6,092 research outputs found
Quantum size effects on the perpendicular upper critical field in ultra-thin lead films
We report the thickness-dependent (in terms of atomic layers) oscillation
behavior of the perpendicular upper critical field in the
ultra-thin lead films at the reduced temperature (). Distinct
oscillations of the normal-state resistivity as a function of film thickness
have also been observed. Compared with the oscillation, the
shows a considerable large oscillation amplitude and a phase shift. The
oscillatory mean free path caused by quantum size effect plays a role in
oscillation.Comment: 4 pages, 4 figure
QED Renormalization Given in A Mass-Dependent Subtraction and The Renormalization Group Approach
The QED renormalization is restudied by using a mass-dependent subtraction
which is performed at a time-like renormalization point. The subtraction
exactly respects necessary physical and mathematical requirements such as the
gauge symmetry, the Lorentz- invariance and the mathematical convergence.
Therefore, the renormalized results derived in the subtraction scheme are
faithful and have no ambiguity. Especially, it is proved that the solution of
the renormalization group equation satisfied by a renormalized wave function,
propagator or vertex can be fixed by applying the renormalization boundary
condition and, thus, an exact S-matrix element can be expressed in the form as
written in the tree diagram approximation provided that the coupling constant
and the fermion mass are replaced by their effective ones. In the one-loop
approximation, the effective coupling constant and the effective fermion mass
obtained by solving their renormalization group equations are given in rigorous
and explicit expressions which are suitable in the whole range of distance and
exhibit physically reasonable asymptotic behaviors.Comment: 29 pages, 4 figure
Controlling Excitations Inversion of a Cooper Pair Box Interacting with a Nanomechanical Resonator
We investigate the action of time dependent detunings upon the excitation
inversion of a Cooper pair box interacting with a nanomechanical resonator. The
method employs the Jaynes-Cummings model with damping, assuming different decay
rates of the Cooper pair box and various fixed and t-dependent detunings. It is
shown that while the presence of damping plus constant detunings destroy the
collapse/revival effects, convenient choices of time dependent detunings allow
one to reconstruct such events in a perfect way. It is also shown that the mean
excitation of the nanomechanical resonator is more robust against damping of
the Cooper pair box for convenient values of t-dependent detunings.Comment: 11 pages, 5 figure
Optical Monitoring of BL Lacertae Object OJ 287: a 40-Day Period?
We present the results of our optical monitoring of the BL Lacertae object OJ
287 during the first half of 2005. The source did not show large-amplitude
variations during this period and was in a relatively quiescent state. A
possible period of 40 days was derived from its light curves in three BATC
wavebands. A bluer-when-brighter chromatism was discovered, which is different
from the extremely stable color during the outburst in 1994--96. The different
color behaviors imply different variation mechanisms in the two states. We then
re-visited the optical data on OJ 287 from the OJ-94 project and found as well
a probable period of 40 days in its optical variability during the late-1994
outburst. The results suggest that two components contribute to the variability
of OJ 287 during its outburst state. The first component is the normal {\sl
blazar} variation. This component has an amplitude similar to that of the
quiescent state and also may share a similar periodicity. The second component
can be taken as a `low-frequency modulation' to the first component. It may be
induced by the interaction of the assumed binary black holes in the center of
this object. The 40-day period may be related to the helical structure of the
magnetic field at the base of the jet, or to the orbital motion close to the
central primary black hole.Comment: 31 pages, 8 figures, accepted by A
The Resolved Stellar Halo of NGC 253
We have obtained Magellan/IMACS and HST/ACS imaging data that resolve red
giant branch stars in the stellar halo of the starburst galaxy NGC 253. The HST
data cover a small area, and allow us to accurately interpret the ground-based
data, which cover 30% of the halo to a distance of 30 kpc, allowing us to make
detailed quantitative measurements of the global properties and structure of a
stellar halo outside of the Local Group. The geometry of the halo is
significantly flattened in the same sense as the disk, with a projected axis
ratio of b/a ~ 0.35 +/- 0.1. The total stellar mass of the halo is estimated to
be M_halo ~ 2.5 +/- 1.5 x 10^9 M_sun, or 6% of the total stellar mass of the
galaxy, and has a projected radial dependence that follows a power law of index
-2.8 +/- 0.6, corresponding to a three-dimensional power law index of ~ -4. The
total luminosity and profile shape that we measure for NGC 253 are somewhat
larger and steeper than the equivalent values for the Milky Way and M31, but
are well within the scatter of model predictions for the properties of stellar
halos built up in a cosmological context. Structure within the halo is seen at
a variety of scales: there is small kpc-scale density variation and a large
shelf-like feature near the middle of the field. The techniques that have been
developed will be essential for quantitatively comparing our upcoming larger
sample of observed stellar halos to models of halo formation.Comment: ApJ, in press. Version with full resolution figures available at
http://www.astro.lsa.umich.edu/~jbailin/papers/bailin_n253halo.pd
The Structure of the Sagittarius Stellar Stream as Traced by Blue Horizontal Branch Stars
We use a sample of blue horizontal branch (BHB) stars from the Sloan Digital
Sky Survey Data Release 7 to explore the structure of the tidal tails from the
Sagittarius Dwarf Galaxy. We use a method yielding BHB star candidates with up
to ~70% purity from photometry alone. The resulting sample has a distance
precision of roughly 5% and can probe distances in excess of 100 kpc. Using
this sample, we identify a possible extension to the trailing arm at distances
of 60-80 kpc from the Sun with an estimated significance of at least 3.8 sigma.
Current models predict that a distant `returning' segment of the debris stream
should exist, but place it substantially closer to the Sun where no debris is
observed in our data. Exploiting the distance precision of our tracers, we
estimate the mean line-of-sight thickness of the leading arm to be ~3 kpc, and
show that the two `bifurcated' branches of the debris stream differ by only 1-2
kpc in distance. With a spectroscopic very pure BHB star subsample, we estimate
the velocity dispersion in the leading arm, 37 km s^-1, which is in reasonable
agreement with models of Sgr disruption. We finally present a sample of
high-probability Sgr BHB stars in the leading arm of Sgr, selected to have
distances and velocities consistent with Sgr membership, to allow further
study.Comment: Astrophysical Journal, in press. 17 pages, 15 figure
Quantifying Kinematic Substructure in the Milky Way's Stellar Halo
We present and analyze the positions, distances, and radial velocities for
over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn
from SDSS DR8. We search for position-velocity substructure in these data, a
signature of the hierarchical assembly of the stellar halo. Using a cumulative
"close pair distribution" (CPD) as a statistic in the 4-dimensional space of
sky position, distance, and velocity, we quantify the presence of
position-velocity substructure at high statistical significance among the BHB
stars: pairs of BHB stars that are close in position on the sky tend to have
more similar distances and radial velocities compared to a random sampling of
these overall distributions. We make analogous mock-observations of 11
numerical halo formation simulations, in which the stellar halo is entirely
composed of disrupted satellite debris, and find a level of substructure
comparable to that seen in the actually observed BHB star sample. This result
quantitatively confirms the hierarchical build-up of the stellar halo through a
signature in phase (position-velocity) space. In detail, the structure present
in the BHB stars is somewhat less prominent than that seen in most simulated
halos, quite possibly because BHB stars represent an older sub-population. BHB
stars located beyond 20 kpc from the Galactic center exhibit stronger
substructure than at kpc.Comment: 29 page, 10 figures, 1 table; accepted by APJ; for related article by
another group see arXiv:1011.192
Bihamiltonian Cohomologies and Integrable Hierarchies I: A Special Case
We present some general results on properties of the bihamiltonian
cohomologies associated to bihamiltonian structures of hydrodynamic type, and
compute the third cohomology for the bihamiltonian structure of the
dispersionless KdV hierarchy. The result of the computation enables us to prove
the existence of bihamiltonian deformations of the dispersionless KdV hierarchy
starting from any of its infinitesimal deformations.Comment: 43 pages. V2: the accepted version, to appear in Comm. Math. Phy
Evidence for phase formation in potassium intercalated 1,2;8,9-dibenzopentacene
We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under
vacuum conditions. The evolution of the electronic excitation spectra upon
potassium addition as measured using electron energy-loss spectroscopy clearly
indicate the formation of particular doped phases with compositions
Kdibenzopentacene ( = 1,2,3). Moreover, the stability of these phases as
a function of temperature has been explored. Finally, the electronic excitation
spectra also give insight into the electronic ground state of the potassium
doped 1,2;8,9-dibenzopentacene films.Comment: 6 pages, 5 figures. arXiv admin note: text overlap with
arXiv:1201.200
Stellar Population Variations in the Milky Way's Stellar Halo
If the stellar halos of disk galaxies are built up from the disruption of
dwarf galaxies, models predict highly structured variations in the stellar
populations within these halos. We test this prediction by studying the ratio
of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor
populations) to main-sequence turn-off stars (MSTO stars; a feature of all
populations) in the stellar halo of the Milky Way using data from the Sloan
Digital Sky Survey. We develop and apply an improved technique to select BHB
stars using ugr color information alone, yielding a sample of ~9000 g<18
candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across
~1/4 of the sky at the distance resolution permitted by the absolute magnitude
distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in
the stellar halo. Previously identified, stream-like halo structures have
distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo
features, e.g., the low-latitude structure, appear to be almost completely
devoid of BHB stars, whereas other structures appear to be rich in BHB stars.
The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio
along its extent, which we interpret in terms of population gradients within
the progenitor dwarf galaxy. Our detection of coherent stellar population
variations between different stellar halo substructures provides yet more
support to cosmologically motivated models for stellar halo growth.Comment: Astronomical Journal, in press. 10 pages, 5 color figures. Much
better printed in colo
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