Disorder and superconductivity : a new phase of bi-particle localized states

We study the two-dimensional, disordered, attractive Hubbard model by the projector quantum Monte Carlo method and Bogoliubov - de Gennes mean-field theory. Our results for the ground state show the appearance of a new phase with charge localization in the metallic regime of the non-interacting model. Contrary to the common lore, we demonstrate that mean-field theory fails to predict this phase and is unable to describe the correct physical picture in this regime.Comment: revtex, 4 pages, 3 figure

Early observations of the Afterglow of GRB000301c

We report multiband observations of the Optical Transient (OT) associated with GRB000301c carried out between 2--4 March 2000 using the 2.34-m Vainu Bappu Telescope (VBT) at Kavalur, India. When combined with other reported data, the initial decline in the R-band magnitude with log ($t-t_0$), the time since the burst is fit with a slope $\alpha_{1}$ = -0.70 $\pm$ 0.07 which steepens after about 6.0 days to a slope of $\alpha_{2}$ = -2.44 $\pm$ 0.29. This change in slope does not occur smoothly but there is an indication for a bimodal distribution. The available measurements of the evolution of (B--R) color do not show any discernible evolution in the first 12 days.Comment: 14 pages, 2 postscript figures, Accepted for Publication in ApJ Letter

Narrow band photometry of selected asteroids

The CCD photometry of selected asteroids was carried out to check for possible cometary activity in them. To distinguish the asteroids with possible cometary activity from those of the main belt, each object of interest was observed in two filters; one centered on the C2 emission band at 5140A (90A bandpass) and the other centered on the nearby continuum at 4845A (65A bandpass). None of the observed asteroids appear to have any C2 emission

Variational Monte Carlo and Configurational Interaction Studies of C60C_{60} and its Fragments

The $C_{60}$ molecule and its fragments are studied using Configuration Interaction (CI) and Variational Monte Carlo (VMC) techniques, within the Hubbard model. Using benzene as a test case, we compare the results of the approximate calculations with exact calculations. The fragments of $C_{60}$ studied are pyracylene, fluoranthene and corannulene. The energies, bond orders, spin-spin and charge-correlation functions of these systems are obtained for various values of the Hubbard parameter, $U$. The analysis of bond orders and correlation functions of these individual molecules allow us to visualise pyracylene as a naphthalene unit with two ethylenic moieties and fluoranthene as weakly bridged benzene and naphthalene units. Corannulene is the largest fragment of $C_{60}$ that we have studied. The hexagon-hexagon(h-h) bond orders are slightly larger than those of the hexagon-pentagon bonds(h-p), a feature also found in other fragments. We also find bonds between two co-ordinated carbon sites to be stronger than bonds involving three coordinated carbon sites. In $C_{60}$, the h-h bonds are stronger than in corannulene and the h-p bonds weaker than in corannulene for all correlation strengths. Introducing bond alternation in the buckyball enhances this difference.Comment: 42 pages, 5 figures available on request, to appear in J. Phys. Che

Transition to an Insulating Phase Induced by Attractive Interactions in the Disordered Three-Dimensional Hubbard Model

We study numerically the interplay of disorder and attractive interactions for spin-1/2 fermions in the three-dimensional Hubbard model. The results obtained by projector quantum Monte Carlo simulations show that at moderate disorder, increasing the attractive interaction leads to a transition from delocalized superconducting states to the insulating phase of localized pairs. This transition takes place well within the metallic phase of the single-particle Anderson model.Comment: revtex, 4 pages, 3 figure

On the Clustering of GRBs on the Sky

The two-point correlation of the 4th (current) BATSE catalog (2494 objects) is calculated. It is shown to be consistent with zero at nearly all angular scales of interest. Assuming that GRBs trace the large scale structure in the universe we calculate the angular correlation function for the standard CDM (sCDM) model. It is shown to be $\le 10^{-4}$ at $\theta \simeq 5^\circ$ if the BATSE catalog is assumed to be a volume-limited sample up to $z \simeq 1$. Combined with the error analysis on the BATSE catalog this suggests that nearly $10^5$ GRBs will be needed to make a positive detection of the two-point angular correlation function at this angular scale.Comment: 5 pages, Latex with aipproc.sty, incl. 1 ps-Fig., Proc. of the 5th Huntsville Gamma Ray Burst Symposium, Oct. 1999, ed. R.M. Kippen, AI

Can local dynamics enhance entangling power?

It is demonstrated here that local dynamics have the ability to strongly modify the entangling power of unitary quantum gates acting on a composite system. The scenario is common to numerous physical systems, in which the time evolution involves local operators and nonlocal interactions. To distinguish between distinct classes of gates with zero entangling power we introduce a complementary quantity called gate-typicality and study its properties. Analyzing multiple applications of any entangling operator interlaced with random local gates, we prove that both investigated quantities approach their asymptotic values in a simple exponential form. This rapid convergence to equilibrium, valid for subsystems of arbitrary size, is illustrated by studying multiple actions of diagonal unitary gates and controlled unitary gates.Comment: 7 pages, 3 figure

Study of mercury transformation with chlorinated species under homogeneous and heterogeneous conditions

Mercury (Hg) transformation under homogeneous (gas-phase oxidation reactions primarily involving chlorine species in flue gases) and heterogeneous (gas-surface oxidation reactions involving surface enhanced Hg oxidation in the presence of flue gases) environments were investigated. Gas phase experiments were performed in the presence of chlorine sources such as Cl2 and HCl. A large body of literature studies indicates that during combustion in coal-fired power plants coal mineral matter components play a major role in Hg transformation. Surface activity of these components with respect to Hg adsorption and overall Hg removal were evaluated using a laboratory-scale, fixed bed flow reactor where initial Hg concentration, temperature, residence time, gas composition, and the metal oxide surface were carefully controlled. The metal oxides of interest were γ-Fe2O3, TiO2, Al2O3, and CaO. These catalytic materials were immobilized between quartz wool in a quartz flow reactor. Homogeneous experiments with different gas compositions, different chlorine sources (HCl or Cl2), and gas-phase residence times of 1 and 2 sec showed no measurable difference in Hg oxidation except at 100°C. Hg removal (oxidation) efficiencies ranged from 2 to 15%. Heterogeneous studies in the presence of metal oxides (with Cl2 and HCl as the chlorine source) indicated that γ-iron oxide showed the highest Hg removal efficiency at 1 sec residence time, compared to other metal oxides under the same experimental conditions. However, the data were highly scattered and occasionally showed inconsistency. A reduction in the surface activity of γ-iron oxide due to aging may have been responsible for the inconsistency in some of the results. TiO2, used in the presence of Cl2 at 100°C, resulted in a 60% Hg removal efficiency which decreased with increasing temperature. TiO2 used in the presence of HCl resulted in a 55% Hg removal efficiency at 400°C. Al2O3 and CaO were ineffective with regard to Hg oxidation in the presence of Cl2 or HCl compared to γ-iron oxide and TiO2. Adsorption and overall Hg removal efficiencies showed the following trend (in descending order of effectiveness): γ- Fe2O3 \u3e TiO2 \u3e Al2O3 \u3e CaO