Generic Metrics and Connections on Spin- and Spin c -Manifolds

We study the dependence of the dimension h ⁰(g,A) of the kernel of the Atyiah-Singer Dirac operator on a spin ^c -manifold M on the metric g and the connection A. The main result is that in the case of spin-structures the value of h ⁰(g) for the generic metric is given by the absolute value of the index provided . In dimension 2 the mod-2 index theorems have to be taken into a account and we obtain an extension of a classical result in the theory of Riemann surfaces. In the spin ^c -case we also discuss upper bounds on h ⁰(g,A) for generic metrics, and we obtain a complete result in dimension 2. The much simpler dependence on the connection A and applications to Seiberg–Witten theory are also discussed. Received: 3 July 1996 / Accepted: 27 February 1997     

Size-dependent melting of self-assembled indium nanostructures

We have measured the melting temperature of nanoscale indium islands on a WSe(2) substrate using perturbed angular correlations combined with scanning tunneling microscopy. The indium islands are self-assembled nanostructures whose diameter can vary between about 5 and 100 nm, depending on deposition conditions. The melting point decreases due to surface energies as the islands get smaller. This decrease depends on the faceting of the crystalline nanostructures and interactions between the islands and the substrate.     

Structuring of material parameters in lithium niobate crystals with low-mass, high-energy ion radiation

Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41?MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index ??n, improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of ??n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, ³He??Nb and ³He??O causing thermal spikes, are identified as origin for the material modifications.     

Fiber interferometer for simultaneous multiwavelength phase measurement with a broadband femtosecond laser

We present a fiber interferometer for the simultaneous measurement of phase at multiple wavelengths from a single broadband femtosecond laser. Narrow-bandwidth fiber Bragg gratings isolate a particular frequency from the broad-bandwidth laser pulse produced. The multiwavelength phase data permit the unambiguous measurement range to be significantly increased compared with the wavelengths used in the interferometer. Preliminary experimental results are presented for a two-frequency sensor with an absolute range of 0.13 mm and associated dynamic range of 43,000:1.     

Upper limits on resonance contributions to proton-proton elastic scattering in the c.m. mass range 2.05-2.85 GeV/ c2

Recently published excitation functions in proton-proton (pp) elastic scattering observables in the laboratory energy range 0.5-2.5GeV provide an excellent data base to establish firm upper limits on the elasticities η_el = Γ_el/Γ_tot of possible isovector resonant contributions to the nucleon-nucleon (NN) system. Such contributions have been predicted to arise from dibaryonic states, with c.m. masses between 2.1-2.9GeV/c², but have not been confirmed experimentally. A method to determine quantitatively the maximum value of η_el compatible with experimental data is presented. We use energy-dependent phase shift fits to the pp data base to model the non-resonant interaction. Based upon the differential cross-section data measured by the EDDA Collaboration an unbiased statistical test is constructed to obtain upper limits on η_el, that exclude larger values with a 99% confidence level. Results in the c.m. mass range 2.05-2.85GeV/c² and total widths of 10-100MeV/c² in the partial waves ¹S₀, ¹D₂, ³P₀, ³P₁, and ³F₃ are presented and discussed.     

Superfine Expanded Graphite with Large Capacity for Lithium Storage

A superfine expanded graphite (s‐EG) fiber material was investigated as an anode material for lithium‐based batteries. The fibers were prepared by decomposition of dicarbon monofluoride‐intercalated graphite. The high resolution transmission electron microscopy (HRTEM) images showed the fiber thickness in range of 2–3 nm with several microns in length. Lithium storage capacity in this material was measured in lithium half cells. High lithium storage capacity of about 1000 mAh · g^–1 at a rate of C/10, corresponding to Li₃C₆ composition was obtained. The material showed fairly good rate capability exhibiting lithium storage capabilities even at 60C. As a effect of ball milling, the s‐EG showed crystallographic ordering in the sample with reduced the lithium storage capacity corresponding to composition of LiC₆. A simple mathematical relation to account for the excess lithium storage capacity in this material is put forward.     

The Effect of Specific Adsorption of Cations and Their Size on the Charge‐Compensation Mechanism in Carbon Micropores: The Role of Anion Desorption

Combined application of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) technique reveals a complicated interplay between the adsorption of ammonium and lower molecular weight tetraalkyl ammonium cations and desorption of Cl^? anions inside carbon micropores at low surface charge densities, which results in failure of their permselectivity. Higher negative surface charge densities induce complete exclusion (desorption) of the Cl^? co‐ions, which imparts purely permselective behavior on the carbon micropores. The second fundamental effect discovered herein relates to the dominant role of anion desorption (as compared to cation adsorption), that is, overwhelming failure of permselectivity extends to high negative charge densities of the electrode in the presence of bulky tetraalkyl ammonium cations, which tend to be confined in the micropores of the carbon. The results obtained are important for advancement of high power density carbon‐based supercapacitors, nanofiltration technologies with porous carbon membranes, and studies of ionic transport across biological membranes.     

Confinement effects for ionic carriers in SrTiO3 ultrathin films: first-principles calculations of oxygen vacancies

One-dimensional confinement effects are modelled within the hybrid HF-DFT LCAO approach considering neutral and single-charged oxygen vacancies in SrTiO(3) ultrathin films. The calculations reveal that confinement effects are surprisingly short-range in this partly covalent perovskite; already for film thickness of 2-3 nm (and we believe, similar size nanoparticles) only the surface-plane defect properties differ from those in the bulk. This includes a pronounced decrease of the defect formation energy (by ～1 eV), a much deeper defect band level and a noticeable change in the electronic density redistribution at the near-surface vacancy site with respect to that in the bulk. The results also show that the size effect pertains to the interactions between the oxygen vacancy and two neighboring titanium atoms and orientation (parallel or perpendicular to the surface) of the Ti-V(O)-Ti complex. In particular, we predict considerable oxygen vacancy segregation towards the surface.