Density controls the kinetic stability of ultrastable glasses

We use a swap Monte Carlo algorithm to numerically prepare bulk glasses with kinetic stability comparable to that of glass films produced experimentally by physical vapor deposition. By melting these systems into the liquid state, we show that some of our glasses retain their amorphous structures longer than 10^5 times the equilibrium structural relaxation time. This exceptional kinetic stability cannot be achieved experimentally for bulk materials. We perform simulations at both constant volume and constant pressure to demonstrate that the density mismatch between the ultrastable glass and the equilibrium liquid accounts for a major part of the observed kinetic stability.Comment: 7 Pages, 6 Figures. Figures 4b) and 5b) updated, revisions to text to improve discussion, missing page numbers added to references, typos correcte

Front-mediated melting of ultrastable glasses

Ultrastable vapor-deposited glasses display uncommon material properties. Most remarkably, upon heating they are believed to melt via a liquid front that originates at the free surface and propagates over a mesoscopic crossover length, before crossing over to bulk melting. We combine swap Monte Carlo with molecular dynamics simulations to prepare and melt isotropic amorphous films of unprecedendtly high kinetic stability. We are able to directly observe both bulk and front melting, and the crossover between them. We measure the front velocity over a broad range of conditions, and a crossover length scale that grows to nearly $400$ particle diameters in the regime accessible to simulations. Our results disentangle the relative roles of kinetic stability and vapor deposition in the physical properties of stable glasses.Comment: 7 pages, 6 figures; accepted for publication in Phys. Rev. Let

Current-induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars

Nanopillar spin valve devices are typically comprised of two ferromagnetic layers: a reference layer and a free layer whose magnetic orientation can be changed by both an external magnetic field and through the introduction of spin-polarized electric current. Here we report the continuous repeated switching behavior of both the reference and free layers of a perpendicular spin valve made of Co/Pd and Co/Ni multilayers that arises for sufficiently large DC currents. This periodic switching of the two layers produces an oscillating signal in the MHz regime but is only observed for one sign of the applied current. The observed behavior agrees well with micromagnetic simulations

Studies in Allophane in Scottish Soils

High amounts of allophane as measured by the sodium fluoride test were found in the brown forest soils and B horizons of podzols developed on basalt and basalt till. A hydrologic sequence of the Darleith association was examined and the soils' strong aggregation, high organic matter levels and phosphate retention were found due to their allophane content. Allophane was found also in the C horizons of podzols of other material. C horizons flocculated on ultra-sonic vibration but were dispersed in ammonia. All horizons however, when vigorously peroxidised, behaved like allophane soils and remained flocculated in alkaline suspension. The relative tendency of a soil to flocculate appeared to depend on the number of aluminous sites exposed ie. on the amount of allophane and its SiO2 : Al2O3 ratio. This correlation was supported by analysis of the alumino-silicate extracted with (a) cold 5% Na2CO3 (b) cation exchange resin (c) 1% citric acid, and by the rise in suspension pH on NaF treatment. NaF released more Al, and Na2CO2 more Al and much more Si when added to dry soil than to suspensions, and this was attributed to energy provided by the heat of wetting. Successful dispersion of flocs , as measured by mechanical analysis , was obtained for a New Zealand Tirau B horizon and the peroxidised Scottish soils by: (1) Vibrating in HCl at pH 4.3. This was only possible for Tirau B, with negligible clay mineral present. (2) Vibrating in zirconium or thorium nitrate solution and pH controlled at 4.3. Adsorption of the hydrolysis products (polycations) on the negative colloids, reversing their charge, was the dispersion mechanism, (3) Shaking with Na-exchange resin, followed by hand shaking in ammonia. The resin took up large quantities of alumina and silica and some iron, in addition to bases. (4) Vibrating in alkaline silica sol. (5) Vibrating in sodium hexametaphosphate. Peroxidised Darleith soil dispersed in calgon. Tirau B was partially dispersed in (NaPO3)6 at pH 7.5. (6) Repeated washing of alkaline flocs followed by shaking in NaOH (efficiency not yet measured). (7) Dialysis at soil pH gave partial dispersion of Tirau B. Alkaline flocculation of allophane could be explained in terms of (a) alumina groups (b) their capacity to adsorb hydroxyl (c) the presence of free electrolyte. Dispersion was achieved by any mechanism which interfered with one of these, viz. by:- masking alumina groups with organic matter or silica; 'destroying' hydroxyl with H+ ions; or removal of electrolyte. This interpretation is Inherent in Mattson's 1922 paper, In which adsorbed hydroxyl ions were shown to be flocculating agents, and a bridging mechanism dependent on electrostatic linkages between them and divalent cations in the free electrolyte was envisaged. A review was made of the concepts of aggregation, coagulation and flocculation. Experiments showed the validity of the distinction between 'coagulation' by electrolytes and 'flocculation' by polyelectrolytes was unsubstantiated. It was suggested that aggregation could be Interpreted in terms of the double layer Verwey-Overbeek theory, but flocculation required another explanation. Mattson's work, together with his postulate of the crucial role of the hydroxyl group should be reappraised. The unusual cation exchange properties of allophane described In the work of New Zealand and Japanese soil scientists were re-interpreted in the light of Mattson's experiments with amphoteric colloids. Anomalies were explained in terms of the acidoid-basoid complex. Sodium fluoride was used to extract allophane-held organic matter. Optimum conditions were established at pH 6 to 7. Approximately 30% of total polysaccharide was extracted, or allowing for non-humified cellulose, about 50% of soil 'combined' polysaccharide. It appeared to be representative in composition. Sodium fluoride extraction of soils high in allophane offers a technique for the systematic study of virtually unaltered organic matter. N.B. Plastic artefacts interfered with organic matter studies: (a) Moist soils stored in plastic bags became impregnated with mobile hydrocarbons. (b) Dialysis tubing dissolved out reducing substances. These gave high values to polysaccharide analyses and interfered with sugar chromatograms. The Forestry Commission finding that spruce checked on basalt brown forest soils, and that the needles showed phosphate deficiency although the soils appeared to contain adequate phosphate, led the writer to postulate that aluminium toxicity was the cause of the phosphate deficiency. Large amounts of alumina were dissolved by 1% citric acid. Mattson and Hester (1933) showed that the pH of injury to wheat seedlings was raised in soils of low silica-sesquioxide ratio due to the greater mobility of aluminium. If the toxicity hypothesis were correct, the condition could be ameliorated by adding organic matter to mask aluminous sites and lower the pH of injury, or by liming. An alternative crop might thrive better than spruce. No allophane reaction to the NaF test was found on Darleith soils under hardwood, even where it was strong in the adjoining grassland

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Efficient swap algorithms for molecular dynamics simulations of equilibrium supercooled liquids

It was recently demonstrated that a simple Monte Carlo (MC) algorithm involving the swap of particle pairs dramatically accelerates the equilibrium sampling of simulated supercooled liquids. We propose two numerical schemes integrating the efficiency of particle swaps into equilibrium molecular dynamics (MD) simulations. We first develop a hybrid MD/MC scheme combining molecular dynamics with the original swap Monte Carlo. We implement this hybrid method in LAMMPS, a software package employed by a large community of users. Secondly, we define a continuous time version of the swap algorithm where both the positions and diameters of the particles evolve via Hamilton's equations of motion. For both algorithms, we discuss in detail various technical issues as well as the optimisation of simulation parameters. We compare the numerical efficiency of all available swap algorithms and discuss their relative merits.Comment: 16 pages, 13 figure