Nonlinear optical probe of tunable surface electrons on a topological insulator

We use ultrafast laser pulses to experimentally demonstrate that the second-order optical response of bulk single crystals of the topological insulator Bi$_2$Se$_3$ is sensitive to its surface electrons. By performing surface doping dependence measurements as a function of photon polarization and sample orientation we show that second harmonic generation can simultaneously probe both the surface crystalline structure and the surface charge of Bi$_2$Se$_3$. Furthermore, we find that second harmonic generation using circularly polarized photons reveals the time-reversal symmetry properties of the system and is surprisingly robust against surface charging, which makes it a promising tool for spectroscopic studies of topological surfaces and buried interfaces

Wrinkling of a bilayer membrane

The buckling of elastic bodies is a common phenomenon in the mechanics of solids. Wrinkling of membranes can often be interpreted as buckling under constraints that prohibit large amplitude deformation. We present a combination of analytic calculations, experiments, and simulations to understand wrinkling patterns generated in a bilayer membrane. The model membrane is composed of a flexible spherical shell that is under tension and that is circumscribed by a stiff, essentially incompressible strip with bending modulus B. When the tension is reduced sufficiently to a value \sigma, the strip forms wrinkles with a uniform wavelength found theoretically and experimentally to be \lambda = 2\pi(B/\sigma)^{1/3}. Defects in this pattern appear for rapid changes in tension. Comparison between experiment and simulation further shows that, with larger reduction of tension, a second generation of wrinkles with longer wavelength appears only when B is sufficiently small.Comment: 9 pages, 5 color figure

Gravity and Light: Combining Gravitational Wave and Electromagnetic Observations in the 2020s

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations

Stochastic Invariants for Probabilistic Termination

Termination is one of the basic liveness properties, and we study the termination problem for probabilistic programs with real-valued variables. Previous works focused on the qualitative problem that asks whether an input program terminates with probability~1 (almost-sure termination). A powerful approach for this qualitative problem is the notion of ranking supermartingales with respect to a given set of invariants. The quantitative problem (probabilistic termination) asks for bounds on the termination probability. A fundamental and conceptual drawback of the existing approaches to address probabilistic termination is that even though the supermartingales consider the probabilistic behavior of the programs, the invariants are obtained completely ignoring the probabilistic aspect. In this work we address the probabilistic termination problem for linear-arithmetic probabilistic programs with nondeterminism. We define the notion of {\em stochastic invariants}, which are constraints along with a probability bound that the constraints hold. We introduce a concept of {\em repulsing supermartingales}. First, we show that repulsing supermartingales can be used to obtain bounds on the probability of the stochastic invariants. Second, we show the effectiveness of repulsing supermartingales in the following three ways: (1)~With a combination of ranking and repulsing supermartingales we can compute lower bounds on the probability of termination; (2)~repulsing supermartingales provide witnesses for refutation of almost-sure termination; and (3)~with a combination of ranking and repulsing supermartingales we can establish persistence properties of probabilistic programs. We also present results on related computational problems and an experimental evaluation of our approach on academic examples.Comment: Full version of a paper published at POPL 2017. 20 page

Parametric and sensitivity analysis for a proposed filtered tailings storage facility in challenging topography

Parametric and sensitivity analyses were performed for a proposed filtered tail- ings storage facility in challenging topography from the Southern Peru region. The purpose was to identify elements which are likely to dictate performance and stability, and to understand the risk profile for the facility. The analyses were performed varying: foundation material physical and hydraulic properties, tailings physical and hydraulic properties, foundation configuration, seismicity, and staged construction. The physical properties of the foundation material were varied by changing the Mohr-Coulomb strength parameters c’ and φ’, whereas the hydraulic properties were varied by moving the groundwater table upwards or saturating a larger amount of the foundation material. A similar approach was adopted in order to vary the physical and hydraulic properties of the tailings. The foundation configuration was varied by hypothetically moving the starter buttress closer to the edge of steep slopes further downhill from the facility. Staged construction stability analyses were simulated by analyzing filtered tailings fill configurations, which followed the overall design slope but varied in height. All analyses presented herein were conducted for the most critical failure mode for each scenario. The results from these parametric analyses showed that the design of the facility was highly sensitive to tailings potential saturation. In the simulation scenarios, saturation of the tailings caused instability in both static and seismic conditions. This sensitivity to moisture conditions highlighted the importance of properly defining the unsaturated filtered tailings parameters and simulating water infiltration processes through the vadose zone.Fil: Preciado, H. F. AMEC S.A; Perú.Fil: Ale, J. AMEC S.A; Perú.Fil: Byler, B. AMEC Englewood; USA.Fil: Perez, F. AMEC S.A; Perú.Fil: Mejía, V. AMEC S.A; Perú.Fil: McIver, D. Minera IRL SA; Perú.Otras Ingeniería Civi

Selective probing of photo-induced charge and spin dynamics in the bulk and surface of a topological insulator

Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator Bi2Se3 to independently measure its photo-induced charge and spin dynamics with bulk and surface selectivity. Our results show that a transient net spin density can be optically induced in both the bulk and surface, which may drive spin transport in topological insulators. By utilizing a novel rotational anisotropy analysis we are able to separately resolve the spin de-polarization, intraband cooling and interband recombination processes following photo-excitation, which reveal that spin and charge degrees of freedom relax on very different time scales owing to strong spin-orbit coupling.Comment: Accepted to Phys. Rev. Let

Women candidates and councillors in Scottish local government, 1974-2012

While significant attention has been paid to the levels of representation of women in both the Westminster Parliament and the Scottish Parliament, much less considered has been given to the position within local government. This article addresses that deficit for Scotland. It shows that for twenty-five years following the reorganisation of local government in Scotland in 1974 there was a slow but relatively steady increase in the numbers of female candidates and councillors, although more recently this appears to have since plateaued somewhat, together with a similar increase in the number of women councillors taking up more senior roles in Scotland’s councils. The article analyses the representation of women in Scottish local government over the period from 1974 to 2012 against the backdrop of significant change in Scotland, including a further restructuring of local government and the introduction of the Single Transferable Vote for council elections, the creation of the Scottish Parliament, the rise of the SNP and the decline of the Conservative Party