Scanning tunneling spectroscopy ͑STS͒ experiments were performed on the ͑001͒ and ͑111͒ surfaces of single crystalline magnetite. Room temperature spectra exhibit a ϳ0.2 eV gap around E f . The importance of perfect surface order to the existence of this gap is illustrated. STS is also carried out on the ͑111͒ surface, at 140 and 95 K, just above and below the Verwey transition temperature ͑T V ϳ 120 K͒, respectively. It is confirmed that above T V a ϳ0.2 eV gap exists in the surface density of states ͑DOS͒ around E f . Furthermore, broad bands are resolved on both sides of E f , with peaks centered on ϳ + 0.5 eV and ϳ−0.45 eV. Below T V it is shown that the value of the gap in the surface DOS remains similar, however, the peaks resolved in the conduction and valence bands shift markedly away from E f . The similarity of the gap value before and after the transition points away from an ionic charge ordering occurring at the magnetite surface below T V . However, the shifting of the bands points to a certain degree of electronic ordering or charge disproportionation playing an integral part in the Verwey transition, at the magnetite surface.
The oxidation of Mo͑110͒ was studied at 1000°C and 1 ϫ 10 −6 Torr oxygen. Low energy electron diffraction and scanning tunneling microscopy data were used to give a detailed analysis of the oxide surface structure. From this data a model was built, and through the use of density functional theory ͑DFT͒ calculations, we show that a strained bulklike MoO 2 ͑010͒ "surface oxide" is in excellent agreement with the experimental data. The stability of this oxide was accounted for by a strong adhesion at the interface. The origin of this strong adhesion between the film and substrate can be related to the charge redistribution at the interface, which is analogous to the macroscopic image charge interaction between the two. Furthermore, we employed DFT calculations to illustrate the charge redistribution at the interface and estimate the work of adhesion for this system. The calculated work of adhesion is around 7 J / m 2 , indicating that there is indeed a strong interaction between the film and substrate as expected.
We report on the formation of equilateral triangular clusters hollow inside with 5-6 atoms per side, selfassembled on Ni adislands grown on Rh(111). The observation of standing wave patterns on the Ni adislands and the Rh(111) indicates that the self-assembly is mediated by Friedel oscillations. In this context, we propose a model based on the energy of interaction between adsorbates, which explains the formation of the clusters as a result of the assembly of rows of 5-6 adatoms.
In mechanical treatment carried out by ball milling, powder particles are subjected to repeated high-energy mechanical loads which induce heavy plastic deformations together with fracturing and cold-welding events. Owing to the continuous defect accumulation and interface renewal, both structural and chemical transformations occur. The nature and the rate of such transformations have been shown to depend on variables, such as impact velocity and collision frequency that depend, in turn, on the whole dynamics of the system. The characterization of the ball dynamics under different impact conditions is then to be considered a necessary step in order to gain a satisfactory control of the experimental set up. In this paper we investigate the motion of a ball in a milling device. Since the ball motion is governed by impulsive forces acting during each collision, no analytical expression for the complete ball trajectory can be obtained. In addition, mechanical systems exhibiting impacts are strongly nonlinear due to sudden changes of velocities at the instant of impact. Many different types of periodic and chaotic impact motions exist indeed even for simple systems with external periodic excitation forces. We present results of the analysis on the ball trajectory, obtained from a suitable numerical model, under growing degree of impact elasticity. A route to high dimensional chaos is obtained. Crisis and attractors merging are also found. Among the others, the scarce knowledge of the dynamics of ball milling devices, where milling bodies undergo a huge number of collisions during the processing. Under such conditions, indeed, any experimental measurement of the exact number of impacts and of the energy transferred to powders at collisions is greatly hindered. It becomes therefore impossible to relate the degree of structural evolution to the mechanical energy dissipated, maybe the most important macroscopic parameter used to characterize the yield of a mechanochemical reaction. Any information on the dynamics of milling bodies is then extremely valuable in order to quantitatively describe and rationalize the kinetic features of mechanically induced transformations. Preliminary investigations have already shown the possible occurrence of chaotic regimes during milling treatments. 6,7 On the other hand, modeling results and experimental evidences demonstrate the occurrence of regular dynamical regimes allowing for the direct measurement of both the average collision frequency and impact energy. 8 It becomes therefore important to study the transition from periodic to chaotic regimes in order to understand when and why transition takes place, so as to avoid it. Experimentalists are indeed mainly interested in periodic and regular regimes, which permit the full control of experimental parameters.
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