44,743 research outputs found
Spectropolarimetry of PKS 0040-005 and the Orientation of Broad Absorption Line Quasars
We have used the Very Large Telescope (VLT) to obtain spectropolarimetry of
the radio-loud, double-lobed broad absorption line (BAL) quasar PKS 0040-005.
We find that the optical continuum of PKS 0040-005 is intrinsically polarized
at 0.7% with an electric vector position angle nearly parallel to that of the
large-scale radio axis. This result is naturally explained in terms of an
equatorial scattering region seen at a small inclination, building a strong
case that the BAL outflow is not equatorial. In conjunction with other recent
results concerning radio-loud BAL quasars, the era of simply characterizing
these sources as ``edge-on'' is over.Comment: 5 Pages, including 2 PostScript figures. Accepted for publication in
MNRAS letter
Universal resonant ultracold molecular scattering
The elastic scattering amplitudes of indistinguishable, bosonic,
strongly-polar molecules possess universal properties at the coldest
temperatures due to wave propagation in the long-range dipole-dipole field.
Universal scattering cross sections and anisotropic threshold angular
distributions, independent of molecular species, result from careful tuning of
the dipole moment with an applied electric field. Three distinct families of
threshold resonances also occur for specific field strengths, and can be both
qualitatively and quantitatively predicted using elementary adiabatic and
semi-classical techniques. The temperatures and densities of heteronuclear
molecular gases required to observe these univeral characteristics are
predicted. PACS numbers: 34.50.Cx, 31.15.ap, 33.15.-e, 34.20.-bComment: 4 pages, 5 figure
Interference of nematic quantum critical quasiparticles: a route to the octet model
Repeated observations of inhomogeneity in cuperate superconductors[1-5] make
one immediately question the existance of coherent quasiparticles(qp's) and the
applicability of a momentum space picture. Yet, obversations of interference
effects[6-9] suggest that the qp's maintain a remarkable coherence under
special circumstances. In particular, quasi-particle interference (QPI) imaging
using scanning tunneling spectroscopy revealed a highly unusual form of
coherence: accumulation of coherence only at special points in momentum space
with a particular energy dispersion[5-7]. Here we show that nematic quantum
critical fluctuations[10], combined with the known extreme velocity
anisotropy[11] provide a natural mechanism for the accumulation of coherence at
those special points. Our results raise the intriguing question of whether the
nematic fluctuations provide the unique mechanism for such a phenomenon.Comment: 4 pages, 3 figure
Absence of Metastable States in Strained Monatomic Cubic Crystals
A tetragonal (Bain path) distortion of a metal with an fcc (bcc) ground state
will initially cause an increase in energy, but at some point along the Bain
path the energy will again decrease until a local minimum is reached. Using a
combination of parametrized tight-binding and first-principles LAPW
calculations we show that this local minimum is unstable with respect to an
elastic distortion, except in the rare case that the minimum is at the bcc
(fcc) point on the Bain path. This shows that body-centered tetragonal phases
of these materials, which have been seen in epitaxially grown thin films, must
be stabilized by the substrate and cannot be free-standing films.Comment: 7 pages, 5 postscript figures, REVTEX, submitted to Phys. Rev.
Self-normalized processes: exponential inequalities, moment bounds and iterated logarithm laws
Self-normalized processes arise naturally in statistical applications.
Being unit free, they are not affected by scale changes. Moreover,
self-normalization often eliminates or weakens moment assumptions. In this
paper we present several exponential and moment inequalities, particularly
those related to laws of the iterated logarithm, for self-normalized random
variables including martingales. Tail probability bounds are also derived. For
random variables B_t>0 and A_t, let Y_t(\lambda)=\exp{\lambda A_t-\lambda
^2B_t^2/2}. We develop inequalities for the moments of A_t/B_{t} or sup_{t\geq
0}A_t/{B_t(\log \log B_{t})^{1/2}} and variants thereof, when EY_t(\lambda
)\leq 1 or when Y_t(\lambda) is a supermartingale, for all \lambda belonging to
some interval. Our results are valid for a wide class of random processes
including continuous martingales with A_t=M_t and B_t=\sqrt _t, and sums of
conditionally symmetric variables d_i with A_t=\sum_{i=1}^td_i and
B_t=\sqrt\sum_{i=1}^td_i^2. A sharp maximal inequality for conditionally
symmetric random variables and for continuous local martingales with values in
R^m, m\ge 1, is also established. Another development in this paper is a
bounded law of the iterated logarithm for general adapted sequences that are
centered at certain truncated conditional expectations and self-normalized by
the square root of the sum of squares. The key ingredient in this development
is a new exponential supermartingale involving \sum_{i=1}^td_i and
\sum_{i=1}^td_i^2.Comment: Published by the Institute of Mathematical Statistics
(http://www.imstat.org) in the Annals of Probability
(http://www.imstat.org/aop/) at http://dx.doi.org/10.1214/00911790400000039
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