Determination of the Thermomigration Force on Adatoms.

Thermal gradients in nanomaterials can cause surface mass transport phenomena. However, the atomic fluxes are challenging to quantify and the underlying atomic mechanisms are complex. Using low energy electron microscopy we have examined in operando, under a thermal gradient of 10^{4} K/m, the thermomigration of supercooled Si(111)-1×1 advacancy islands. The islands move in the direction of the thermal gradient at 0.26±0.06 nm/s. This reveals that the adatoms move toward the cold region and the effective force exerted on Si adatoms is 1.4±0.4×10^{-8} eV/nm. We quantify the heat of transport of Si atoms Q^{*}=1.2±0.4 eV and show that it corresponds to the combined effects of adatom creation at step edges and adatom diffusion on atomically flat terraces.

Atomic Transport in Au-Ge Droplets: Brownian and Electromigration Dynamics.

The deposition of Au on Ge(111)-sqrt[3]×sqrt[3]-Au above the eutectic temperature results in the formation of AuGe liquid droplets that reach the liquidus composition by digging a hole in the Ge substrate. The combination of low-energy electron microscopy and atomic force microscopy measurements shows that AuGe droplets randomly migrate or electromigrate under an applied electric current dragging their underneath hole. The droplet motion is due to a mass transport phenomenon based on Ge dissolution at the droplet front and Ge crystallization at its rear. At high temperature the mass transport is limited by attachment or detachment at the solid-liquid interface and the activation energy is 1.05±0.3 eV. At low temperature the effective activation energy increases as a function of the droplet radius. This behavior is attributed to the nucleation of 2D layers at the faceted liquid-solid interface.

Spatial inhomogeneity and temporal dynamics of a 2D electron gas in interaction with a 2D adatom gas.

Fundamental interest for 2D electron gas (2DEG) systems has been recently renewed with the advent of 2D materials and their potential high-impact applications in optoelectronics. Here, we investigate a 2DEG created by the electron transfer from a Ag adatom gas deposited on a Si(111) [Formula: see text]-Ag surface to an electronic surface state. Using low-energy electron microscopy (LEEM), we measure the Ag adatom gas concentration and the 2DEG-induced charge transfer. We demonstrate a linear dependence of the surface work function change on the Ag adatom gas concentration. A breakdown of the linear relationship is induced by the occurrence of the Ag adatom gas superstructure identified as Si(111) [Formula: see text]-Ag only observed below room temperature. We evidence below room temperature a confinement of the 2DEG on atomic terraces characterised by spatial inhomogeneities of the 2DEG-induced charge transfer along with temporal fluctuations. These variations mirror the Ag adatom gas concentration changes induced by the growth of 3D Ag islands and the occurrence of an Ehrlich-Schwoebel diffusion barrier of 155 ± 10 meV.

Quantum Hall resistance standard in graphene devices under relaxed experimental conditions.

The quantum Hall effect provides a universal standard for electrical resistance that is theoretically based on only the Planck constant h and the electron charge e. Currently, this standard is implemented in GaAs/AlGaAs, but graphene's electronic properties have given hope for a more practical device. Here, we demonstrate that the experimental conditions necessary for the operation of devices made of high-quality graphene grown by chemical vapour deposition on silicon carbide can be extended and significantly relaxed compared with those for state-of-the-art GaAs/AlGaAs devices. In particular, the Hall resistance can be accurately quantized to within 1 × 10(-9) over a 10 T wide range of magnetic flux density, down to 3.5 T, at a temperature of up to 10 K or with a current of up to 0.5 mA. This experimental simplification highlights the great potential of graphene in the development of user-friendly and versatile quantum standards that are compatible with broader industrial uses beyond those in national metrology institutes. Furthermore, the measured agreement of the quantized Hall resistance in graphene and GaAs/AlGaAs, with an ultimate uncertainty of 8.2 × 10(-11), supports the universality of the quantum Hall effect. This also provides evidence of the relation of the quantized Hall resistance with h and e, which is crucial for the new Système International d'unités to be based on fixing such fundamental constants of nature.

Combining low-energy electron microscopy and scanning probe microscopy techniques for surface science: development of a novel sample-holder.

We introduce an experimental facility dedicated to surface science that combines Low-Energy Electron Microscopy/Photo-Electron Emission Microscopy (LEEM/PEEM) and variable-temperature Scanning Probe Microscopy techniques. A technical challenge has been to design a sample-holder that allows to exploit the complementary specifications of both microscopes and to preserve their optimal functionality. Experimental demonstration is reported by characterizing under ultrahigh vacuum with both techniques: Au(111) surface reconstruction and a two-layer thick graphene on 6H-SiC(0001). A set of macros to analyze LEEM/PEEM data extends the capabilities of the setup.

Dimensionality crossover in magnetism: from domain walls (2D) to vortices (1D).

Dimensionality crossover is a classical topic in physics. Surprisingly, it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was confirmed experimentally based on micron-sized flux-closure dots.

Contacting individual Fe(110) dots in a single electron-beam lithography step.

We report on a new approach, entirely based on an electron-beam lithography technique, to contact electrically, in a four-probe scheme, single nanostructures obtained by self-assembly. In our procedure, nanostructures of interest are located and contacted in the same fabrication step. This technique has been developed to study the field-induced reversal of an internal component of an asymmetric Bloch domain wall observed in elongated structures such as Fe(110) dots. We have focused on the control, using an external magnetic field, of the magnetization orientation within Néel caps that terminate the domain wall at both interfaces. Preliminary magneto-transport measurements are discussed demonstrating that single Fe(110) dots have been contacted.

Controlled switching of Néel caps in flux-closure magnetic dots.

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Néel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.