Oxidation of Nb(110): atomic structure of the NbO layer and its influence on further oxidation.

NbO terminated Nb(110) and its oxidation are examined by scanning tunneling microscopy and spectroscopy (STS). The oxide structures are strongly influenced by the structural and electronic properties of the underlying NbO substrate. The NbO is terminated by one-dimensional few-nanometer nanocrystals, which form an ordered pattern. High-resolution STS measurements reveal that the nanocrystals and the regions between the nanocrystals exhibit different electronic characters. Low-dosage oxidation, sufficient for sub-monolayer coverage of the NbO, with subsequent UHV annealing results in the formation of resolved sub-nanometer clusters, positioned in-between the nanocrystals. Higher dosage oxidation results in the formation of a closed Nb2O5-y layer, which is confirmed by X-ray photoelectron spectroscopy measurements. The pentoxide is amorphous at the atomic-scale. However, large scale (tens of nanometers) structures are observed with their symmetry matching that of the underlying nanocrystals

Step bunching with both directions of the current: Vicinal W(110) surfaces versus atomistic scale model

We report for the first time the observation of bunching of monoatomic steps on vicinal W(110) surfaces induced by step up or step down currents across the steps. Measurements reveal that the size scaling exponent {\gamma}, connecting the maximal slope of a bunch with its height, differs depending on the current direction. We provide a numerical perspective by using an atomistic scale model with a conserved surface flux to mimic experimental conditions, and also for the first time show that there is an interval of parameters in which the vicinal surface is unstable against step bunching for both directions of the adatom drift.Comment: 17 pages, 10 figure

Self-assembly of Fe nanocluster arrays on templated surfaces

The growth of Fe nanoclusters on the Ge(001) and MoO2/Mo(110) surfaces has been studied using low-temperature scanning tunneling microscopy (STM) and X-ray magnetic circular dichroism (XMCD). STM results indicate that at low coverage Fe atoms self-assemble on both surfaces into well-separated nanoclusters, which nucleate at equivalent surface sites. Their size, shape, and the observed spatial separation are dictated by the substrate and depend on preparation conditions. Annealing the Fe nanoclusters on Ge(001) at 420 K leads to the formation of linear nanocluster arrays, which follow the Ge dimer rows of the substrate, due to cluster mobility at such temperature. In turn, linear Fe nanocluster arrays are formed on the MoO2/Mo(110) surface at room temperature at a surface coverage greater than 0.5 monolayer. This is due to the more pronounced row pattern of the MoO2/Mo(110) surface compared to Ge(001). These nanocluster arrays follow the direction of the oxide rows of the strained MoO2/Mo(110) surface. The Fe nanoclusters formed on both surfaces show a superparamagnetic behavior as measured by XMCD. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676207

Deformation and fracture of crystalline tungsten and fabrication of composite STM probes

Fracturing microscale constrictions in metallic wires, such as tungsten, platinum, or platinum-iridium, is a common fabrication method used to produce atomically sharp tips for scanning tunneling microscopy (STM), field-emission microscopy and field ion microscopy. Typically, a commercial polycrystalline drawn wire is locally thinned and then fractured by means of a dislocation slip inside the constriction. We examine a special case where a dislocation-free microscale constriction is created and fractured in a single crystal tungsten rod with a long side parallel to the [100] direction. In the absence of dislocations, vacancies become the main defects in the constriction which breaks under the tensile stress of approximately 10 GPa, which is close to the theoretical fracture strength for an ideal monocrystalline tungsten. We propose that the vacancies are removed early in the tensile test by means of deformation annealing, creating a defect-free tungsten constriction which cleaves along the W(100) plane. This approach enables fabrication of new composite STM probes which demonstrate excellent stability, atomic resolution and magnetic contrast that cannot be attained using conventional methods

REVEALING ELECTROMIGRATION ON DIELECTRICS AND METALS THROUGH THE STEP-BUNCHING INSTABILITY

Electromigration, due to its technological and scientific significance, has been a subject of extensive studies for many years. We present evidence of electromigration in dielectric materials, namely C -plane sapphire, obtained from direct experimental observation of an atomic step-bunching instability driven by electromigration. We further expand upon our previously reported findings of electromigration induced step-bunching transformation of a metal surface. The only system where electromigration driven step bunching has been observed and comprehensively investigated is the low index surfaces of silicon. In this study we show that electromigration driven SB can be induced on a variety of crystallographic surfaces, including metals and insulating oxides, and may be more prevalent than previously thought. Electric fields were applied at high temperature to W(110) and A l 2 O 3 ( 0001 ) crystals whereupon their surface reordered to a morphology closely resembling that of Si(111) with atomic steps bunched by electromigration. This suggests that the mechanism of step bunching on the W(110), A l 2 O 3 ( 0001 ) , and Si(111) can be fundamentally the same. Annealing W(110) offcut in the [001] direction with an up-step current produced a morphology with the bunch edges composed of zigzag segments meeting at a right angle

Revealing electromigration on dielectrics and metals through the step-bunching instability

Electromigration, due to its technological and scientific significance, has been a subject of extensive studies for many years. We present evidence of electromigration in dielectric materials, namely C-plane sapphire, obtained from direct experimental observation of an atomic step-bunching instability driven by electromigration. We further expand upon our previously reported findings of electromigration induced step-bunching transformation of a metal surface. The only system where electromigration driven step bunching has been observed and comprehensively investigated is the low index surfaces of silicon. In this study we show that electromigration driven SB can be induced on a variety of crystallographic surfaces, including metals and insulating oxides, and may be more prevalent than previously thought. Electric fields were applied at high temperature to W(110) and Al2O3(0001) crystals whereupon their surface reordered to a morphology closely resembling that of Si(111) with atomic steps bunched by electromigration. This suggests that the mechanism of step bunching on the W(110), Al2O3(0001), and Si(111) can be fundamentally the same. Annealing W(110) offcut in the [001] direction with an up-step current produced a morphology with the bunch edges composed of zigzag segments meeting at a right angle

Observation of a giant mass enhancement in the ultrafast electron dynamics of a topological semimetal

Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics