Temperature dependence of spin pumping and Gilbert damping in thin Co/Pt bilayers.

We report on the temperature dependence of the spin-pumping effect and the Gilbert damping in Co/Pt bilayers grown on Silicon oxide by measuring the change of the linewidth in a ferromagnetic resonance (FMR) experiment. By varying the Co thickness d(Co) between 1.5 nm and 50 nm we find that the damping increases inversely proportional to d(Co) at all temperatures between 300 K and 5 K, showing that the spin pumping effect does not depend on temperature. We also find that the linewidth increases with decreasing temperature for all thicknesses down to about 30 K, before leveling off to a constant, or even decreasing again. This behavior is similar to what is found in bulk ferromagnets, leading to the conclusion that in thin films a conductivity-like damping mechanism is present similar to what is known in crystals.

Transition voltage spectroscopy and the nature of vacuum tunneling.

Transition voltage spectroscopy (TVS) has been proposed as a tool to analyze charge transport through molecular junctions. We extend TVS to Au-vacuum-Au junctions and study the distance dependence of the transition voltage V(t)(d) for clean electrodes in cryogenic vacuum. On the one hand, this allows us to provide an important reference for V(t)(d) measurements on molecular junctions. On the other hand, we show that TVS forms a simple and powerful test for vacuum tunneling models.

Conductance switching in Ag(2)S devices fabricated by in situ sulfurization.

We report a simple and reproducible method to fabricate switchable Ag(2)S devices. The alpha-Ag(2)S thin films are produced by a sulfurization process after silver deposition on an Si substrate. Structure and composition of the Ag(2)S are characterized using XRD and RBS. Our samples show semiconductor behaviour at low bias voltages, whereas they exhibit reproducible bipolar resistance switching at higher bias voltages. The transition between both types of behaviour is observed by hysteresis in the I-V curves, indicating decomposition of the Ag(2)S, increasing the Ag(+) ion mobility. The as-fabricated Ag(2)S samples are a good candidate for future solid state memory devices, as they show reproducible memory resistive properties and they are fabricated by an accessible and reliable method.

Highly conductive molecular junctions based on direct binding of benzene to platinum electrodes.

Highly conductive molecular junctions were formed by direct binding of benzene molecules between two Pt electrodes. Measurements of conductance, isotopic shift in inelastic spectroscopy, and shot noise compared with calculations provide indications for a stable molecular junction where the benzene molecule is preserved intact and bonded to the Pt leads via carbon atoms. The junction has a conductance comparable to that for metallic atomic junctions (around 0.1-1G0), where the conductance and the number of transmission channels are controlled by the molecule's orientation at different interelectrode distances.

Electron-vibration interaction in single-molecule junctions: from contact to tunneling regimes.

Point contact spectroscopy on a H(2)O molecule bridging Pt electrodes reveals a clear crossover between enhancement and reduction of the conductance due to electron-vibration interaction. As single-channel models predict such a crossover at a transmission probability of tau=0.5, we used shot noise measurements to analyze the transmission and observed at least two channels across the junction where the dominant channel has a tau=0.51 +/- 0.01 transmission probability at the crossover conductance, which is consistent with the predictions for single-channel models.

Formation of a metallic contact: jump to contact revisited.

The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determine the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through a combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers, or double-bond contacts.

Evidence for a single hydrogen molecule connected by an atomic chain.

Stable, single-molecule conducting-bridge configurations are typically identified from peak structures in a conductance histogram. In previous work on Pt with H2 at cryogenic temperatures it has been shown that a peak near 1G0 identifies a single-molecule Pt-H2-Pt bridge. The histogram shows an additional structure with lower conductance that has not been identified. Here, we show that it is likely due to a hydrogen decorated Pt chain in contact with the H2 molecular bridge.

The effect of bonding of a CO molecule on the conductance of atomic metal wires.

We have measured the effect of bonding of a CO molecule on the conductance of Au, Cu, Pt and Ni atomic contacts at 4.2 K. When CO gas is admitted to the metal nanocontacts, a conductance feature appears in the conductance histogram near 0.5 of the quantum unit of conductance, for all metals. For Au, the intensity of this fractional conductance feature can be tuned with the bias voltage, and it disappears at high bias voltage (above approximately 200 mV). The bonding of CO to Au appears to be weakest, and associated with the formation of monatomic Au wire.

Electronic and atomic shell structure in aluminium nanowires.

We report experiments on aluminium nanowires in ultra-high vacuum at room temperature that reveal a periodic spectrum of exceptionally stable structures. Two 'magic' series of stable structures are observed: at low conductance, the formation of stable nanowires is governed by electronic shell effects whereas for larger contacts atomic packing dominates. The crossover between the two regimes is found to be smooth. A detailed comparison of the experimental results to a theoretical stability analysis indicates that, while the main features of the observed electron-shell structure are similar to those of alkali and noble metals, a sequence of extremely stable wires plays a unique role in aluminium. This series appears isolated in conductance histograms and can be attributed to 'superdeformed' non-axisymmetric nanowires.

Vibrationally induced two-level systems in single-molecule junctions.

Single-molecule junctions are found to show anomalous spikes in dI/dV spectra. The position in energy of the spikes is related to local vibration mode energies. A model of vibrationally induced two-level systems reproduces the data very well. This mechanism is expected to be quite general for single-molecule junctions. It acts as an intrinsic amplification mechanism for local vibration mode features and may be exploited as a new spectroscopic tool.