Karsten Pohl

Identifying the forces responsible for self-organization of nanostructures at crystal surfaces

The spontaneous formation of organized surface structures at nanometre scales, has the potential to augment or surpass standard materials patterning technologies. Many observations of self-organization of nanoscale clusters at surfaces have been reported, but the fundamental mechanisms underlying such behaviour — and in particular, the nature of the forces leading to and stabilizing self-organization — are not well understood. The forces between the many-atom units in these structures, with characteristic dimensions of one to tens of nanometres, must extend far beyond the range of typical interatomic interactions. One commonly accepted source of such mesoscale forces is the stress field in the substrate around each unit,. This, however, has not been confirmed, nor have such interactions been measured directly. Here we identify and measure the ordering forces in a nearly perfect triangular lattice of nanometre-sized vacancy islands that forms when a single monolayer of silver on the ruthenium (0001) surface is exposed to sulphur at room temperature. By using time-resolved scanning tunnelling microscopy to monitor the thermal fluctuations of the centres of mass of the vacancy islands around their final positions in the self-organized lattice, we obtain the elastic constants of the lattice and show that the weak forces responsible for its stability can be quantified. Our results are consistent with general theories of strain-mediated interactions between surface defects in strained films.

Low-energy acoustic plasmons at metal surfaces

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons—higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

Spatially-Resolved Structure and Electronic Properties of Graphene on Polycrystalline Ni

We have used in situ low-energy electron microscopy (LEEM) to correlate the atomic and electronic structure of graphene films on polycrystalline Ni with nm-scale spatial resolution. Spatially resolved electron scattering measurements show that graphene monolayers formed by carbon segregation do not support the π-plasmon of graphene, indicating strong covalent bonding to the Ni. Graphene bilayers have the Bernal stacking characteristic of graphite and show the expected plasmon loss at 6.5 eV. The experimental results, in agreement with first-principles calculations, show that the π-band structure of free-standing graphene appears only in films with a thickness of at least two layers and demonstrate the sensitivity of the plasmon loss to the electronic structure.

Acoustic surface plasmon on Cu(111)

Contrary to previous reports we show that the acoustic surface plasmon (ASP) exists also at noble-metal surfaces, thus demonstrating the generality of this phenomenon in the presence of partially filled Shockley surface states. Angle-resolved high-resolution electron energy loss spectroscopy measurements and calculations of the surface loss function indicate that for Cu(111) the ASP is a sharp feature up to a loss energy of about 0.4 eV. The dispersion is indeed linear (acoustic) with a slope (sound velocity) of (4.33±0.33) eVA in good agreement with recent theoretical predictions. The ASP can play important roles down to the meV regime, precluded to ordinary surface plasmons, for electron, phonon and adsorbate dynamics, as well as chemical reactions and advanced microscopies.

Multilayer relaxation of the Mg(0001) surface

Abstract We have Investigated the interplanar relaxation of the clean (0001)-(1 × 1) surface of magnesium at 100 K using a dynamical LEED I - V analysis. In contrast to almost all other metal surfaces, an expansion has been observed for the first interlayer spacing of this clean surface. Using an extended database, the results indicate that the first three interlayer spacings are relaxed from the bulk value by Δ d 12 = +1.9 ± 0.3%, Δ d 23 = +0.8 ± 0.4%, and Δ d 34 = −0.4 ± 0.5%. A comparison of this observed multi relaxation with experimental and theoretical results for similar free-electron closepacked metal surfaces, e.g. Al(111), suggests that a surface expansion is a normal property of high electron density simple metals.

An ultrahigh vacuum fast-scanning and variable temperature scanning tunneling microscope for large scale imaging

We describe the design and performance of a fast-scanning, variable temperature scanning tunneling microscope (STM) operating from 80 to 700 K in ultrahigh vacuum (UHV), which routinely achieves large scale atomically resolved imaging of compact metallic surfaces. An efficient in-vacuum vibration isolation and cryogenic system allows for no external vibration isolation of the UHV chamber. The design of the sample holder and STM head permits imaging of the same nanometer-size area of the sample before and after sample preparation outside the STM base. Refractory metal samples are frequently annealed up to 2000 K and their cooldown time from room temperature to 80 K is 15 min. The vertical resolution of the instrument was found to be about 2 pm at room temperature. The coarse motor design allows both translation and rotation of the scanner tube. The total scanning area is about 8 x 8 microm(2). The sample temperature can be adjusted by a few tens of degrees while scanning over the same sample area.

Highly Ordered Assembly of Single-Domain Dichloropentacene over Large Areas on Vicinal Gold Surfaces

Defining pathways to assemble long-range-ordered 2D nanostructures of specifically designed organic molecules is required in order to optimize the performance of organic thin-film electronic devices. We report on the rapid fabrication of a nearly perfect self-assembled monolayer (SAM) composed of a single-domain 6,13-dichloropentacene (DCP) brick-wall pattern on Au(788). Scanning tunneling microscopy (STM) results show the well-ordered DCP SAM extends over hundreds of nanometers. Combining STM results with insights from density functional theory, we propose that a combination of unique intermolecular and molecule-step interactions drives the DCP SAM formation.

Band structure effects on the Be(0001) acoustic surface plasmon energy dispersion

We report first-principles calculations of acoustic surface plasmons on the (0001) surface of Be, as obtained in the randomphase approximation of many-body theory. The energy dispersion of these collective excitations has been obtained along two symmetry directions. Our results show a considerable anisotropy of acoustic surface plasmons, and underline the capability of experimental measurements of these plasmons to map the electron-hole excitation spectrum of the two-dimensional Shockley surface state band that is present on the Be(0001) surface.

Electronic Structure of the Metastable Epitaxial Rock-Salt SnSe {111} Topological Crystalline Insulator

Wencan Jin, ∗ Suresh Vishwanath, ∗ Jianpeng Liu, Lingyuan Kong, Rui Lou, Zhongwei Dai, Jerzy T. Sadowski, Xinyu Liu, Huai-Hsun Lien, Alexander Chaney, Yimo Han, Micheal Cao, Junzhang Ma, Tian Qian, Jerry I. Dadap, Shancai Wang, Malgorzata Dobrowolska, Jacek Furdyna, David A. Muller, Karsten Pohl, Hong Ding, Huili Grace Xing, 8, † and Richard M. Osgood, Jr ‡ Columbia University, New York, New York 10027, USA Cornell University, Ithaca, New York 14853, USA Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106, USA Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Renmin University of China, Beijing 100872, China University of New Hampshire, Durham, NH 03824, USA Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA University of Notre Dame, Notre Dame, Indiana 46556, USATopological crystalline insulators have been recently predicted and observed in rock-salt structure SnSe

Highly uniform step and terrace structures on SiC(0001) surfaces

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