Evaporite-karst processes, landforms, and environmental problems

Evaporite karst is widespread, but relatively unknown when compared with carbonate karst; this special issue addresses that lack of familiarity. Evaporite rocks have much higher solubilities and faster dissolution rates than carbonate rocks and they also commonly have lower mechanical strengths and more ductile reheologies. Many of these factors are dependent on the local hydrogeology, and when combined they can result in areas where karst features evolve on a human time scale, rather than a geological timescale. Karst collapse and subsidence are common in such areas, making them problematical for the local population. The evaporite-karst environment is very sensitive to changes in the local hydrology and hydrogeology, so that human factors such as groundwater extraction, drainage, and irrigation can act as triggering events for karst collapses. Some evaporite-karst features such as caves and saline springs have been beneficially exploited, but most of them, including sinkholes, subsidence, and groundwater degradation, pose a threat to the local environment and a hazard to development. The papers in this special issue of Environmental Geology arose from a successful session on Evaporite Karst convened by us at the Sixth International Conference on Geomorphology. This was held in Zaragoza, Spain, in September 2005 and was organised by Zaragoza University and the International Association of Geomorphologists (IAG; http://www.geomorph.org/). Authors of the twenty-eight presented abstracts were invited to submit full papers to this special issue. Nineteen papers were proposed and sixteen papers have been accepted and are published here

Identification, prediction and mitigation of sinkhole hazards in evaporite karst areas

Abstract Sinkholes usually have a higher probability of occurrence and a greater genetic diversity in evaporite terrains than in carbonate karst areas. This is because evaporites have a higher solubility, and commonly a lower mechanical strength. Subsidence damage resulting from evaporite dissolution generates substantial losses throughout the world, but the causes are only well-understood in a few areas. To deal with these hazards, a phased approach is needed for sinkhole identification, investigation, prediction, and mitigation. Identification techniques include field surveys, and geomorphological mapping combined with accounts from local people and historical sources. Detailed sinkhole maps can be constructed from sequential historical maps, recent topographical maps and digital elevation models (DEMs) complemented with building-damage surveying, remote sensing, and high-resolution geodetic surveys. On a more detailed level, information from exposed paleosubsidence features (paleokarst), speleological explorations, geophysical investigations, trenching, dating techniques, and boreholes, may help to recognize dissolution and subsidence features. Information on the hydrogeological pathways including caves, springs and swallow holes, are particularly important especially when corroborated by tracer tests. These diverse data sources make a valuable database - the karst inventory. From this dataset, sinkhole susceptibility zonations (relative probability) may be produced based on the spatial and temporal distribution of the features and good knowledge of the local geology. Sinkhole distribution can be investigated by spatial distribution analysis techniques including studies of preferential elongation, alignment and nearest neighbor analysis. More objective susceptibility models may be obtained by analyzing the statistical relationships between the known sinkholes and the conditioning factors, such as weather conditions. Chronological information on sinkhole formation is required to estimate the probability of occurrence of sinkholes (number of sinkholes/km² year). Such spatial and temporal predictions, derived from limited records and based on the assumption that past sinkhole activity may be extrapolated to the future, are non-corroborated hypotheses. Validation methods allow us to assess the predictive capability of the susceptibility maps and to transform them into probability maps. Avoiding the most hazardous areas by preventive planning is the safest strategy for development in sinkhole-prone areas. Corrective measures could be to reduce the dissolution activity and subsidence processes, but these are difficult. A more practical solution for safe development is to reduce the vulnerability of the structures by using subsidence-proof designs

Threshold voltage and space charge in organic transistors

We investigate rubrene single-crystal field-effect transistors, whose stability and reproducibility are sufficient to measure systematically the shift in threshold voltage as a function of channel length and source-drain voltage. The shift is due to space-charge transferred from the contacts, and can be modeled quantitatively without free fitting parameters, using Poisson's equation, and by assuming that the density of states in rubrene is that of a conventional inorganic semiconductor. Our results demonstrate the consistency, at the quantitative level, of a variety of recent experiments on rubrene crystals, and show how the use of FET measurements can enable the determination of microscopic parameters (e.g., the effective mass of charge carriers).Comment: 4 pages, 4 figure

Two-dimensional Einstein manifolds in geometrothermodynamics

We present a class of thermodynamic systems with constant thermodynamic curvature which, within the context of geometric approaches of thermodynamics, can be interpreted as constant thermodynamic interaction among their components. In particular, for systems constrained by the vanishing of the Hessian curvature we write down the systems of partial differential equations. In such a case it is possible to find a subset of solutions lying on a circumference in an abstract space constructed from the first derivatives of the isothermal coordinates. We conjecture that solutions on the characteristic circumference are of physical relevance, separating them from those of pure mathematical interest. We present the case of a one-parameter family of fundamental relations that -- when lying in the circumference -- describe a polytropic fluid

Ambipolar Light-Emitting Transistors on Chemical Vapor Deposited Monolayer MoS2

We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap {\Delta} of monolayer MoS2 directly from the device transfer curves and find {\Delta} $\approx$ 2.4-2.7 eV. In the ambipolar injection regime, we observe electroluminescence due to exciton recombination in MoS2, originating from the region close to the hole-injecting contact. Both the observed transport properties and the behavior of the electroluminescence can be consistently understood as due to the presence of defect states at an energy of 250-300 meV above the top of the valence band, acting as deep traps for holes. Our results are of technological relevance, as they show that devices with useful optoelectronic functionality can be realized on large-area MoS2 monolayers produced by controllable and scalable techniques