Rare top decay t-> c l+l- as a probe of new physics

The rare top decay t-> c l+l-, which involves flavor violation, is studied as a possible probe of new physics. This decay is analyzed with the simplest Standard Model extensions with additional gauge symmetry formalism. The considered extension is the Left-Right Symmetric Model, including a new neutral gauge boson Z' that allows to obtain the decay at tree level through Flavor Changing Neutral Currents (FCNC) couplings. The neutral gauge boson couplings are considered diagonal but family non-universal in order to induce these FCNC. We find the $BR(t-> c l+l-)~10^{-13} for a range 1 TeV < M_{Z'} < 3 TeV.Comment: 9 pages, 6 figure

Interaction-induced chaos in a two-electron quantum-dot system

A quasi-one-dimensional quantum dot containing two interacting electrons is analyzed in search of signatures of chaos. The two-electron energy spectrum is obtained by diagonalization of the Hamiltonian including the exact Coulomb interaction. We find that the level-spacing fluctuations follow closely a Wigner-Dyson distribution, which indicates the emergence of quantum signatures of chaos due to the Coulomb interaction in an otherwise non-chaotic system. In general, the Poincar\'e maps of a classical analog of this quantum mechanical problem can exhibit a mixed classical dynamics. However, for the range of energies involved in the present system, the dynamics is strongly chaotic, aside from small regular regions. The system we study models a realistic semiconductor nanostructure, with electronic parameters typical of gallium arsenide.Comment: 4 pages, 3ps figure

Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN

We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN thin films. The saturation magnetization moments in our best films of Cr-GaN and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively, indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated conduction that follows the exponential law expected for variable range hopping between localized states. Hall measurements on a Cr-GaN sample indicate a mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping conduction, and a free carrier density (1.4E20/cm^3), which is similar in magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A large negative magnetoresistance is attributed to scattering from loose spins associated with non-ferromagnetic impurities. The results indicate that ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange mechanism as a result of hopping between near-midgap substitutional Cr impurity bands.Comment: 14 pages, 4 figures, submitted to AP