Electrically driven spin resonance in a bent disordered carbon nanotube

Resonant manipulation of carbon nanotube valley-spin qubits by an electric field is investigated theoretically. We develop a new analysis of electrically driven spin resonance exploiting fixed physical characteristics of the nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity electron spin, and features that coincide with a recent measurement are identified. We show that the same mechanism allows resonant control of the full four-dimensional spin-valley space.Comment: 11 pages, 7 figure

The X-ray luminosity function of AGN at z~3

We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN. Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z=1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ~3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA

Particle Dark Energy

We explore the physics of a gas of particles interacting with a condensate that spontaneously breaks Lorentz invariance. The equation of state of this gas varies from 1/3 to less than -1 and can lead to the observed cosmic acceleration. The particles are always stable. In our particular class of models these particles are fermions with a chiral coupling to the condensate. They may behave as relativistic matter at early times, produce a brief period where they dominate the expansion with w<0 today, and behave as matter at late time. There are no small parameters in our models, which generically lead to dark energy clustering and, depending on the choice of parameters, smoothing of small scale power.Comment: 8 pages, 5 figures; minor update with added refs; version appearing in Phys. Rev.

Weighted-density approximation for general nonuniform fluid mixtures

In order to construct a general density-functional theory for nonuniform fluid mixtures, we propose an extension to multicomponent systems of the weighted-density approximation (WDA) of Curtin and Ashcroft [Phys. Rev. A 32, 2909 (1985)]. This extension corrects a deficiency in a similar extension proposed earlier by Denton and Ashcroft [Phys. Rev. A 42, 7312 (1990)], in that that functional cannot be applied to the multi-component nonuniform fluid systems with spatially varying composition, such as solid-fluid interfaces. As a test of the accuracy of our new functional, we apply it to the calculation of the freezing phase diagram of a binary hard-sphere fluid, and compare the results to simulation and the Denton-Ashcroft extension.Comment: 4 pages, 4 figures, to appear in Phys. Rev. E as Brief Repor

Direct calculation of the hard-sphere crystal/melt interfacial free energy

We present a direct calculation by molecular-dynamics computer simulation of the crystal/melt interfacial free energy, $\gamma$, for a system of hard spheres of diameter $\sigma$. The calculation is performed by thermodynamic integration along a reversible path defined by cleaving, using specially constructed movable hard-sphere walls, separate bulk crystal and fluid systems, which are then merged to form an interface. We find the interfacial free energy to be slightly anisotropic with $\gamma$ = 0.62$\pm 0.01$, 0.64$\pm 0.01$ and 0.58$\pm 0.01 k_BT/\sigma^2$ for the (100), (110) and (111) fcc crystal/fluid interfaces, respectively. These values are consistent with earlier density functional calculations and recent experiments measuring the crystal nucleation rates from colloidal fluids of polystyrene spheres that have been interpreted [Marr and Gast, Langmuir {\bf 10}, 1348 (1994)] to give an estimate of $\gamma$ for the hard-sphere system of $0.55 \pm 0.02 k_BT/\sigma^2$, slightly lower than the directly determined value reported here.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

The CAZyome of Phytophthora spp.: A comprehensive analysis of the gene complement coding for carbohydrate-active enzymes in species of the genus Phytophthora

<p>Abstract</p> <p>Background</p> <p>Enzymes involved in carbohydrate metabolism include Carbohydrate esterases (CE), Glycoside hydrolases (GH), Glycosyl transferases (GT), and Polysaccharide lyases (PL), commonly referred to as carbohydrate-active enzymes (CAZymes). The CE, GH, and PL superfamilies are also known as cell wall degrading enzymes (CWDE) due to their role in the disintegration of the plant cell wall by bacterial and fungal pathogens. In <it>Phytophthora infestans</it>, penetration of the plant cells occurs through a specialized hyphal structure called appressorium; however, it is likely that members of the genus <it>Phytophthora </it>also use CWDE for invasive growth because hyphal forces are below the level of tensile strength exhibited by the plant cell wall. Because information regarding the frequency and distribution of CAZyme coding genes in <it>Phytophthora </it>is currently unknown, we have scanned the genomes of <it>P. infestans, P. sojae</it>, and <it>P. ramorum </it>for the presence of CAZyme-coding genes using a homology-based approach and compared the gene collinearity in the three genomes. In addition, we have tested the expression of several genes coding for CE in cultures grown <it>in vitro</it>.</p> <p>Results</p> <p>We have found that <it>P. infestans, P. sojae </it>and <it>P. ramorum </it>contain a total of 435, 379, and 310 CAZy homologs; in each genome, most homologs belong to the GH superfamily. Most GH and PL homologs code for enzymes that hydrolyze substances present in the pectin layer forming the middle lamella of the plant cells. In addition, a significant number of CE homologs catalyzing the deacetylation of compounds characteristic of the plant cell cuticle were found. In general, a high degree of gene location conservation was observed, as indicated by the presence of sequential orthologous pairs in the three genomes. Such collinearity was frequently observed among members of the GH superfamily. On the other hand, the CE and PL superfamilies showed less collinearity for some of their putative members. Quantitative PCR experiments revealed that all genes are expressed in <it>P. infestans </it>when this pathogen grown <it>in vitro</it>. However, the levels of expression vary considerably and are lower than the expression levels observed for the constitutive control.</p> <p>Conclusions</p> <p>In conclusion, we have identified a highly complex set of CAZy homologs in the genomes of <it>P. infestans, P. sojae</it>, and <it>P. ramorum</it>, a significant number of which could play roles critical for pathogenicity, by participating in the degradation of the plant cell wall.</p