Rehybridization of electronic structure in compressed two-dimensional quantum dot superlattices

Two-dimensional superlattices of organically passivated 2.6-nm silver quantum dots were prepared as Langmuir monolayers and transferred to highly oriented pyrolytic graphite substrates. The structural and electronic properties of the films were probed with variable temperature scanning tunneling microscopy. Particles passivated with decanethiol (interparticle separation distance of ∼1.1±0.2 nm) exhibited Coulomb blockade and staircase. For particles passivated with hexanethiol or pentanethiol (interparticle separation distance of ∼0.5±0.2 nm), the single-electron charging was quenched, and the redistribution of the density of states revealed that strong quantum mechanical exchange, i.e., wave-function hybridization, existed among the particles in these films

Nonextensivity in the solar magnetic activity during the increasing phase of solar Cycle 23

In this paper we analyze the behavior of the daily Sunspot Number from the Sunspot Index Data Center (SIDC), the mean Magnetic Field strength from the National Solar Observatory/Kitt Peak (NSO/KP) and Total Solar Irradiance means from Virgo/SoHO, in the context of the $q$--Triplet which emerges within nonextensive statistical mechanics. Distributions for the mean solar Magnetic Field show two different behaviors, with a $q$--Gaussian for scales of 1 to 16 days and a Gaussian for scales longer than 32 days. The latter corresponds to an equilibrium state. Distributions for Total Solar Irradiance also show two different behaviors (approximately Gaussian) for scales of 128 days and longer, consistent with statistical equilibrium and $q$--Gaussian for scales $<$ 128 days. Distributions for the Sunspot Number show a $q$--Gaussian independent of timescales, consistent with a nonequilibrium state. The values obtained ("$q$--Triplet"$\equiv$$\{$$q$$_{stat}$,$q$$_{sen}$,$q$$_{rel}$$\}$) demonstrate that the Gaussian or $q$--Gaussian behavior of the aforementioned data depends significantly on timescales. These results point to strong multifractal behavior of the dataset analyzed, with the multifractal level decreasing from Sunspot Number to Total Solar Irradiance. In addition, we found a numerically satisfied dual relation between $q_{stat}$ and $q_{sen}$.Comment: 6 pages, 4 figure

On the link between rotation, chromospheric activity and Li abundance in subgiant stars

The connection rotation-CaII emission flux-lithium abundance is analyzed for a sample of bona fide subgiant stars, with evolutionary status determined from HIPPARCOS trigonometric parallax measurements and from the Toulouse-Geneva code.Comment: 9 pages, 8 figure

Four-dimensional topological Einstein-Maxwell gravity

The complete on-shell action of topological Einstein-Maxwell gravity in four-dimensions is presented. It is shown explicitly how this theory for SU(2) holonomy manifolds arises from four-dimensional Euclidean N=2 supergravity. The twisted local BRST symmetries and twisted local Lorentz symmetries are given and the action and stress tensor are shown to be BRST-exact. A set of BRST-invariant topological operators is given. The vector and antisymmetric tensor twisted supersymmetries and their algebra are also found.Comment: Published version. Expanded discussion of new results in the introduction and some clarifying remarks added in later sections. 22 pages, uses phyzz

A snapshot of the inner dusty regions of a RCrB-type variable

R Coronae Borealis variable stars are suspected to sporadically eject optically thick dust clouds causing, when one of them lies on the line-of-sight, a huge brightness decline in visible light. Mid-infrared interferometric observations of RYSgr allowed us to explore the circumstellar regions very close to the central star (~20-40 mas) in order to look for the signature of any heterogeneities. Using the VLTI/MIDI instrument, five dispersed visibility curves were recorded with different projected baselines oriented towards two roughly perpendicular directions. The large spatial frequencies visibility curves exhibit a sinusoidal shape whereas, at shorter spatial frequencies visibility curves follow a Gaussian decrease. These observations are well interpreted with a geometrical model consisting in a central star surrounded by an extended circumstellar envelope in which one bright cloud is embedded. Within this simple geometrical scheme, the inner 110AU dusty environment of RYSgr is dominated at the time of observations by a single dusty cloud which, at 10mic represents ~10% of the total flux of the whole system. The cloud is located at about 100stellar radii from the centre toward the East-North-East direction (or the symmetric direction with respect to centre) within a circumstellar envelope which FWHM is about 120stellar radii. This first detection of a cloud so close to the central star, supports the classical scenario of the RCrB brightness variations in the optical spectral domain

Experimental Demonstration of a Structured Material with Extreme Effective Parameters at Microwaves

Following our recent theoretical studies [M. G. Silveirinha, C. A. Fernandes, Phys. Rev. B, 78, 033108, 2008], it is experimentally verified that an array of crossed metallic wires may behave as a nonresonant material with extremely large index of refraction at microwaves, and may enable the realization of ultra-subwavelength waveguides.Comment: accepted for publication in Applied Physics Letters (in press). Applied Physics Letters (in press) (2008

Observational constraints for Lithium depletion before the RGB

Precise Li abundances are determined for 54 giant stars mostly evolving across the Hertzsprung gap. We combine these data with rotational velocity and with information related to the deepening of the convective zone of the stars to analyse their link to Li dilution in the referred spectral region. A sudden decline in Li abundance paralleling the one already established in rotation is quite clear. Following similar results for other stellar luminosity classes and spectral regions, there is no linear relation between Li abundance and rotation, in spite of the fact that most of the fast rotators present high Li content. The effects of convection in driving the Li dilution is also quite clear. Stars with high Li content are mostly those with an undeveloped convective zone, whereas stars with a developed convective zone present clear sign of Li dilution.Comment: 5 pages, 4 figures. accepted for publicatio

Non-Chern-Simons Topological Mass Generation in (2+1) Dimensions

By dimensional reduction of a massive BF theory, a new topological field theory is constructed in (2+1) dimensions. Two different topological terms, one involving a scalar and a Kalb-Ramond fields and another one equivalent to the four-dimensional BF term, are present. We constructed two actions with these topological terms and show that a topological mass generation mechanism can be implemented. Using the non-Chern-Simons topological term, an action is proposed leading to a classical duality relation between Klein-Gordon and Maxwell actions. We also have shown that an action in (2+1) dimensions with the Kalb-Ramond field is related by Buscher's duality transformation to a massive gauge-invariant Stuckelberg-type theory.Comment: 8 pages, no figures, RevTE