11,381 research outputs found
Hot Electron Effects in the 2D Superconductor-Insulator Transition
The parallel magnetic field tuned two-dimensional superconductor-insulator
transition has been investigated in ultrathin films of amorphous Bi. The
resistance is found to be independent of temperature on both sides of the
transition below approximately 120 mK. Several observations suggest that this
regime is not intrinsically "metallic" but results from the failure of the
films' electrons to cool. The onset of this temperature-independent regime can
be moved to higher temperatures by either increasing the measuring current or
the level of electromagnetic noise. Temperature scaling is successful above 120
mK. Electric field scaling can be mapped onto temperature scaling by relating
the electric fields to elevated electron temperatures. These results cast doubt
on the existence of an intrinsic metallic regime and on the independent
determination of the correlation length and dynamical critical exponents
obtained by combining the results of electric field and temperature scaling.Comment: 4 pages, 4 figure
Neutrino masses or new interactions
Recent proposals to study the mass of the "electron" neutrino at a
sensitivity of 0.3 eV can be used to place limits on the right handed and
scalar charged currents at a level which improves on the present experimental
limits. Indeed the neglect of the possibility of such interactions can lead to
the inference of an incorrect value for the mass, as we illustrate.Comment: 12 pages and 3 figures. Contributed to the XX International Symposium
on Lepton and Photon Interactions at High Energies, Rome, July 2001, and to
the International Europhysics Conference on High Energy Physics, Budapest,
July 2001. Preprint numbers added, misprints correcte
Neutrino clustering and the Z-burst model
The possibility that the observed Ultra High Energy Cosmic Rays are generated
by high energy neutrinos creating "Z-bursts" in resonant interactions with the
background neutrinos has been proposed, but there are difficulties in
generating enough events with reasonable incident neutrino fluxes.
We point out that this difficulty is overcome if the background neutrinos
have coalesced into "neutrino clouds" --- a possibility previously suggested by
some of us in another context. The limitations that this mechanism for the
generation of UHECRs places on the high energy neutrino flux, on the masses of
the background neutrinos and the characteristics of the neutrino clouds are
spelled out.Comment: 13 pages and 3 figures. Contributed to the XX International Symposium
on Lepton and Photon Interactions at High Energies, Rome, July 2001, and to
the International Europhysics Conference on High Energy Physics, Budapest,
July 2001. Preprint numbers added, misprints correcte
Spectroscopic and photometric studies of white dwarfs in the Hyades
The Hyades cluster is known to harbour ten so-called classical white dwarf
members. Numerous studies through the years have predicted that more than twice
this amount of degenerate stars should be associated with the cluster. Using
the PPMXL catalog of proper motions and positions, a recent study proposed 17
new white dwarf candidates. We review the membership of these candidates by
using published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to the ten
classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades
origin and three more objects that have an uncertain membership status due to
their unknown or imprecise atmospheric parameters. Among those, two to three
are expected as field stars contamination. Accurate radial velocity
measurements will confirm or reject the candidates. One consequence is that the
longstanding problem that no white dwarf older than ~340 Myr appears to be
associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and
Astrophysics journa
Electrostatic Tuning of the Superconductor-Insulator Transition in Two Dimensions
Superconductivity has been induced in insulating ultra-thin films of
amorphous bismuth using the electric field effect. The screening of
electron-electron interaction was found to increase with electron concentration
in a manner correlated with the tendency towards superconductivity. This does
not preclude an increase in the density of states being important in the
development of superconductivity. The superconductor-insulator transition
appears to belong to the universality class of the three dimensional XY model.Comment: Four pages, three figures. Revised slightly to reflect referees'
comment
Epoch Dependent Dark Energy
We present a model in which the equation of state parameter w approaches -1
near a particular value of z, and has significant negative values in a
restricted range of z. For example, one can have w ~ -1 near z = 1, and w >
-0.2 from z = 0 to z = 0.3, and for z > 9. The ingredients of the model are
neutral fermions (which may be neutrinos, neutralinos, etc) which are very
weakly coupled to a light scalar field. This model emphasises the importance of
the proposed studies of the properties of dark energy into the region z > 1.Comment: 7pp., 2 figs. Invited talk at the 5th Int'l. Wkshp. on the Dark Side
of the Universe, 1-5 June 2009 Melbourne, DSU09; to appear in the proceeding
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