287,725 research outputs found
-adic exponential sums of polynomials in one variable
The -adic exponential sum of a polynomial in one variable is studied. An
explicit arithmetic polygon in terms of the highest two exponents of the
polynomial is proved to be a lower bound of the Newton polygon of the
-function of the T-adic exponential sum. This bound gives lower bounds for
the Newton polygon of the -function of exponential sums of -power order
Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy
The previous proposal (by two of us) of chain inflation with the QCD axion is
shown to fail. The proposal involved a series of fast tunneling events, yet
here it is shown that tunneling is too slow. We calculate the bubble nucleation
rates for phase transitions in the thick wall limit, approximating the barrier
by a triangle. A similar problem arises in realization of chain inflation in
the string landscape that uses series of minima along the monodromy staircase
around the conifold point. The basic problem is that the minima of the
potential are too far apart to allow rapid enough tunneling in these two
models. We entertain the possibility of overcoming this problem by modifying
the gravity sector to a Brans-Dicke theory. However, one would need extremely
small values for the Brans-Dicke parameter. Many successful alternatives exist,
including other "axions" (with mass scales not set by QCD) or potentials with
comparable heights and widths that do not suffer from the problem of slow
tunneling and provide successful candidates for chain inflation.Comment: 6 pages, 1 figur
Internal Friction of Amorphous Silicon in a Magnetic Field
The internal friction of e-beam amorphous silicon was measured in a magnetic
field between 0 and 6 T, from 1.5-20 K, and was found to be independent of the
field to better than 8%. It is concluded that the low energy excitations
observed in this experiment are predominantly atomic in nature.Comment: 4 pages, 4 figures, REVTe
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Correlations and Renormalization of the Electron-Phonon Coupling in the Honeycomb Hubbard Ladder and Superconductivity in Polyacene
We have performed extensive density matrix renormalization group (DMRG)
studies of the Hubbard model on a honeycomb ladder. The band structure (with
Hubbard U=0) exhibits an unusual quadratic band touching at half filling, which
is associated with a quantum Lifshitz transition from a band insulator to a
metal. %SAK as a function of a third-neighbor hopping parameter. For one
electron per site, non-zero drives the system into an insulating state in
which there is no pair-binding between added electrons; this implies that
superconductivity driven directly by the repulsive electron-electron
interactions is unlikely in the regime of small doping, . However, the
divergent density of states as , the large values of the phonon
frequencies, and an unusual correlation induced enhancement of the
electron-phonon coupling imply that lightly doped polyacenes, which
approximately realize this structure, are good candidates for high temperature
electron-phonon driven superconductivity
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