New twist field couplings from the partition function for multiply wrapped D-branes

We consider toroidal compactifications of bosonic string theory with particular regard to the phases (cocycles) necessary for a consistent definition of the vertex operators, the boundary states and the T-duality rules. We use these ingredients to compute the planar multi-loop partition function describing the interaction among magnetized or intersecting D-branes, also in presence of open string moduli. It turns out that unitarity in the open string channel crucially depends on the presence of the cocycles. We then focus on the 2-loop case and study the degeneration limit where this partition function is directly related to the tree-level 3-point correlators between twist fields. These correlators represent the main ingredient in the computation of Yukawa couplings and other terms in the effective action for D-brane phenomenological models. By factorizing the 2-loop partition function we are able to compute the 3-point couplings for abelian twist fields on generic non-factorized tori, thus generalizing previous expressions valid for the 2-torus.Comment: 36 pages, 1 figure; v2: typos corrected, proof in the Appendix improve

Template Assisted Nucleation of Cobalt and Gold Nano-clusters on an Ultrathin Iron Oxide Film

Being the basic building blocks for nano-magnetic and nano-catalytic devices, regular arrays of nano-clusters play a crucial role in modern nanotechnology. One of the possible fabrication methods of periodic nanostructures consists in exploiting nano-patterned substrates as templates for the self-assembly of the deposited atoms. Here, we have investigated the templating properties of a Moiré superlattice formed at the interface between a FeO(111)-like ultrathin film and a Ni/Fe(001) substrate. Co and Au, representative of elements with high and low oxygen affinity, respectively, have been deposited on the iron oxide film. Scanning tunneling microscopy reveals that Co nucleates preferentially along the corrugated regions of the Moiré superstructure, forming stripes with high aspect ratio. On the other hand, Au atoms nucleate randomly distributed three-dimensional islands on the FeO(111) surface

Intercalation from the Depths: Growth of a Metastable Chromium Carbide between Epitaxial Graphene and Ni(111) by Carbon Segregation from the Bulk

The intercalation of atoms or small molecules underneath graphene epitaxially grown on single metal surfaces is a widely exploited method for modifying the interaction between the carbon monolayer and the substrate. Despite it would be highly desirable to expand the class of the intercalants by including also metal compounds such as oxides, nitrides, or carbides, their use as decoupling layers is a much more challenging task. Here, we demonstrate that it is possible to intercalate an ultrathin layer of Cr-carbide at the graphene/Ni(111) interface by using the carbon dissolved in the bulk of the substrate as a reservoir. Auger electron spectroscopy reveals that Cr deposition on the graphene/Ni(111) interface triggers C segregation from the Ni bulk, while the graphene layer floats on top of the growing film. Scanning tunneling microscopy shows the presence of a periodic superstructure on the surface, due to a coincidence lattice at the graphene/carbide boundary or alternatively to a dislocation network developing at the carbide/Ni(111) interface. Scanning tunneling spectra normalized to the total conductance indicate that the density of states around the Fermi level depends linearly on the energy, suggesting that the graphene layer is electronically decoupled from the Cr-carbide film

Controlling the Electronic and Structural Coupling of C 60

C60 molecules coupled to metals form hybrid systems exploited in a broad range of emerging fields, such as nanoelectronics, spintronics, and photovoltaic solar cells. The electronic coupling at the C60/metal interface plays a crucial role in determining the charge and spin transport in C60-based devices; therefore, a detailed understanding of the interface electronic structure is a prerequisite to engineering the device functionalities. Here, we compare the electronic and structural properties of C60 monolayers interfaced with Fe(001) and oxygen-passivated Fe(001)-p(1 × 1)O substrates. By combining scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy, photoemission and inverse photoemission spectroscopies, we are able to elucidate the striking effect of oxygen on the interaction between Fe(001) and C60. Upon C60 deposition on the oxygen-passivated surface, the oxygen layer remains buried at the C60/Fe(001)-p(1 × 1)O interface, efficiently decoupling the fullerene film from the metallic substrate. Tunneling and photoemission spectroscopies reveal the presence of well-defined molecular resonances for the C60/Fe(001)-p(1 × 1)O system, with a large HOMO-LUMO gap of about 3.4 eV. On the other hand, for the C60/Fe(001) interface, a strong hybridization between the substrate states and the C60 orbitals occurs, resulting in broader molecular resonances

Graphene as an Ideal Buffer Layer for the Growth of High-Quality Ultrathin Cr 2 O 3 Layers on Ni(111)

Metal-oxide nanostructures play a fundamental role in a large number of technological applications, ranging from chemical sensors to data storage devices. As the size of the devices shrinks down to the nanoscale, it is mandatory to obtain sharp and good quality interfaces. Here, it is shown that a two-dimensional material, namely, graphene, can be exploited as an ideal buffer layer to tailor the properties of the interface between a metallic substrate and an ultrathin oxide. This is proven at the interface between an ultrathin film of the magnetoelectric antiferromagnetic oxide Cr 2 O 3 and a Ni(111) single crystal substrate. The chemical composition of the samples has been studied by means of X-ray photoemission spectroscopy, showing that the insertion of graphene, which remains buried at the interface, is able to prevent the oxidation of the substrate. This protective action leads to an ordered and layer-by-layer growth, as revealed by scanning tunneling microscopy data. The structural analysis performed by low-energy electron diffraction indicates that the oxide layer grown on graphene experiences a significant compressive strain, which strongly influences the surface electronic structure observed by scanning tunneling spectroscopy

Incipient Anion Intercalation of Highly Oriented Pyrolytic Graphite Close to the Oxygen Evolution Potential: A Combined X-ray Photoemission and Raman Spectroscopy Study

In the present work, we used two different electrochemical (EC) techniques, namely, cyclic voltammetry and normal pulsed voltammetry, applied to a highly oriented pyrolytic graphite (HOPG) electrode for anion intercalation in two different aqueous electrolytes (i.e., perchloric and sulphuric acid). We performed comparative X-ray photoemission (XPS) and Raman spectroscopy studies at various EC potentials. The chemical analysis obtained by XPS and Raman spectroscopy, the latter applied in situ and in real time during the EC processes, indicates that at oxygen evolution potential (i.e., before reaching the well-known intercalation stage potentials), the HOPG intercalation process is already active. These results suggest that the intercalated compound is efficiently obtained before reaching higher potentials, which usually cause a detriment of the graphite crystal

Enhanced Magnetic Hybridization of a Spinterface through Insertion of a Two-Dimensional Magnetic Oxide Layer

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. Here we propose an approach consisting in the insertion of a two-dimensional magnetic oxide layer at the interface with the aim of both increasing the reproducibility of the interface preparation and offering a way for a further fine control over the electronic and magnetic properties. We have inserted a two-dimensional Cr4O5 layer at the C60/Fe(001) interface and have characterized the corresponding morphological, electronic, and magnetic properties. Scanning tunneling microscopy and electron diffraction show that the film grows well-ordered both in the monolayer and multilayer regimes. Electron spectroscopies confirm that hybridization of the electronic states occurs at the interface. Finally, magnetic dichroism in X-ray absorption shows an unprecedented spin-polarization of the hybridized fullerene states. The latter result is discussed also in light of an ab initio theoretical analysis