A reactive force-field (ReaxFF) Monte Carlo study of surface enrichment and step structure on yttria-stabilized zirconia

To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) while yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. We found that it is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, suggesting that the stability of YSZ surface steps observed experimentally is due to kinetic barriers for surface re-ordering.     

Adsorbate-substrate bonding and the growth of naphthalene thin films on Ag(1 1 1)

The adsorption kinetics, energetics and growth of naphthalene thin films, from submonolayer to about 10 layers, on a Ag(1 1 1) surface at low temperature in a ultrahigh vacuum chamber are examined by using temperature programmed desorption spectroscopy. The first layer adsorption occurs with a desorption energy of 85 ± 5 kJ/mole and results in an interface dipole of 5 ± 1 D, from charge transfer of approximately 0.2 e from naphthalene to Ag. The surface dipole induced inter-adsorbate repulsion causes the lowering of the adsorption energy within the first layer near the saturation coverage so that the second layer deposition begins before the completion of the first layer. The second layer is a metastable phase with desorption energy, 74 ± 3 kJ/mole, smaller than the multilayer desorption energy of 79 ± 5 kJ/mole. Fractional order desorption kinetics were found for both the metastable and the multilayer phases, suggesting desorption from 2-D islanding and 3-D islanding, respectively.     

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer.     

Observation of a Griffiths-like phase in the magnetocaloric compound Tb5Si2Ge2

The onset of a Griffiths-like phase has been observed in Tb5Si2Ge2 (TC=110 K) by means of magnetic susceptibility and small-angle neutron scattering experiments. We show the growth of a ferromagnetic cluster system characterized by an inverse susceptibility exponent lower than unity at TC相似文献   

Strain relief and disorder in commensurate water layers formed on Pd(111)

Water adsorbs and desorbs intact on Pd(111), forming a hydrogen-bonded wetting layer whose structure we examine by low energy electron diffraction (LEED) and He atom scattering (HAS). LEED shows that water forms commensurate (√3 × √3)R30° clusters that aggregate into a partially ordered, approximately (7 × 7) superstructure as the layer completes. HAS indicates that the water layer remains disordered on a local (approximately 10 ?) scale. Based on workfunction measurements and density functional theory simulations we propose that water forms small, flat domains of a commensurate (√3 × √3)R30° water network, separated by disordered domain boundaries containing largely H-down water. This arrangement allows the water layer to adapt its density and relieve the lateral strain associated with adsorbing water in the optimum flat atop adsorption site. We discuss different possibilities for the structure of these domain walls and compare this strain relief mechanism to the highly ordered, large unit cell structures formed on surfaces such as Pt(111).     

Strain relief of heteroepitaxial bcc-Fe(001) films

The strain relief of heteroepitaxial bcc-Fe(001) films, deposited at 520-570 K onto MgO(001), has been investigated by scanning tunneling microscopy. In accordance with real-time stress measurements, the tensile misfit strain is relieved during coalescence of flat, mainly 2-3 monolayers (ML) high Fe islands at the high thickness of approximately 20 ML. To accommodate the misfit between merging strain-relaxed islands, a network of 1/2[111] screw dislocations is formed. A strong barrier for dislocation glide--which is typical for bcc metals--is most likely responsible for the big delay in strain relief of Fe/MgO(001), since only the elastic energy of the uppermost layer(s) is available for the formation of an energy-costly intermediate layer.     

Thickness periodicity in the Auger line shape from epitaxial (111)Cu films

The 61 eV MMM Cu Auger line doublet was recorded in the derivative mode as a function of thickness for epitaxial (111)Cu films approximately 1500 Å thick. The overlap of the doublet lines makes it possible to define a measure of the doublet profile called the “R -factor” as a ratio of the peak-to-peak heights of the small overlap oscillation to that of the major oscillation. To within the experimental error, it was found that the R -factor varies with a periodicity of approximately one monoatomic layer as the film thickens. Since these films grow by a layer growth mechanism, the surface topography varies periodically with the number of monolayers deposited, going from a smooth to a rough to a smooth, etc. surface. It is believed that the occurrence of such a periodicity implies that there is a difference in the electronic structure at the surface of the flat areas of the film from that at the edges of monolayer high, flat islands. The amplitude of the oscillation in R is interpreted to be a measure of the relative amounts of edge area compared to flat area. These results show that it is possible to use Auger electron spectroscopy to monitor surface topography and the electronic structure changes that accompany the topographical changes occurring when epitaxial films grow by a layer growth mechanism.     

Stranski-Krastanov growth of CuCl on MgO(001)

Initial growth of CuCl, a zinc-blende-structure I–VII ionic compound, on MgO(001) with NaCl-type structure has been studied using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). An intermediate layer of adsorbed CuCl, which has a surface structure strongly influenced by the host lattice, was formed before islanding. This layer is easily distinguished from (111) top surfaces of the CuCl islands with trigonal symmetry which have been grown subsequently. This implies that CuCl grows in the Stranski-Krastanov growth mode, which also explains why the nucleation of the islands is insensitive to the step edges of MgO(001).     

Surface property enhancement by RE-borosulphurizing on high-carbon steel

Pack boronizing and rare-earth (RE)-borosulphurizing of high-carbon steel (T8) were conducted at 950 ^°C for 6 h. Characterizations of the layers formed on the surface of the high carbon steel were carried out by metallographic techniques, scanning electron microscopy, Auger electron spectroscopy and wear and corrosion resistance tests. It has been revealed that the diffusion front of the boride layer (BL) has a sawtooth shape, while that of the RE-borosulfide layer (RBSL) is flat. Different from the BL layer, the RBSL layer is compact, continuous and flat. The formation of FeS, Fe₂B and FeB phases on the substrates was confirmed by Auger electron spectroscopy analysis. The wear resistance test indicated that within a certain range, the abrasion resistance of the RBSL layer is better than that of the BL layer, especially under high-load conditions. The corrosion resistance test using the weight loss method has shown that the corrosion resistance of the RBSL layer is better but decreases faster with time extension than that of the BL layer.     

Air-Driven Segregation in Binary Granular Mixtures with Same Size but Different Densities

We investigate the segregation effect of binary granular mixtures with the same size but different densities under vibration at different air pressures. Our experiments show that the segregation state is seriously dependent on the air pressure and there is a new type of partially segregated state at high air pressure, which has the characteristic that the lighter grains tend to stay at the bottom and form a pure layer, while heavier grains and remained lighter ones tend to rise and to form a mixed layer on the top of the system. We redefine the order parameter to study the variation of the segregation effect with the air pressure and vibration parameter in detail. Finally, the mechanism of the air-driven segregation is illustrated by the faster acceleration due to the airflow through the granular bed for lighter particles.     

Observation of anomalous spin segregation in a trapped Fermi gas

We report the observation of spin segregation, i.e., time-dependent separation of the spin density profiles of two spin states, in a trapped, coherently prepared Fermi gas of 6Li with a magnetically tunable scattering length a12 close to zero. For |a12| approximately = 5 bohr, as the cloud profiles evolve, the measured difference in the densities at the cloud center increases in 200 ms from 0 to approximately = 60% of the initial mean density and changes sign with a12. The data are in disagreement in both amplitude and temporal evolution with a spin-wave theory for a Fermi gas. In contrast, for a Bose gas, an analogous theory has successfully described previous observations of spin segregation. The observed segregated atomic density profiles are far from equilibrium, yet they persist for approximately = 5 s, long compared to the axial trapping period of 6.9 ms. We find the zero crossing in a12=0, where spin segregation ceases, at 527.5+/-0.2 G.     

In在Al(001)表面偏析的模拟

通过一种空位模型详细的描述了In在Al(001)表面的扩散偏析过程,利用周期性密度泛函理论方法计算了这个偏析过程中每步构型的能量和In原子扩散的能量壁垒,并对可能的偏析机理进行分析.结果表明:In原子从Al(001)表面第二层扩散偏析至表面层时,系统的能量降低了0.64 eV,最大的扩散迁移壁垒为0.34 eV;而从表面更内层向表面第二层扩散时系统能量基本保持不变,扩散需要克服的能量壁垒为0.65 eV,说明In原子在Al(001)表面只能由体内向表面扩散偏析.In在Al(001)的清洁表面具有强烈的偏析趋势,在热力学上是容易进行的. 关键词： 密度泛函理论 表面偏析 扩散 Al合金     

Characterisation of Interfaces Between Liquid Tin and Alumina in the Presence of Titanium Alloy Additions

Neutron reflection spectroscopy has been used to characterise the composition of interfaces between liquid Sn-Ti alloys and Al₂O₃. The reflectivity profiles for both 1% and 3% Ti alloys are consistent with the presence of a layer approximately 2 nm thick at the liquid/solid interface, showing extensive segregation of Ti. The composition of this layer is not pure Ti but shows a greater Ti content than any known Ti oxide composition. In all interfaces exposed to liquid metals at elevated temperature (including pure Sn) there is a layer of thickness 20–100 nm on the alumina surface with slightly lower neutron scattering length density than pure alumina. This is interpreted as evidence for surface roughening of the Al₂O₃ surface during exposure to the liquid metal.     

Search for a stochastic background of 100-MHz gravitational waves with laser interferometers

This Letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of approximately 10;{-16} Hz;{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h{100};{2}Omega_{gw}<6 x 10;{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.     

Surface composition changes caused by Ar ion bombardment of Au-Ni alloys

Surface composition changes caused by 1 keV Ar ion bombardment of a series of Au-Ni alloys at room temperature were studied. Using 11 combinations of different energy Auger spectra, the variation of the composition with depth from 0.4 to 1.5 nm in a bombarded Au-Ni alloy was obtained. We found that a drastic variation in concentration took place within the first few layers. This is evidently indicating that a bombardment-induced Gibbsian segregation exists in the Au-Ni system. We also found that the degree of gold depletion at the second layer depended strongly on the bulk composition, thus this depletion became slight as the gold content in alloy was high. In addition, we have roughly estimated the concentration of Au at the topmost layer of a bombarded Au-Ni alloy and found that this value tentatively might be smaller than the bulk one. This result differs from two known examples: C(Au, top) > C(Au, bulk) in a bombarded Au-Cu alloy and C(Cu, top) = C(Cu, bulk) in a bombarded Cu-Ni alloy. Comparing our experimental results with a recent theory presented by Kelly, we suggest that a more sophisticated theory should take this new phenomenon into account.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Moiré flat bands of twisted few-layer graphite

We report that the twisted few layer graphite (tFL-graphite) is a new family of moiré heterostructures (MHSs), which has richer and highly tunable moiré flat band structures entirely distinct from all the known MHSs. A tFL-graphite is composed of two few-layer graphite (Bernal stacked multilayer graphene), which are stacked on each other with a small twisted angle. The moiré band structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers. Near the magic angle, a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive (parabolic or linear) bands at the Fermi level, thus, enhances the DOS at E_F . This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems. Therefore, we expect strong multiband correlation effects in tFL-graphite. Meanwhile, a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites, indicating that tFL-graphite is also a novel topological flat band system.     

Dynamics of a penta-hepta defect in a hexagonal pattern

The structure and dynamics of a penta-hepta defect in a hexagonal pattern are studied experimentally. The hexagonal pattern is formed by placing a layer of soap bubbles (diameter approximately 1 mm) on a flat glass plate. We find that an isolated penta-hepta defect in a bubble raft with free boundary always moves along the direction perpendicular to the wave vector of the nonsingular mode and towards the nearest boundary. The structure of the penta-hepta defect is found to be similar to that found in nonequilibrium pattern forming systems.     

Stress development and impurity segregation during oxidation of the Si(1 0 0) surface

We have studied the segregation of P and B impurities during oxidation of the Si(1 0 0) surface by means of combined static and dynamical first-principles simulations based on density functional theory. In the bare surface, dopants segregate to chemically stable surface sites or to locally compressed subsurface sites. Surface oxidation is accompanied by development of tensile surface stress up to 2.9 Nm⁻¹ at a coverage of 1.5 monolayers of oxygen and by formation of oxidised Si species with charges increasing approximately linearly with the number of neighbouring oxygen atoms. Substitutional P and B defects are energetically unstable within the native oxide layer, and are preferentially located at or beneath the Si/SiO_x interface. Consistently, first-principles molecular dynamics simulations of native oxide formation on doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds, suggesting a surface oxidation mechanism whereby impurities remain trapped at the Si/SiO_x interface. This seems to preclude a direct influence of impurities on the surface electrostatics and, hence, on the interactions with an external environment.     

Competitive segregation of gallium and indium at heterophase Cu-MnO interfaces studied with transmission electron microscopy

This paper concentrates on the possible segregation of indium and gallium and competitive segregation of gallium and indium at atomically flat parallel {111}-oriented Cu-MnO interfaces. The segregation of gallium at Cu-MnO interfaces after introduction of gallium in the copper matrix of internally oxidized Cu-1 at.% Mn could be hardly detected with energy-dispersive spectrometry in a field emission gun transmission electron microscope. After a heat treatment to dissolve indium in the copper matrix, gallium has a weak tendency to segregate, that is 2.5 at.% Ga per monolayer at the interface compared with 2 at.% in the copper matrix. The striking result is that this gallium segregation is observable because it does not occur at the metal side of the interface but in the first two monolayers at the oxide side. Using the same heat treatment as for introducing indium in the sample, but without indium present, gallium segregates strongly at the oxide side of the Cu-MnO interface with a concentration of about 14.3 at.% in each monolayer of the two. In contrast, the presence of gallium has no influence on the segregation of indium towards Cu-MnO interfaces, because the outermost monolayer at the metal side of the interface contains 17.6 at.% In, that is similar to previously found results. This leads to the intriguing conclusions, firstly, that, in contrast with antimony and indium, gallium segregates at the oxide side of the interface and, secondly, that the presence of indium strongly hampers gallium segregation. The results from analytical transmission electron microscopy on gallium segregation are supported by high-resolution transmission electron microscopy observations.