Diffractive wave guiding of hot electrons by the Au (111) herringbone reconstruction

The surface potential of the herringbone reconstruction on Au(111) is known to guide surface-state electrons along the potential channels. Surprisingly, we find by scanning tunneling spectroscopy that hot electrons with kinetic energies twenty times larger than the potential amplitude (38 meV) are still guided. The efficiency even increases with kinetic energy, which is reproduced by a tight binding calculation taking the known reconstruction potential and strain into account. The guiding is explained by diffraction at the inhomogeneous electrostatic potential and strain distribution provided by the reconstruction.Comment: 10 pages, 9 figure

Bistability and oscillatory motion of natural nano-membranes appearing within monolayer graphene on silicon dioxide

The recently found material graphene is a truly two-dimensional crystal and exhibits, in addition, an extreme mechanical strength. This in combination with the high electron mobility favours graphene for electromechanical investigations down to the quantum limit. Here, we show that a monolayer of graphene on SiO2 provides natural, ultra-small membranes of diameters down to 3 nm, which are caused by the intrinsic rippling of the material. Some of these nano-membranes can be switched hysteretically between two vertical positions using the electric field of the tip of a scanning tunnelling microscope (STM). They can also be forced to oscillatory motion by a low frequency ac-field. Using the mechanical constants determined previously, we estimate a high resonance frequency up to 0.4 THz. This might be favorable for quantum-electromechanics and is prospective for single atom mass spectrometers.Comment: 9 pages, 4 figure

Shock Induced Damages on alumina and zirconia hip prostheses

International audienceCeramics mostly used in orthopaedics are alumina (Al²O³) and zirconia (ZrO²), the rate of fracture bioceramics hip prostheses being around 0.01% [1]. Some in vivo fractures have been reported and some of them have occurred in overweight patients [2]. One may suggest that prostheses may fail because of important repeated mechanical shocks. Moreover, the microseparation between the head and cup that may occur in vivo [3,4] seems to be another key parameter influencing the lifetime of the prostheses. Only few walking hip simulators have been modified to take into account this phenomenon and it has been shown that microseparation increases the wear and creates the so-called wear bands that have been observed in vivo on ceramic heads [5,6]. The main goal of this work was to use a specially designed shock machine to test bioceramics heads and cups under microseparation

Dark matter annihilation and decay profiles for the Reticulum II dwarf spheroidal galaxy

The dwarf spheroidal galaxies (dSph) of the Milky Way are among the most attractive targets for indirect searches of dark matter. In this work, we reconstruct the dark matter annihilation (J-factor) and decay profiles for the newly discovered dSph Reticulum II. Using an optimized spherical Jeans analysis of kinematic data obtained from the Michigan/Magellan Fiber System (M2FS), we find Reticulum II's J-factor to be among the largest of any Milky Way dSph. We have checked the robustness of this result against several ingredients of the analysis. Unless it suffers from tidal disruption or significant inflation of its velocity dispersion from binary stars, Reticulum II may provide a unique window on dark matter particle properties.Comment: 5 pages, 4 figures. Match the ApJL accepted versio

Wave function mapping in graphene quantum dots with soft confinement

Using low-temperature scanning tunneling spectroscopy, we map the local density of states (LDOS) of graphene quantum dots supported on Ir(111). Due to a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphene-like. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moir{\'e} potential and a 10% penetration of an Ir surface resonanceComment: 7 pages, 11 figures, DFT calculations directly showing the origin of soft confinment, correct identification of the state penetrating from Ir(111) into graphen

Cosmic-ray antiproton constraints on light dark matter candidates

Some direct detection experiments have recently collected excess events that could be interpreted as a dark matter (DM) signal, pointing to particles in the $\sim$10 GeV mass range. We show that scenarios in which DM can self-annihilate with significant couplings to quarks are likely excluded by the cosmic-ray (CR) antiproton data, provided the annihilation is S-wave dominated when DM decouples in the early universe. These limits apply to most of supersymmetric candidates, eg in the minimal supersymmetric standard model (MSSM) and in the next-to-MSSM (NMSSM), and more generally to any thermal DM particle with hadronizing annihilation final states.Comment: Contribution to the proceedings of TAUP-2011 (Munich, 5-9 IX 2011). 4 page

Galvanically enhanced fretting-crevice corrosion of cemented femoral stems

The Ultima TPS MoM THR was designed and developed as a 2nd generation MoM THR specifically aimed at younger more active patients due to the anticipated low wear rates and increased longevity of MoM THRs. In 2010, published clinical data highlighted the early failure of the Ultima TPS MoM due to fretting-crevice corrosion at the stem-cement interface. Since 2010 similar observations have been reported by other clinical centres implicating competitor products as well as the Ultima TPS MoM THR. In an attempt to replicate the electrochemical reaction and interactions established across MoM THR systems, fretting-crevice corrosion tests subjected to galvanic coupling were conducted. Galvanic coupling was seen to significantly increase the rates of corrosion under static and dynamic conditions. This was due to the large potential differences developed across the system between active and passive areas, increasing the rates of corrosion and metallic ion release from the stem-cement interface

Hysteresis of Electronic Transport in Graphene Transistors

Graphene field effect transistors commonly comprise graphene flakes lying on SiO2 surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene transistors. These findings improved our understanding of the electrical response of graphene to its surroundings. The unique sensitivity to environment and related phenomena in graphene deserve further studies on nonvolatile memory, electrostatic detection and chemically driven applications.Comment: 13 pages, 6 Figure