8,033 research outputs found
Motion of a sphere through an aging system
We have investigated the drag on a sphere falling through a clay suspension
that has a yield stress and exhibits rheological aging. The drag force
increases with both speed and the rest time between preparation of the system
and the start of the experiment, but there exists a nonzero minimum speed below
which steady motion is not possible. We find that only a very thin layer of
material around the sphere is fluidized when it moves, while the rest of
suspension is deformed elastically. This is in marked contrast to what is found
for yield-stress fluids that do not age.Comment: latex, 4 figure
Quantum Monte Carlo simulation of overpressurized liquid 4He
A diffusion Monte Carlo simulation of superfluid He at zero temperature
and pressures up to 275 bar is presented. Increasing the pressure beyond
freezing ( 25 bar), the liquid enters the overpressurized phase in a
metastable state. In this regime, we report results of the equation of state
and the pressure dependence of the static structure factor, the condensate
fraction, and the excited-state energy corresponding to the roton. Along this
large pressure range, both the condensate fraction and the roton energy
decrease but do not become zero. The roton energies obtained are compared with
recent experimental data in the overpressurized regime.Comment: 5 pages, accepted for publication in Phys. Rev. Let
Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs
In this paper we analyze the properties of a dc SQUID when the London
penetration depth \lambda is larger than the superconducting film thickness d.
We present equations that govern the static behavior for arbitrary values of
\Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The
SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic
flux through a contour surrounding the central hole plus a term proportional to
the line integral of the current density around this contour. While it is well
known that the SQUID inductance depends upon \Lambda, we show here that the
focusing of magnetic flux from applied fields and vortex-generated fields into
the central hole of the SQUID also depends upon \Lambda. We apply this
formalism to the simplest case of a linear SQUID of width 2w, consisting of a
coplanar pair of long superconducting strips of separation 2a, connected by two
small Josephson junctions to a superconducting current-input lead at one end
and by a superconducting lead at the other end. The central region of this
SQUID shares many properties with a superconducting coplanar stripline. We
calculate magnetic-field and current-density profiles, the inductance
(including both geometric and kinetic inductances), magnetic moments, and the
effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions
being to denote the effective flux by \Phi rather than
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
LOFAR: A new radio telescope for low frequency radio observations: Science and project status
LOFAR, the Low Frequency Array, is a large radio telescope consisting about
100 soccer field sized antenna stations spread over a region of 400 km in
diameter. It will operate in the frequency range from ~10 to 240 MHz, with a
resolution at 240 MHz of better than an arcsecond. Its superb sensitivity will
allow for a broad range of astrophysical studies. In this contribution we first
discuss four major areas of astrophysical research in which LOFAR will
undoubtedly make important contributions: reionisation, distant galaxies and
AGNs, transient radio sources and cosmic rays. Subsequently, we will discuss
the technical concept of the instrument and the status of the LOFAR projectComment: 8 pages, 2 figures, to appear in the proceedings of the XXI Texas
Symposium on Relativistic Astrophysics held on December 9--13 2002, in
Florence, Ital
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