Superconducting diamagnetic fluctuations in ropes of carbon nanotubes

We report low-temperature magnetisation measurements on a large number of purified ropes of single wall carbon nanotubes. In spite of a large superparamagnetic contribution due to the small ferromagnetic catalytical particles still present in the sample, at low temperature ($T < 0.5K$) and low magnetic field ($H < 80 Oe$), a diamagnetic signal is detectable. This low temperature diamagnetism can be interpreted as the Meissner effect in ropes of carbon nanotubes which have previously been shown to exhibit superconductivity from transport measurements.Comment: 10 pages 3 figure

Alteration of superconductivity of suspended carbon nanotubes by deposition of organic molecules

We have altered the superconductivity of a suspended rope of single walled carbon nanotubes, by coating it with organic polymers. Upon coating, the normal state resistance of the rope changes by less than 20 percent. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below 300 mK, is gradualy suppressed. We correlate this to the suppression of radial breathing modes, measured with Raman Spectroscopy on suspended Single and Double-walled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in carbon nanotubes

Remarks on the tight-binding model of graphene

We address a simple but fundamental issue arising in the study of graphene, as well as of other systems that have a crystalline structure with more than one atom per unit cell. For these systems, the choice of the tight-binding basis is not unique. For monolayer graphene two bases are widely used in the literature. While the expectation values of operators describing physical quantities should be independent of basis, the form of the operators may depend on the basis, especially in the presence of disorder or of an applied magnetic field. Using the inappropriate form of certain operators may lead to erroneous physical predictions. We discuss the two bases used to describe monolayer graphene, as well as the form of the most commonly used operators in the two bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure

On postglacial sea level—III. Incorporating sediment redistribution

We derive a generalized theory for gravitationally self-consistent, static sea level variations on earth models of arbitrary complexity that takes into account the redistribution of sediments. The theory is an extension of previous work that incorporated, into the governing equations, shoreline migration due to local sea level variations and changes in the geometry of grounded, marine-based ice. In addition, we use viscoelastic Love number theory to present a version of the new theory valid for spherically symmetric earth models. The Love number theory accounts for the gravitational, deformational and rotational effects of the sediment redistribution. As a first, illustrative application of the new theory, we compute the perturbation in sea level driven by an idealized pulse of sediment transport into the Gulf of Mexico. We demonstrate that incorporating a gravitationally self-consistent water load in this case significantly improves the accuracy of sea level predictions relative to previous simplified treatments of the sediment redistribution

Pinning and switching of magnetic moments in bilayer graphene

We examine the magnetic properties of the localized states induced by lattice vacancies in bilayer graphene with an unrestricted Hartree-Fock calculation. We show that with realistic values of the parameters and for experimentally accessible gate voltages we can have a magnetic switching between an unpolarized and a fully polarized system.Comment: 9 pages, 4 figure

The key physical parameters governing frictional dissipation in a precipitating atmosphere

Precipitation generates small-scale turbulent air flows the energy of which ultimately dissipates to heat. The power of this process has previously been estimated to be around 2-4 W m-2 in the tropics: a value comparable in magnitude to the dynamic power of the global circulation. Here we suggest that this previous power estimate is approximately double the true figure. Our result reflects a revised evaluation of the mean precipitation path length Hp. We investigate the dependence of Hp on surface temperature,relative humidity,temperature lapse rate and degree of condensation in the ascending air. We find that the degree of condensation,defined as the relative change of the saturated water vapor mixing ratio in the region of condensation, is a major factor determining Hp. We estimate from theory that the mean large-scale rate of frictional dissipation associated with total precipitation in the tropics lies between 1 and 2 W m-2 and show that our estimate is supported by empirical evidence. We show that under terrestrial conditions frictional dissipation constitutes a minor fraction of the dynamic power of condensation-induced atmospheric circulation,which is estimated to be at least 2.5 times larger. However,because Hp increases with surface temperature Ts, the rate of frictional dissipation would exceed that of condensation-induced dynamics, and thus block major circulation, at Ts >~320 K in a moist adiabatic atmosphere.Comment: 12 pp, 2 figure

The amphioxus genome and the evolution of the chordate karyotype

Lancelets ('amphioxus') are the modern survivors of an ancient chordate lineage, with a fossil record dating back to the Cambrian period. Here we describe the structure and gene content of the highly polymorphic approx520-megabase genome of the Florida lancelet Branchiostoma floridae, and analyse it in the context of chordate evolution. Whole-genome comparisons illuminate the murky relationships among the three chordate groups (tunicates, lancelets and vertebrates), and allow not only reconstruction of the gene complement of the last common chordate ancestor but also partial reconstruction of its genomic organization, as well as a description of two genome-wide duplications and subsequent reorganizations in the vertebrate lineage. These genome-scale events shaped the vertebrate genome and provided additional genetic variation for exploitation during vertebrate evolution