (DFT). The standard self-consistent charge (SCC) DFTB approximates the charge fluctuations in a system using a multipole expansion truncated to the monopole term. For systems with asymmetric charge distributions, such as might be induced by an applied external field, higher terms in the multipole expansion are likely to be important. We have extended the formalism to include dipoles (SCCD), have implemented the method computationally, and test it by calculating the response of various carbon nanotubes and fullerenes to an applied electric field. A comparison of polarizabilities with experimental data or more sophisticated DFT calculations indicates a substantial improvement over standard SCC-DFTB. We also discuss the issues surrounding parameterization of the new SCCD-DFTB scheme

Initiating and imaging the coherent surface dynamics of charge carriers in real space

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out, it can induce adsorbed molecules to react. By comparing large-scale 'before' and 'after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here, we measure the nonlocal manipulation of toluene molecules on the Si(111)-7 × 7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5-15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (that is, ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼10 fs.</p

Open-boundary reflection of quantum well states at Pb(111)

Using a scanning tunneling microscope, confined electron states are studied that exist above subsurface nanometer-sized voids at Pb(111), where potential barriers at the parallel vacuum–Pb(111) and Pb(111)–void interfaces establish a principal series of quantum well states that are further confined laterally by strong reflection at the open boundaries at the edges of the void. The influence of the size, depth and shape of the voids on the effectiveness of the lateral confinement is discussed. Standing wave patterns observed in differential conductance maps unravel the dispersion of the relevant underlying Pb electron states

Raman spectra of monolayer, few-layer, and bulk ReSe₂:An anisotropic layered semiconductor

Rhenium diselenide (ReSe₂) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe₂ is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group <i>P</i>1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe₂ show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe₂ as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe₂

Lateral electron confinement with open boundaries:quantum well states above nanocavities at Pb(111)

We have studied electron states present at the Pb(111) surface above Ar-fillednanocavities created by ion beam irradiation and annealing. Vertical confinement between the parallel crystal and nanocavity surfaces creates a series of quantum well state subbands. Differential conductance data measured by scanning tunneling spectroscopy contain additional spectroscopic features within the highest occupied subband, revealing additional quantization. Unexpectedly, reflection at the open boundary where the thin Pb film recovers its bulk thickness gives rise to lateral confinement of electrons