Complex weathering in drylands: implications of 'stress' history for rock debris breakdown and sediment release

Weathering studies have often sought to explain features in terms of a prevailing set of environmental conditions. However, it is clear that in most present-day hot desert regions, the surface rock debris has been exposed to a range of weathering environments and processes. These different weathering conditions can arise in two ways, either from the effects of long-term climate change acting on debris that remains relatively static within the landscape or through the spatial relocation of debris from high to low altitude. Consequently, each fragment of rock may contain a unique weathering-related legacy of damage and alteration — a legacy that may greatly influence its response to present-day weathering activity. Experiments are described in which blocks of limestone, sandstone, granite and basalt are given ‘stress histories’ by subjecting them to varying numbers of heating and freezing cycles as a form of pre-treatment. These imposed stress histories act as proxies for a weathering history. Some blocks were used in a laboratory salt weathering simulation study while others underwent a 2 year field exposure trial at high, mid and low altitude sites in Death Valley, California. Weight loss and ultrasonic pulse velocity measurements suggest that blocks with stress histories deteriorate more rapidly than unstressed samples of the same rock type exposed to the same environmental conditions. Laboratory data also indicate that the result of imposing a known ‘weathering history’ on samples by pre-stressing them is an increase in the amount of fine sediment released during salt weathering over a given period of time in comparison to unstressed samples.

Symmetry, delocalization, and molecular conductance

Molecules bonded between two metal contacts form the simplest possible molecular devices. Coupled by the molecule, the left and right contact-based states form symmetric and antisymmetric pairs near the Fermi level. We relate the size of the resulting energy splitting DeltaE to the symmetry and degree of delocalization of the coupling molecular orbital. Qualitative trends in molecular conductances are then estimated from the variations in DeltaE. We examine benzenedithiol and other molecules of interest in transport. (C) 2005 American Institute of Physics.

Broken symmetry and pseudogaps in ropes of carbon nanotubes

Since the discovery of carbon nanotubes, it has been speculated that these materials should behave like nanoscale wires with unusual electronic properties and exceptional strength. Recently, 'ropes' of close-packed single-wall nanotubes have been synthesized in high yield. The tubes in these ropes are mainly of the (10,10) type3, which is predicted to be metallic. Experiments on individual nanotubes and ropes indicate that these systems indeed have transport properties that qualify them to be viewed as nanoscale quantum wires at low temperature. It has been expected that the close-packing of individual nanotubes into ropes does not change their electronic properties significantly. Here, however, we present first-principles calculations which show that a broken symmetry of the (10,10) tube caused by interactions between tubes in a rope induces a pseudogap of about 0.1 eV at the Fermi level. This pseudogap strongly modifies many of the fundamental electronic properties: we predict a semimetal-like temperature dependence of the electrical conductivity and a finite gap in the infrared absorption spectrum. The existence of both electron and hole charge carriers will lead to qualitatively different thermopower and Hall-effect behaviours from those expected for a normal metal.

Broken symmetry and pseudogaps in ropes of carbon nanotubes

We investigate the influence of tube-tube interactions in ropes of (10,10) carbon nanotubes, and find that these effects induce a pseudogap in the density of state (DOS) of the rope of width 0.1 eV at the Fermi level. In an isolated (n,n) carbon nanotube there are two bands that cross in a linear fashion at the Fermi level, making the nanotube metallic with a DOS that is constant in a 1.5 eV wide window around the Fermi energy. The presence of the neighbouring tubes causes these two bands to repel, opening up a band gap that can be as large as 0.3 eV. The small dispersion in the plane perpendicular to the rope smears out this gap for a rope with a large cross-sectional area, and we see a pseudogap at the Fermi energy in the DOS where the DOS falls to one third of its value for the isolated tube. This phenomenon should affect many properties of the behavior of ropes of (n,n) nanotubes, which should display a more semimetallic character than expected in transport and doping experiments, with the existence of both hole and electron carriers leading to qualitatively different thermopower and Hall-effect behaviors from those expected for a normal metal. Band repulsion like this can be expected to occur for any tube perturbed by a sufficiently strong interaction, for example, from contact with a surface or with other tubes.

Complete Sudoku - extra rules for more symmetry

We propose some extra rules to add to the well-known Sudoku puzzle and present an argument to justify their inclusion. The rules mean that puzzles can be created with fewer cells completed initially yet which still have only one solution. We have created a Web-based program which can be used to generate and solve both standard and extended (Complete) puzzles.

Presentism and ontological symmetry

In this paper, I argue that there is an inconsistency between two presentist doctrines: that of ontological symmetry and asymmetry of fixity. The former refers to the presentist belief that the past and future are equally unreal. The latter refers to the A-Theoretic intuition that the past is closed or actual, and the future is open or potential. My position in this paper is that the presentist is unable to account for the temporal asymmetry that is so fundamentally a part of her theory. In Section I, I briefly outline a recent defence of presentism due to Craig, and argue that a flaw in this defence highlights the tension between the presentist's doctrines of ontological symmetry and asymmetry of fixity. In Section II, I undertake an investigation, on the presentist's behalf, in order to determine whether she is capable of reconciling these two doctrines. In the course of the investigation, I consider different asymmetries, other than that of ontology, which might be said fundamentally to constitute temporal asymmetry, and the asymmetry of fixity in particular. In Section III, I also consider whether the presentist is able to avail herself of some of the standard B-Theoretic accounts of the asymmetry of fixity, and argue that she cannot. Finally, I conclude that temporal asymmetry cannot be accounted for (or explained) other than through the postulation of an ontological asymmetry.