Absence of a proximity effect for a thin-films of a Bi2Se3 topological insulator grown on top of a Bi2Sr2CaCu2O(8+δ) cuprate superconductor.

Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional s-wave superconductors put hard constraints on these experiments. Significantly larger gaps and higher transition temperatures in cuprate superconductors might be an attractive alternative to considerably relax these constraints, but it is not clear whether the proximity effect would be effective in heterostructures involving cuprates and topological insulators. Here, we present angle-resolved photoemission studies of thin Bi(2)Se(3) films grown in situ on optimally doped Bi(2)Sr(2)CaCu(2)O(8+δ) substrates that show the absence of proximity-induced gaps on the surfaces of Bi(2)Se(3) films as thin as a 1.5 quintuple layer. These results suggest that the superconducting proximity effect between a cuprate superconductor and a topological insulator is strongly suppressed, likely due to a very short coherence length along the c axis, incompatible crystal and pairing symmetries at the interface, small size of the topological surface state's Fermi surface, and adverse effects of a strong spin-orbit coupling in the topological material.

The mechanism of caesium intercalation of graphene.

Properties of many layered materials, including copper- and iron-based superconductors, topological insulators, graphite and epitaxial graphene, can be manipulated by the inclusion of different atomic and molecular species between the layers via a process known as intercalation. For example, intercalation in graphite can lead to superconductivity and is crucial in the working cycle of modern batteries and supercapacitors. Intercalation involves complex diffusion processes along and across the layers; however, the microscopic mechanisms and dynamics of these processes are not well understood. Here we report on a novel mechanism for intercalation and entrapment of alkali atoms under epitaxial graphene. We find that the intercalation is adjusted by the van der Waals interaction, with the dynamics governed by defects anchored to graphene wrinkles. Our findings are relevant for the future design and application of graphene-based nano-structures. Similar mechanisms can also have a role for intercalation of layered materials.

Growth mode and oxidation state analysis of individual cerium oxide islands on Ru(0001).

The growth of cerium oxide on Ru(0001) by reactive molecular beam epitaxy has been investigated using low-energy electron microscopy (LEEM) and diffraction as well as local valence band photoemission. The oxide islands are found to adopt a carpet-like growth mode, which depending on the local substrate morphology and misorientation leads to deviations from the otherwise almost perfect equilateral shape at a growth temperature of 850 °C. Furthermore, although even at this high growth temperature the micron-sized CeO2(111) islands are found to exhibit different lattice registries with respect to the hexagonal substrate, the combination of dark-field LEEM and local intensity-voltage analysis reveals that the oxidation state of the islands is homogeneous down to the 10 nm scale.

The electron spectro-microscopy beamline at National Synchrotron Light Source II: a wide photon energy range, micro-focusing beamline for photoelectron spectro-microscopies.

A comprehensive optical design for a high-resolution, high-flux, wide-energy range, micro-focused beamline working in the vacuum ultraviolet and soft x-ray photon energy range is proposed. The beamline is to provide monochromatic radiation to three photoelectron microscopes: a full-field x-ray photoelectron emission microscope and two scanning instruments, one dedicated to angle resolved photoemission spectroscopy (µ-ARPES) and one for ambient pressure x-ray photoelectron spectroscopy and scanning photoelectron microscopy (AP-XPS/SPEM). Microfocusing is achieved with state of the art elliptical cylinders, obtaining a spot size of 1 µm for ARPES and 0.5 µm for AP-XPS/SPEM. A detailed ray tracing analysis quantitatively evaluates the overall beamline performances.

Electronic structure of few-layer epitaxial graphene on Ru(0001).

The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.

Comparative studies of pentacene and perfluoropentacene grown on a Bi(0001) surface.

We report two distinct growth modes of pentacene (PEN) and perfluoropentacene (PFP) films on a Bi(0001) substrate investigated by scanning tunneling microscopy (STM). PEN grows epitaxially on Bi(0001) at room temperature (RT), resulting in the formation of bulk-like crystalline films. In contrast, submonolayer PFP forms a two-dimensional (2D) liquid-like phase with PFP molecules loosely bound on Bi(0001). Beyond one monolayer, the PFP molecules diffuse over very long distances to aggregate into three-dimensional (3D) islands, leading to a rough film morphology. Utilizing the stacking interaction at the PFP/PEN interface, we deposited PFP on the template of an ordered PEN monolayer formed on Bi. It is found that PFP molecules nucleate into ordered crystalline islands with PFP molecules standing-up. The different morphologies of PEN and PFP overlayers can be understood in terms of perfluorination induced decoupling of PFP molecules from the Bi substrate below.

Temperature dependent low energy electron microscopy study of Ge island growth on bare and Ga terminated Si(112).

The pre-adsorption of Ga on Si(112) leads to a drastic change of the morphology of subsequently grown Ge islands. In contrast to the case for Ge growth on bare Si(112), even nanowire growth can be achieved on Ga terminated Si(112). Employing low energy electron microscopy and low energy electron diffraction, the initial phase of Ge nucleation and Ge island growth was systematically analysed for growth temperatures between 420 and 610 °C, both on clean and on Ga terminated Si(112). In both cases the island density exhibits an Arrhenius-like behaviour, from which diffusion barrier heights of about 1.3 and 1.0 eV can be estimated for growth with and without Ga pre-adsorption, respectively. The Ge island shape on the bare Si(112) surface is found to be nearly circular over the whole temperature range, whereas the shapes of the Ge islands on the Ga terminated Si(112) become highly anisotropic for higher temperatures. Ge nanowires with sizes of up to 2 µm along the [Formula: see text] direction are observed.

Fluorine diffusion assisted by diffusing silicon on the Si(111)-(7x7) surface.

The diffusion process of fluorine (F) atoms on the Si(111)-(7x7) surface is investigated using high-temperature scanning tunneling microscopy. The kinetic parameters of F hopping agree well with those of the diffusing silicon (Si) atoms, which implies that of all reaction processes, the Si diffusion serves as the rate-determining one. Deposition of Si on the surface is found to enhance F hopping, which supports the above-mentioned observation. Theory reveals that the replacement of F adsorption sites by diffusing Si atoms is the key process in the diffusion mechanism.

Single-nucleus polycrystallization in thin film epitaxial growth.

We have observed, by use of low-energy electron microscopy, the first direct evidence of self-driven polycrystallization evolved from a single nucleus in the case of epitaxial pentacene growth on the Si(111)-H terminated surface. In this Letter we demonstrate that such polycrystallization can develop in anisotropic systems (in terms of crystal structure and/or the intermolecular interactions) when kinetic growth conditions force the alignment of the intrinsic preferential growth directions along the density gradient of diffusing molecules. This finding gives new insight into the crystallization of complex molecular systems, elucidating the importance of nanoscale control of the growth conditions.

Role of surface electronic structure in thin film molecular ordering.

We show that the orientation of pentacene molecules is controlled by the electronic structure of the surface on which they are deposited. We suggest that the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene pi orbitals. A reduction of this density as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5 x 2)Au surfaces, results in pentacene standing up. Interestingly, pentacene grown on Bi(001) is highly ordered, yielding the first vertically oriented epitaxial pentacene thin films observed to date.

Nanofilm allotrope and phase transformation of ultrathin Bi film on Si(111)-7x7.

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the Si(111)-7x7 surface. This pseudocubic [012]-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) phase, as the bulk contribution in the cohesion becomes dominant. Based on ab initio calculations, we propose that the new allotrope consists of black phosphorus-like puckered layers stabilized by saturating all the p(z) dangling bonds in the film.