4,596 research outputs found
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
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