Adatom diffusion on vicinal surfaces with permeable steps

We study the behavior of single atoms on an infinite vicinal surface assuming certain degree of step permeability. Assuming complete lack of re-evaporation an ruling out nucleation the atoms will inevitably join kink sites at the steps but can do many attempts before that. Increasing the probability of step permeability or the kink spacing lead to increase of the number of steps crossed before incorporation of the atoms into kink sites. The asymmetry of the attachment-detachment kinetics (Ehrlich-Schwoebel effect) suppresses the step permeability and completely eliminates it in the extreme case of infinite Ehrlich-Schwoebel barrier. The average number of permeability events per atom scales with the average kink spacing. A negligibly small drift of the adatoms in a direction perpendicular to the steps leads to a significant asymmetry of the distribution of the permeability events the atoms thus visiting more distant steps in the direction of the drift.Comment: 12 pages, 6 figure

Clay resources and technical choices for neolithic pottery (Chalain, Jura, France): chemical, mineralogical and grain-size analyses

Many authors have considered pottery manufacturing constraints and sociocultural elements as factors in change in past civilizations over time. The main issue of this research is to better understand the reasons for changes, or choices, in pottery raw materials. The very precise and detailed stratigraphy and cultural succession of occupations is based on dendrochronological data from the lake-dwelling sites of Chalain (Jura, France). Petrographic, palaeontological and chemical analyses were used to determine the nature and origins of the raw materials used by the Neolithic potters. Stratigraphy and dendrochronological data were used to reconstruct in detail the evolution dynamics of fabric changes. Several raw material sources were identified for many of the pottery groups. Each of them was sampled for qualitative experimental tests of pottery forming. The experimental results show a high variability between the sediments tested. This variability was quantitatively estimated by XRF, XRD, the Rietveld method, calcium carbonate quantification and laser grain-size analyses of matrices, indirect measures of plasticity. These analytical results allow a better understanding of the differences observed in the experimental tests. On the basis of these experimental and analytical results, changing parameters such as pottery manufacturing constraints, mineralogical characteristics of raw materials and sociocultural factors are considered. In conclusion, all the social and technical parameters, in each archaeological context, must be taken into account for a better understanding of the changes occurring throughout the chronological sequence

Optical properties of self-assembled plasmonic hyperbolic metasurfaces and metamaterials extracted by (Mueller matrix) spectroscopic ellipsometry

Hyperbolic metamaterials use the concept of controlling the propagative modes through the engineering of the dispersion relation, and are considered highly promising to reach different meta-properties. Spectroscopic Mueller Matrix Ellipsometry with variable angle of incidence and full azimuthal rotation of the sample is a powerful optical technique to characterize both anisotropic and bi-anisotropic materials. We here discuss the experimentally extracted uniaxial and biaxial optical properties of two self-assembled plasmonic systems that appear to have the appropriate meta-dispersion relations. The metasurface was produced by oblique incidence angle ion beam sputtering of glass followed by shadow deposition of Au [1]. The second bulk metamaterial was a block-copolymer based self-assembled hyperbolic metamaterial of nanocomposites based on metal nanoparticles embedded in a self-assembled anisotropic polymer host, presenting a strong spectrally selective optical anisotropy [2]. The extracted effective dielectric functions and the resulting dispersion relations are presented

Drake Passage gateway opening and Antarctic Circumpolar Current onset 31 Ma ago: The message of foraminifera and reconsideration of the Neodymium isotope record

International audienceThe establishment of the Antarctic Circumpolar Current (ACC) is one of the most important events of the Cenozoic for both global oceanic circulation and climate. The onset of this major current hinges on the opening of two major oceanic passages, the Drake Passage and the Tasmanian gateways that connect Pacific, Atlantic and Indian oceans, allowing a modern-like thermohaline circulation. For decades, the ACC onset has been considered as the trigger of the Oligocene glaciation at 33.7 Ma, which marks the beginning of the modern icehouse climate. Today, this scenario is debated. The main obstacle to evaluate the ACC influence on the Oligocene glaciation remains the ill-constrained timing of the Drake Passage gateway opening. Here, we analyse the geochemical composition and Sr isotope ratio of dated planktonic and benthic foraminifera from two IODP and ODP legs in the Southern Atlantic and Pacific oceans (SAO and PO, respectively) to assess the variability of seawater masses' chemical composition through time and to better constrain the timing of the Drake Passage gateway opening along the Eocene-Oligocene interval. These results, based on seawater paleo temperature (Mg/Ca molar ratios), redox (Ce/Ce* anomaly) and provenance (87Sr/86Sr) proxies, highlight a gradual seawater mass mixing between the SAO and PO from 31 Ma to 26 Ma. Combined with a reconsideration of the fossil fish teeth Neodymium isotope records, these geochemical tracers evidencing the SAO-PO interconnection depicts the Drake Passage gateway opening and deepening during this 31–26 Ma interval and thus, the timing of the ACC onset. Hence, antecedence of the Oligocene glaciation onset (at 33.7 Ma) relative to the ACC onset (31–26 Ma) implies that the ACC did not trigger the Oligocene glaciation and that the role of atmospheric pCO2 should be further considered

Eocene-Oligocene southwest Pacific Ocean paleoceanography new insights from foraminifera chemistry (DSDP site 277, Campbell Plateau)

International audienceDespite its major role in the Earth’s climate regulation, the evolution of high-latitude ocean dynamics through geological time remains unclear. Around Antarctica, changes in the Southern Ocean (SO) circulation are inferred to be responsible for cooling from the late Eocene and glaciation in the early Oligocene. Here, we present a geochemical study of foraminifera from DSDP Site 277 (Campbell Plateau), to better constrain thermal and redox evolution of the high latitude southwest Pacific Ocean during this time interval. From 56 to 48 Ma, Mg/Ca- and δ 18 O-paleothermometers indicate high surface and bottom water temperatures (24–26°C and 12–14°C, respectively), while weak negative Ce anomalies indicate poorly oxygenated bottom waters. This is followed by a cooling of ∼4° between 48 and 42 Ma, possibly resulting from a weakening of a proto-EAC (East Australian Current) and concomitant strengthening of a proto-Ross gyre. This paleoceanographic change is associated with better ventilation at Site 277, recorded by an increasing negative Ce anomaly. Once this proto-Ross gyre was fully active, increasing biogenic sedimentation rates and decreasing Subbotina sp. δ 13 C values indicate enhanced productivity. This resulted in a shoaling of the oxygen penetration in the sediment pile recorded by increasing the foraminiferal U/Ca ratio. The negative Ce anomaly sharply increased two times at ∼35 and ∼31 Ma, indicating enhanced seawater ventilation synchronously with the opening of the Tasmanian and Drake Passage gateways, respectively. The Oligocene glaciation is recorded by a major increase of bottom seawater δ 18 O during the EOT (Eocene-Oligocene Transition) while Mg/Ca-temperatures remain rather constant. This indicates a significant ice control on the δ 18 O record

Measurement of the circular dichroism in the second harmonic optical signal produced by Au covered self ordered dielectric nanospheresNonlinear Optics and Applications VII

Here we present the measurements of the second harmonic generation (SHG) signal raised by self ordered dielectric nanospheres partially covered by thin (10nm) Au layer. The measurement were performed by studying the SHG efficiency in different polarization states of the light. In particular measurement performed with circular polarized light show the presence of chiral response of the nanospheres that is induced by the particular geometry of the metasurface

Phase transition of dissociatively adsorbed oxygen on Ag(001)

We show that dissociative oxygen adsorption on Ag(001) induces below room temperature a missing row 2 root 2x root 2 reconstruction of the substrate. As demonstrated by the analysis of the photoelectron diffraction patterns, the oxygen atoms sit thereby in a c(2x2) arrangement in the previous fourfold hollow sites nearly coplanar with the Ag atoms, while rows of substrate atoms are removed along the [100] directions. Annealing the crystal above 350 K restores the p(1X1) symmetry and the oxygen moves to 0.6 Angstrom above the fourfold hollow site. It becomes then more oxidic in nature, as demonstrated by the shift of the O1s level from 530.3 eV to 528.3 eV. The phase transition affects also the O2s and O2p levels as well as the surface component of Ag 3d(5/2). The vibrational frequency of the oxygen adatoms against the surface decreases at the phase transition, in accord with the larger adsorption distance. The highs temperature phase is active towards CO and C(2)H(4) oxidation, while the low-temperature phase is nor. When cooling the sample below room temperature the reconstructed phase is restored. The time constant of this process as well as the chemical reactivity of the high-temperature phase are weakly reproducible since they depend on the previous history, i.e., presumably on the subsurface oxygen content of the sample