26,260 research outputs found
Continuous Crystallization in Hexagonally-Ordered Materials
We demonstrate that the phase transition from columnar-hexagonal liquid
crystal to hexagonal-crystalline solid falls into an unusual universality
class, which in three-dimensional allows for both discontinuous transitions as
well as continuous transitions, characterized by a single set of exponents. We
show by a renormalization group calculation (to first order in ) that the critical exponents of the continuous transition are precisely
those of the XY model, which gives rise to a continuous evolution of elastic
moduli. Although the fixed points of the present model are found to be
identical to the XY model, the elastic compliance to deformations in the plane
of hexagonal order, , is nonetheless shown to critically influence the
crystallization transition, with the continuous transition being driven to
first order by fluctuations as the in plane response grows weaker, .Comment: 4 pages, 2 figures (revised version
Wigner function of noninteracting trapped fermions
We study analytically the Wigner function of
noninteracting fermions trapped in a smooth confining potential in
dimensions. At zero temperature, is constant over a
finite support in the phase space and vanishes outside.
Near the edge of this support, we find a universal scaling behavior of
for large . The associated scaling function is
independent of the precise shape of the potential as well as the spatial
dimension . We further generalize our results to finite temperature .
We show that there exists a low temperature regime where
is an energy scale that depends on and the confining potential , where the Wigner function at the edge again takes a universal scaling
form with a -dependent scaling function. This temperature dependent scaling
function is also independent of the potential as well as the dimension . Our
results generalize to any and the and results
obtained by Bettelheim and Wiegman [Phys. Rev. B , 085102 (2011)].Comment: 16 pages, 4 figure
Soft-Pulse Dynamical Decoupling with Markovian Decoherence
We consider the effect of broadband decoherence on the performance of
refocusing sequences, having in mind applications of dynamical decoupling in
concatenation with quantum error correcting codes as the first stage of
coherence protection. Specifically, we construct cumulant expansions of
effective decoherence operators for a qubit driven by a pulse of a generic
symmetric shape, and for several sequences of - and -pulses. While,
in general, the performance of soft pulses in decoupling sequences in the
presence of Markovian decoherence is worse than that of the ideal
-pulses, it can be substantially improved by shaping.Comment: New version contains minor content clarification
Momentum Driving: which physical processes dominate AGN feedback?
The deposition of mechanical feedback from a supermassive black hole (SMBH)
in an active galactic nucleus (AGN) into the surrounding galaxy occurs via
broad-line winds which must carry mass and radial momentum as well as energy.
The effect can be summarized by the dimensionless parameter
where
(\epslion_w \equiv dot{E}_w/(dot{M_accretion} c^2)) is the efficiency by
which accreted matter is turned into wind energy in the disc surrounding the
central SMBH. The outflowing mass and omentum are proportional to , and
many prior treatments have essentially assumed that . We perform one-
and two-dimensional simulations and find that the growth of the central SMBH is
very sensitive to the inclusion of the mass and momentum driving but is
insensitive to the assumed mechanical efficiency. For example in representative
calculations, the omission of momentum and mass feedback leads to an hundred
fold increase in the mass of the SMBH to over 10^{10} \Msun. When allowance
is made for momentum driving, the final SMBH mass is much lower and the wind
efficiencies which lead to the most observationally acceptable results are
relatively low with .Comment: 10 pages, 8 figures, resubmitted to ApJ, added reference
AGN feedback in an isolated elliptical galaxy: the effect of strong radiative feedback in the kinetic mode
Based on two-dimensional high resolution hydrodynamic numerical simulation,
we study the mechanical and radiative feedback effects from the central AGN on
the cosmological evolution of an isolated elliptical galaxy. Physical processes
such as star formation and supernovae are considered. The inner boundary of the
simulation domain is carefully chosen so that the fiducial Bondi radius is
resolved and the accretion rate of the black hole is determined
self-consistently. In analogy to previous works, we assume that the specific
angular momentum of the galaxy is low. It is well-known that when the accretion
rates are high and low, the central AGNs will be in cold and hot accretion
modes, which correspond to the radiative and kinetic feedback modes,
respectively. The emitted spectrum from the hot accretion flows is harder than
that from the cold accretion flows, which could result in a higher Compton
temperature accompanied by a more efficient radiative heating, according to
previous theoretical works. Such a difference of the Compton temperature
between the two feedback modes, the focus of this study, has been neglected in
previous works. Significant differences in the kinetic feedback mode are found
as a result of the stronger Compton heating and accretion becomes more chaotic.
More importantly, if we constrain models to correctly predict black hole growth
and AGN duty cycle after cosmological evolution, we find that the favored model
parameters are constrained: mechanical feedback efficiency diminishes with
decreasing luminosity (the maximum efficiency being ) and
X-ray Compton temperature increases with decreasing luminosity, although models
with fixed mechanical efficiency and Compton temperature can be found that are
satisfactory as well. We conclude that radiative feedback in the kinetic mode
is much more important than previously thought.Comment: 35 pages, 7 figures, accepted by the Ap
The beginnings of geography teaching and research in the University of Glasgow: the impact of J.W. Gregory
J.W. Gregory arrived in Glasgow from Melbourne in 1904 to take up the post of foundation Professor of Geology in the University of Glasgow. Soon after his arrival in Glasgow he began to push for the setting up of teaching in Geography in Glasgow, which came to pass in 1909 with the appointment of a Lecturer in Geography. This lecturer was based in the Department of Geology in the University's East Quad. Gregory's active promotion of Geography in the University was matched by his extensive writing in the area, in textbooks, journal articles and popular books. His prodigious output across a wide range of subject areas is variably accepted today, with much of his geomorphological work being judged as misguided to varying degrees. His 'social science' publications - in the areas of race, migration, colonisation and economic development of Africa and Australia - espouse a viewpoint that is unacceptable in the twenty-first century. Nonetheless, that viewpoint sits squarely within the social and economic traditions of Gregory's era, and he was clearly a key 'Establishment' figure in natural and social sciences research in the first half of the twentieth century. The establishment of Geography in the University of Glasgow remains enduring testimony of J.W. Gregory's energy, dedication and foresight
Bayesian evidence for two companions orbiting HIP 5158
We present results of a Bayesian analysis of radial velocity (RV) data for
the star HIP 5158, confirming the presence of two companions and also
constraining their orbital parameters. Assuming Keplerian orbits, the
two-companion model is found to be e^{48} times more probable than the
one-planet model, although the orbital parameters of the second companion are
only weakly constrained. The derived orbital periods are 345.6 +/- 2.0 d and
9017.8 +/- 3180.7 d respectively, and the corresponding eccentricities are 0.54
+/- 0.04 and 0.14 +/- 0.10. The limits on planetary mass (m \sin i) and
semimajor axis are (1.44 +/- 0.14 M_{J}, 0.89 +/- 0.01 AU) and (15.04 +/- 10.55
M_{J}, 7.70 +/- 1.88 AU) respectively. Owing to large uncertainty on the mass
of the second companion, we are unable to determine whether it is a planet or a
brown dwarf. The remaining `noise' (stellar jitter) unaccounted for by the
model is 2.28 +/- 0.31 m/s. We also analysed a three-companion model, but found
it to be e^{8} times less probable than the two-companion model.Comment: 5 pages, 4 figures, 3 tables. Added a couple of figures showing the
residuals after one and two companion fits. Accepted for publication in MNRAS
Letter
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