ALBERT

Description: Laboratory experiments suggest that the evolution of in-plane shear rupture along an interface separating two elastic blocks typically shows a transition from slow to fast slip. In contrast to the commonly used continuum mechanics-based approaches, here we study the shear rupture process along a weak interface using the discontinuous deformation analysis (DDA) method. We incorporate a slip-weakening constitutive friction law to simulate the initiation and propagation of shear rupture under external conditions of a constant normal load and a steadily increased shear load. As the shear load increases, our modeling results reveal a sharp transition from episodic expansion and arrest to unstable runaway rupture, consistent with previous experimental results. In the stage of dynamic runaway, rupture velocity is limited by the Rayleigh wave velocity. We further investigate the effects of external loading conditions including load point velocity and normal stress on rupture behavior. We find that the dynamic rupture velocity increases with load point velocity and normal stress, also consistent with previous studies. Our results indicate that the DDA method can well capture some of the general characteristics of shear rupture process and, hence, can be applied to study other aspects of dynamic shear ruptures.

Description: Typical resistant REE phosphates xenotime and monazite are important REE carriers in various types of rocks but the supergene mobility of REE in these minerals remains controversial. In this study, we hypothesize that microbes drive the natural weathering of these resistant REE phosphates. We conducted room-temperature bio-weathering experiments of xenotime concentrate with a common soil bacterium (Bacillus thuringiensis, Bt) isolated from a regolith-hosted REE deposit. Our results showed that Bt was able to promote the dissolution of xenotime and monazite in the concentrate, and the release of REE was enhanced by up to two orders of magnitude. In the bio-weathering medium buffered at pH = 6, the apparent release rates of total REE were in the range of 10−13–10−12 mol·m−2·s−1, with Y releasing at the fastest rates of ∼10−13 mol·m−2·s−1. Furthermore, the estimated dissolution rate of monazite (∼10−9 g·m−2·s−1) was one order of magnitude higher than that of xenotime (∼10−10 g·m−2·s−1) due to a more refractory nature of xenotime determined by its chemical and mineralogical characteristics. On account of the extremely low solubility of REE phosphates, portions of the released REE could be re-precipitated as meta-stable phosphates during mineral dissolution, resulting in the underestimation of the release of REE from primary minerals. Bt could produce various organic acids and acidify the media, promoting the dissolution of resistant phosphates through proton- and ligand-promoted mechanisms. The results of our study suggest that microbes have a high potential to facilitate REE liberation from resistant xenotime and monazite, posing new insight into the biogeochemical cycling of REE on Earth's surface.

Description: Contemporary quantum plasmonics capture subtle corrections to the properties of plasmonic nano-objects in equilibrium. Here, we demonstrate non-equilibrium spill-out redistribution of the electronic density at the ultrafast time scale. As revealed by time-resolved 2D spectroscopy of nanoplasmonic Fe/Au bilayers, an injection of the laser-excited non-thermal electrons induces transient electron spill-out thus changing the plasma frequency. The response of the local electronic density switches the electronic density behavior from spill-in to strong (an order of magnitude larger) spill-out at the femtosecond time scale. The superdiffusive transport of hot electrons and the lack of a direct laser heating indicate significantly non-thermal origin of the underlying physics. Our results demonstrate an ultrafast and non-thermal way to control surface plasmon dispersion through transient variations of the spatial electron distribution at the nanoscale. These findings expand quantum plasmonics into previously unexplored directions by introducing ultrashort time scales in the non-equilibrium electronic systems.

Description: We have developed a novel set of (Mn3O4 and/or MnxNi1-xO)/NiO heterostructured nanocrystals (HNCs) that show promise for multifunctionality. A two-step procedure was used to synthesize our HNC samples. Thermal decomposition was used first to synthesize NiO core nanoparticles, while hydrothermal synthesis under varying pH conditions was then used to produce overgrowth of a MnxNi1-xO shell and/or Mn3O4 islands on the NiO core. In this work, we report on the investigation of the defects and surface/interface chemistry of our HNC samples using x-ray photoelectron spectroscopy (XPS). The results from a detailed analysis of the Mn 2p XPS spectra show that the Mn3+:Mn2+ ratio increases with the increasing pH value used in synthesis of the samples. This is consistent with a trend toward a predominance of Mn3O4 islands at higher pH values, a predominance of MnxNi1-xO shell at the lowest pH values, and a mixture of both for intermediate pH values. A reduction of the satellite features of the Ni 2p XPS with increasing pH of the synthesis medium is attributed predominantly to surface/interface defects of the HNCs. Fitting of the O 1 s XPS spectra shows that Ni-OH and Mn-OH are likely the dominant contributions to the lateral peak, whereas defects such as oxygen in an oxygen-deficient environment and/or oxygen vacancies comprise a smaller contribution. Analysis of both the Ni 2p and O 1 s XPS measured from our samples only shows evidence for a Ni2+ chemical environment (i.e., negligible Ni3+) in octahedral coordination, consistent with a rocksalt structure and well-ordered NiO or MnxNi1-xO nanomaterial. The presence of point defects and the nature of the surface/interface chemistry as determined using XPS suggests that our HNC samples may also be suitable for heterogeneous catalysis applications.

Description: Heterostructures increasingly attracted attention over the past several years to enable various optoelectronic and photonic applications. In this work, atomically thin interfaces of Ir/Al2O3 heterostructures compatible with micro-optoelectronic technologies are reported. Their structural and optical properties were determined by spectroscopic and microscopic techniques (XRR, XPS, HRTEM, spectroscopic ellipsometry, and UV/vis/NIR spectrophotometry). The XRR and HRTEM analyses reveal a layer-by-layer growth mechanism of Ir in atomic scale heterostructures, which is different from the typical island-type growth of metals on dielectrics. Alongside, XPS investigations imply the formation of Ir–O–Al bonding at the interfaces for lower Ir concentrations, in contrast to the nanoparticle core–shell structure formation. Precisely tuning the ratio of the constituents ensures the control of the dispersion profile along with a transition from effective dielectric to metallic heterostructures. The Ir coating thickness was varied ranging from a few angstroms to films of about 7 nm in the heterostructures. The transition has been observed in the structures containing individual Ir coating thicknesses of about 2–4 nm. Following this, we show epsilon-near-zero metamaterials with tunable dielectric constants by precisely varying the composition of such heterostructures. Overall, a comprehensive study on structural and optical properties of the metal–dielectric interfaces of Ir/Al2O3 heterostructures was addressed, indicating an extension of the material portfolio available for novel optical functionalities.

Description: Locked areas of subduction megathrusts are increasingly found to coincide with landscape features sculpted over hundreds of thousand years, yet the mechanisms that underlie such correlations remain elusive. We show that interseismic locking gradients induce increments of irreversible strain across the overriding plate manifested predominantly as distributed seismicity. Summing these increments over hundreds of earthquake cycles produces a spatially variable field of uplift representing the unbalance of co-, post-, and interseismic strain. This long-term uplift explains first-order geomorphological features of subduction zones such as the position of the continental erosive shelf break, the distribution of marine terraces and peninsulas, and the profile of forearc rivers. Inelastic yielding of the forearc thus encodes short-term locking patterns in subduction landscapes, hinting that megathrust locking is stable over multiple earthquake cycles and highlighting the role geomorphology can play in constraining Earth’s greatest source of seismic hazard.

Description: We present transient simulations of the last glacial inception using the Earth system model CLIMBER-X with dynamic vegetation, interactive ice sheets, and visco-elastic solid Earth responses. The simulations are initialized at the middle of the Eemian interglacial (125 kiloyears before present, ka) and run until 100 ka, driven by prescribed changes in Earth's orbital parameters and greenhouse gas concentrations from ice core data. CLIMBER-X simulates a rapid increase in Northern Hemisphere ice sheet area through MIS5d, with ice sheets expanding over northern North America and Scandinavia, in broad agreement with proxy reconstructions. While most of the increase in ice sheet area occurs over a relatively short period between 119 and 117 ka, the larger part of the increase in ice volume occurs afterwards with an almost constant ice sheet extent. We show that the vegetation feedback plays a fundamental role in controlling the ice sheet expansion during the last glacial inception. In particular, with prescribed present-day vegetation the model simulates a global sea level drop of only ∼ 20 m, compared with the ∼ 35 m decrease in sea level with dynamic vegetation response. The ice sheet and carbon cycle feedbacks play only a minor role during the ice sheet expansion phase prior to ∼ 115 ka but are important in limiting the deglaciation during the following phase characterized by increasing summer insolation. The model results are sensitive to climate model biases and to the parameterization of snow albedo, while they show only a weak dependence on changes in the ice sheet model resolution and the acceleration factor used to speed up the climate component. Overall, our simulations confirm and refine previous results showing that climate–vegetation–cryosphere feedbacks play a fundamental role in the transition from interglacial to glacial states characterizing Quaternary glacial cycles.

Description: During the Soviet Virgin Lands Campaign, approximately 23 million hectares (Mha) of Eurasian steppe grassland were converted into cropland in Northern Kazakhstan from 1954 to 1963. As a result Kazakhstan became an important breadbasket of the former Soviet Union. However, the collapse of the Soviet Union in 1991 triggered widespread agricultural abandonment, and much cropland reverted to grasslands. Our goal in this study was to reconstruct and analyze agricultural land-cover change since the eve of the Virgin Lands Campaign, from 1953 to 2010 in Kostanay Province, a region that is representative of Northern Kazakhstan. Further, we assessed the potential of currently idle cropland for re-cultivation. We reconstructed the cropland extent before and after the Virgin Lands Campaign using archival maps, and we mapped the agricultural land cover in the late Soviet and post-Soviet period using multi-seasonal Landsat TM/ETM+ images from circa 1990, 2000 and 2010. Cropland extent peaked at approximately 3.1 Mha in our study area in 1990, 38% of which had been converted from grasslands from 1954 to 1961. After the collapse of the Soviet Union, 45% of the Soviet cropland was abandoned and had reverted to grassland by 2000. After 2000, cropland contraction and re-cultivation were balanced. Using spatial logistic regressions we found that cropland expansion during the Virgin Lands Campaign was significantly associated with favorable agro-environmental conditions. In contrast, cropland expansion after the Campaign until 1990, as well as cropland contraction after 1990, occurred mainly in areas that were less favorable for agriculture. Cropland re-cultivation after 2000 was occurring on lands with relatively favorable agro-environmental conditions in comparison to remaining idle croplands, albeit with much lower agro-environmental endowment compared to stable croplands from 1990 to 2010. In sum, we found that cropland production potentials of the currently uncultivated areas are much lower than commonly believed, and further cropland expansion is only possible at the expense of marginal lands. Our results suggest if increasing production is a goal, improving crop yields in currently cultivated lands should be a focus, whereas extensive livestock grazing as well as the conservation of non-provisioning ecosystem services and biodiversity should be priority on more marginal lands.