178,453 research outputs found
Effective squirmer models for self-phoretic chemically active spherical colloids
Various aspects of self-motility of chemically active colloids in Newtonian
fluids can be captured by simple models for their chemical activity plus a
phoretic slip hydrodynamic boundary condition on their surface. For particles
of simple shapes (e.g., spheres) -- as employed in many experimental studies --
which move at very low Reynolds numbers in an unbounded fluid, such models of
chemically active particles effectively map onto the well studied so-called
hydrodynamic squirmers [S. Michelin and E. Lauga, J. Fluid Mech. \textbf{747},
572 (2014)]. Accordingly, intuitively appealing analogies of
"pusher/puller/neutral" squirmers arise naturally. Within the framework of
self-diffusiophoresis we illustrate the above mentioned mapping and the
corresponding flows in an unbounded fluid for a number of choices of the
activity function (i.e., the spatial distribution and the type of chemical
reactions across the surface of the particle). We use the central collision of
two active particles as a simple, paradigmatic case for demonstrating that in
the presence of other particles or boundaries the behavior of chemically active
colloids may be \textit{qualitatively} different, even in the far field, from
the one exhibited by the corresponding "effective squirmer", obtained from the
mapping in an unbounded fluid. This emphasizes that understanding the
collective behavior and the dynamics under geometrical confinement of
chemically active particles necessarily requires to explicitly account for the
dependence of the hydrodynamic interactions on the distribution of chemical
species resulting from the activity of the particles.Comment: 26 pages, 11 figure
Molecular hydrogen in the galaxy and galactic gamma rays
Recent surveys of 2.6 mm CO emission and 100 MeV gamma-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H2 in the inner galaxy. Both the gamma ray and CO data suggest a prominent ring or arm consisting of cool clouds of H2 at a galactocentric distance of approximately 5 kpc with a mean density of approximately 4 atoms/cu cm. The importance of H2 in understanding galactic gamma ray observations is also reflected in the correlation of galactic latitude distribution of gamma rays and dense dust clouds. A detailed calculation of the gamma ray flux distribution in the 0 deg to 180 deg range using the CO data to obtain the average distribution of molecular clouds in the galaxy shows that most of the enhancement in the inner galaxy is due to pion-decay radiation and the 5 kpc ring plays a major role. Detailed agreement with the gamma ray data is obtained with the additional inclusion of contributions from bremsstrahlung and Compton radiation of secondary electrons and Compton radiation from the intense radiation field near the galactic center
Constraining and Dark Energy with Gamma-Ray Bursts
An relationship with a small
scatter for current -ray burst (GRB) data was recently reported, where
is the beaming-corrected -ray energy and
is the peak energy in the local observer frame. By considering this
relationship for a sample of 12 GRBs with known redshift, peak energy, and
break time of afterglow light curves, we constrain the mass density of the
universe and the nature of dark energy. We find that the mass density
(at the confident level) for a flat
universe with a cosmological constant, and the parameter of an assumed
static dark-energy equation of state ().
Our results are consistent with those from type Ia supernovae. A larger sample
established by the upcoming {\em Swift} satellite is expected to provide
further constraints.Comment: 8 pages including 4 figures, to appear in ApJ Letters, typos
correcte
Fibre bundle formulation of relativistic quantum mechanics. I. Time-dependent approach
We propose a new fibre bundle formulation of the mathematical base of
relativistic quantum mechanics. At the present stage the bundle form of the
theory is equivalent to its conventional one, but it admits new types of
generalizations in different directions.
In the present first part of our investigation we consider the time-dependent
or Hamiltonian approach to bundle description of relativistic quantum
mechanics. In it the wavefunctions are replaced by (state) liftings of paths or
sections along paths of a suitably chosen vector bundle over space-time whose
(standard) fibre is the space of the wavefunctions. Now the quantum evolution
is described as a linear transportation (by means of the evolution transport
along paths in the space-time) of the state liftings/sections in the (total)
bundle space. The equations of these transportations turn to be the bundle
versions of the corresponding relativistic wave equations.Comment: 16 standard LaTeX pages. The packages AMS-LaTeX and amsfonts are
required. The paper continuous the application of fibre bundle formalism to
quantum physics began in the series of works quant-ph/9803083,
quant-ph/9803084, quant-ph/9804062, quant-ph/9806046, quant-ph/9901039,
quant-ph/9902068, and quant-ph/0004041. For related papers, view
http://theo.inrne.bas.bg/~bozho
NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBaCuO
Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned
Ortho-II YBaCuO (YBCO6.5) single crystals reveal unusual and
remarkable properties. The chain Cu resonance broadens significantly, but
gradually, on cooling from room temperature. The lineshape and its temperature
dependence are substantially different from that of a conventional spin/charge
density wave (S/CDW) phase transition. Instead, the line broadening is
attributed to small amplitude static spin and charge density oscillations with
spatially varying amplitudes connected with the ends of the finite length
chains. The influence of this CuO chain phenomenon is also clearly manifested
in the plane Cu NMR.Comment: 4 pages, 3 figures, refereed articl
Semimetal to semimetal charge density wave transition in 1T-TiSe
We report an infrared study on 1-TiSe, the parent compound of the
newly discovered superconductor CuTiSe. Previous studies of this
compound have not conclusively resolved whether it is a semimetal or a
semiconductor: information that is important in determining the origin of its
unconventional CDW transition. Here we present optical spectroscopy results
that clearly reveal that the compound is metallic in both the high-temperature
normal phase and the low-temperature CDW phase. The carrier scattering rate is
dramatically different in the normal and CDW phases and the carrier density is
found to change with temperature. We conclude that the observed properties can
be explained within the scenario of an Overhauser-type CDW mechanism.Comment: 4 pages, 4 page
Determination of the magnetic anisotropy axes of single-molecule magnets
Simple methods are presented allowing the determination of the magnetic
anisotropy axes of a crystal of a single-molecule magnet (SMM). These methods
are used to determine an upper bound of the easy axis tilts in a standard
Mn12-Ac crystal. The values obtained in the present study are significately
smaller than those reported in recent high frequency electron paramagnetic
resonance (HF-EPR) studies which suggest distributions of hard-axes tilts.Comment: 10 pages, 6 figure
An All-Cryogenic THz Transmission Spectrometer
This paper describes a THz transmission spectrometer for the spectral range
of 2-65 cm^-1 (100 GHz to 2 THz) with a spectral resolution of at least 1.8
cm^-1 (50 GHz) where the source, sample, and detector are all fully contained
in a cryogenic environment. Cyclotron emission from a two-dimensional electron
gas heated with an electrical current serves as a magnetic field tunable
source. The spectrometer is demonstrated at 4.2 K by measuring the resonant
cyclotron absorption of a second two dimensional electron gas. Unique aspects
of the spectrometer are that 1) an ultra-broadband detector is used and 2) the
emitter is run quasi-continuously with a chopping frequency of only 1 Hz. Since
optical coupling to room temperature components is not necessary, this
technique is compatible with ultra-low temperature (sub 100 mK) operation.Comment: 7 pages, 5 figures. Author affiliation and funding acknowledgements
clarifie
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