Beats of the Magnetocapacitance Oscillations in Lateral Semiconductor Superlattices

We present calculations on the magnetocapacitance of the two-dimensional electron gas in a lateral semiconductor superlattice under two-dimensional weak periodic potential modulation in the presence of a perpendicular magnetic field. Adopting a Gaussian broadening of magnetic-field-dependent width in the density of states, we present explicit and simple expressions for the magnetocapacitance, valid for the relevant weak magnetic fields and modulation strengths. As the modulation strength in both directions increase, beats of the magnetocapacitance oscillations are observed, in the low magnetic field range (Weiss-oscillations regime), which are absent in the one-dimensional weak modulation case.Comment: 11 pages, 7 figures, accepted by Mod. Phys. Lett. B (March 2007

UBVRI twilight sky brightness at ESO-Paranal

Twilight studies have proved to be important tools to analyze the atmospheric structure with interesting consequences on the characterization of astronomical sites. Active discussions on this topic have been recently restarted in connection with the evaluation of Dome C, Antarctica as a potential astronomical site and several site-testing experiments, including twilight brightness measurements, are being prepared. The present work provides for the first time absolute photometric measurements of twilight sky brightness for ESO-Paranal (Chile), which are meant both as a contribution to the site monitoring and as reference values in the analysis of other sites, including Dome C. The UBVRI twilight sky brightness was estimated on more than 2000 FORS1 archival images, which include both flats and standard stars observations taken in twilight, covering a Sun zenith distance range 94-112 deg. The comparison with a low altitude site shows that Paranal V twilight sky brightness is about 30% lower, implying that some fraction of multiple scattering has to take place at an altitude of a few km above the sea level.Comment: 11 pages, 13 figures, accepted for publication in A&

Isophots of Zodiacal Light and Its Terrestrial Component

Isophotes of zodiacal light and its terrestrial component due to interplanetary dust in vicinity of earth-moon syste

Localized Wavefunctions and Magnetic Band Structure for Lateral Semiconductor Superlattices

In this paper we present calculations on the electronic band structure of a two-dimensional lateral superlattice subject to a perpendicular magnetic field by employing a projection operator technique based on the ray-group of magnetotranslation operators. We construct a new basis of appropriately symmetrized Bloch-like wavefunctions as linear combination of well-localized magnetic-Wannier functions. The magnetic field was consistently included in the Wannier functions defined in terms of free-electron eigenfunctions in the presence of external magnetic field in the symmetric gauge. Using the above basis, we calculate the magnetic energy spectrum of electrons in a lateral superlattice with bi-directional weak electrostatic modulation. Both a square lattice and a triangular one are considered as special cases. Our approach based on group theory handles the cases of integer and rational magnetic fluxes in a uniform way and the provided basis could be convenient for further both analytic and numerical calculations.Comment: 19 pages, 5 figures. accepted to Int. J. Mod. Phys. B (April 2006

Direct detection of supersymmetric dark matter- Theoretical rates for transitions to excited states

The recent WMAP data have confirmed that exotic dark matter together with the vacuum energy (cosmological constant) dominate in the flat Universe. Supersymmetry provides a natural dark matter candidate, the lightest supersymmetric particle (LSP). Thus the direct dark matter detection is central to particle physics and cosmology. Most of the research on this issue has hitherto focused on the detection of the recoiling nucleus. In this paper we study transitions to the excited states, focusing on the first excited state at 50 keV of Iodine A=127. We find that the transition rate to this excited state is about 10 percent of the transition to the ground state. So, in principle, the extra signature of the gammai ray following its de-excitation can be exploited experimentally.Comment: LaTex, 13 pages, 3 postscript figures, 1 table, to appear in IJMP

Dark Matter Spin-Dependent Limits for WIMP Interactions on 19-F by PICASSO

The PICASSO experiment at SNOLAB reports new results for spin-dependent WIMP interactions on $^{19}$F using the superheated droplet technique. A new generation of detectors and new features which enable background discrimination via the rejection of non-particle induced events are described. First results are presented for a subset of two detectors with target masses of $^{19}$F of 65 g and 69 g respectively and a total exposure of 13.75 $\pm$ 0.48 kgd. No dark matter signal was found and for WIMP masses around 24 GeV/c$^2$ new limits have been obtained on the spin-dependent cross section on $^{19}$F of $\sigma_F$ = 13.9 pb (90% C.L.) which can be converted into cross section limits on protons and neutrons of $\sigma_p$ = 0.16 pb and $\sigma_n$ = 2.60 pb respectively (90% C.L). The obtained limits on protons restrict recent interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent interactions.Comment: Revised version, accepted for publication in Phys. Lett. B, 20 pages, 7 figure

The modulation effect for supersymmetric dark matter detection with asymmetric velocity dispersion

The detection of the theoretically expected dark matter is central to particle physics cosmology. Current fashionable supersymmetric models provide a natural dark matter candidate which is the lightest supersymmetric particle (LSP). Such models combined with fairly well understood physics like the quark substructure of the nucleon and the nuclear form factor and the spin response function of the nucleus, permit the evaluation of the event rate for LSP-nucleus elastic scattering. The thus obtained event rates are, however, very low or even undetectable. So it is imperative to exploit the modulation effect, i.e. the dependence of the event rate on the earth's annual motion. In this review we study such a modulation effect in directional and undirectional experiments. We calculate both the differential and the total rates using symmetric as well as asymmetric velocity distributions. We find that in the symmetric case the modulation amplitude is small, less than 0.07. There exist, however, regions of the phase space and experimental conditions such that the effect can become larger. The inclusion of asymmetry, with a realistic enhanced velocity dispersion in the galactocentric direction, yields the bonus of an enhanced modulation effect, with an amplitude which for certain parameters can become as large as 0.46.Comment: 35 LATEX pages, 7 Tables, 8 PostScript Figures include

Searching for Supersymmetric Dark Matter - The Directional Rate and the Modulation Effect Due to Caustic Rings

The detection of the theoretically expected dark matter is central to particle physics and cosmology. Current fashionable supersymmetric models provide a natural dark matter candidate which is the lightest supersymmetric particle (LSP). The allowed parameter space of such models combined with fairly well understood physics (quark substructure of the nucleon and nuclear structure) permit the evaluation of the event rate for LSP-nucleus elastic scattering. The thus obtained event rates, which sensitively depend on the allowed parameter space parameters, are usually very low or even undetectable. So, for background reduction, one would like to exploit two nice features of the reaction, the directional rate, which depends on the sun's direction of motion and the modulation effect, i.e. the dependence of the event rate on the earth's annual motion. In the present paper we study these phenomena in a specific class of non isothermal models, which take into account the late in-fall of dark matter into our galaxy, producing flows of caustic rings. We find that the modulation effect arising from such models is smaller than that found previously with isothermal symmetric velocity distributions and much smaller compared to that obtained using a realistic asymmetric distribution with enhanced dispersion in the galactocentric direction.Comment: 31 LATEX pages, 2 tables and 1 figure included. Accepted for publication in Physical Review

Theoretical Directional and Modulated Rates for Direct SUSY Dark Matter Detection

Exotic dark matter together with the vacuum energy (cosmological constant) seem to dominate in the flat Universe. Thus direct dark matter detection is central to particle physics and cosmology. Supersymmetry provides a natural dark matter candidate, the lightest supersymmetric particle (LSP). Furthermore from the knowledge of the density and velocity distribution of the LSP, the quark substructure of the nucleon and the nuclear structure (form factor and/or spin response function), one is able to evaluate the event rate for LSP-nucleus elastic scattering. The thus obtained event rates are, however, very low. So it is imperative to exploit the two signatures of the reaction, namely the modulation effect, i.e. the dependence of the event rate on the Earth's motion, and the directional asymmetry, i.e. the dependence of the rate on the the relative angle between the direction of the recoiling nucleus and the sun's velocity. These two signatures are studied in this paper employing various velocity distributions and a supersymmetric model with universal boundary conditions at large tan(beta).Comment: 11 LATEX pages, 1 table and 4 ps figures included. Paper presented in DARK2002, Fourth Heidelberg International Conference on Dark Matter in Astro- and Particle Physics, Cape Town, South Africa, 4-9 February, 2002, to appear in the proceedings (to be published by Springer Verlag