565 research outputs found
Direct determination of the collective pinning radius in high temperature superconductors
We study finite-size effects at the onset of the irreversible magnetic
behaviour of micron-sized BiSrCaCuO single crystals by
using silicon micro-oscillators. We find an irreversibility line appearing well
below the thermodynamic Bragg-glass melting line at a magnetic field which
increases both with increasing the sample radius and with decreasing the
temperature. We show that this size-dependent irreversibility line can be
identified with the crossover between the Larkin and the random manifold
regimes of the vortex lattice transverse roughness. Our method allows to
determine the three-dimensional weak collective pinning Larkin radius in a {\it
direct way}.Comment: 4 pages, 3 fig
High resolution imaging of the early-type galaxy NGC 1380: an insight into the nature of extended extragalactic star clusters
NGC 1380 is a lenticular galaxy located near the centre of the Fornax Cluster
northeast of NGC 1399. The globular cluster system of this galaxy was
previously studied only from the ground. Recent studies of similar early-type
galaxies, specially lenticular ones, reveal the existence of star clusters that
apparently break up the traditional open/globular cluster dichotomy. With
higher quality photometry from HST/WFPC2 we study the star clusters in NGC
1380, measuring their magnitudes, colours, sizes and projected distances from
the centre of the galaxy. We used deep archival HST/WFPC2 in the B and V bands.
We built colour magnitude diagrams from which we selected a sample of cluster
candidates. We also analysed their colour distribution and measured their
sizes. Based on their location in the luminosity-size diagram we estimated
probabilities of them being typical globular clusters as those found in the
Galaxy. A total of about 570 cluster candidates were found down to V=26.5. We
measured sizes for approximately 200 of them. The observed colour distribution
has three apparent peaks. Likewise for the size distribution. We identified the
smaller population as being mainly typical globular clusters, while the more
extended objects have small probabilities of being such objects. Different
correlations between absolute magnitudes, sizes, colours and location were
inferred for these cluster sub-populations. Most extended clusters (Reff > 4
pc) share similar properties to the diffuse star clusters reported to inhabit
luminous early-type galaxies in the Virgo galaxy cluster such as being of low
surface brightness and fainter than MV ~ -8. We also report on a small group of
(Reff ~ 10 pc), -8< MV < -6, red clusters located near the centre of NGC 1380,
which may be interpreted as faint fuzzies.Comment: accepted for publication in A&
Vortex matter freezing in BiSrCaCuO samples with a very dense distribution of columnar defects
We show that the dynamical freezing of vortex structures nucleated at diluted
densities in BiSrCaCuO samples with a dense
distribution of columnar defects, with
\,kG, results in configurations with liquid-like correlations. We
propose a freezing model considering a relaxation dynamics dominated by
double-kink excitations driven by the local stresses obtained directly from
experimental images. With this model we estimate the relaxation barrier and the
freezing temperature. We argue that the low-field frozen vortex structures
nucleated in a dense distribution of columnar defects thus correspond to an
out-of-equilibrium non-entangled liquid with strongly reduced mobility rather
than to a snapshot of a metastable state with divergent activation barriers as
for instance expected for the Bose-glass phase at equilibrium.Comment: 12 pages, 7 figure
Field galaxies at intermediate redshift (0.2 < z < 0.8) in the direction of the galaxy cluster LCDCS-S001
We present spectroscopic and photometric analysis for eight field galaxies in
the direction of the galaxy cluster LCDCS-S001. The spectra were obtained with
the GMOS instrument in the Gemini South Observatory. The objects were selected
in an i' band image and the multi-object spectroscopic observations were
centered at 7500 A. For the galaxies ID 440 and ID 461 we have determined
redshifts of z=0.7464 and z=0.7465, respectively. For the other six galaxies we
have confirmed the redshift calculated by Rembold & Pastoriza (2006). The
redshifts of the field galaxies are in the range of 0.2201 < z < 0.7784. We
determined the blue and visual luminosities and they are brighter than
M_B=-18.64. The galaxies ID 180, ID 266, ID 461 follow the Faber-Jackson
relation of the Coma and Virgo early-type galaxies, and therefore do not
present a brightening of the B luminosity as observed in galaxies at higher
redshifts. The stellar velocity dispersion was measured for five galaxies and
estimated to be in the range of 200 < sigma < 346 km/s. Lick indices were
measured and used to determine the stellar population properties of galaxies ID
120 and ID 146, by means of spectral synthesis. The first galaxy, ID 120,
presents in its spectrum absorption and emission lines, and we have found that
the main contribution in the flux at lambda 5870 A is of a 0.1 Gyr stellar
population of solar metallicity. For ID 146, the dominant flux contribution at
lambda 4200 A, is provided by a stellar population of 10 Gyr of subsolar
metallicity. From stellar population synthesis we estimated reddening values of
E(B-V)=0.90 and E(B-V)=0.82 for ID 120 and ID 146, respectively. According to
classical diagnostic diagrams the emission lines present in the spectrum of ID
120 indicate that it is a starburst galaxy.Comment: Accepted for publication in Monthly Notices of the Royal
Astronomical. 6 Figures and 4 table
Vortex phase transformations probed by the local ac response of Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} single crystals with various doping
The linear ac response of the vortex system is measured locally in Bi-2212
single crystals at various doping, using a miniature two-coil mutual-inductance
technique. It was found that a step-like change in the local ac response takes
place exactly at the first-order transition (FOT) temperature T_{FOT}(H)
determined by a global dc magnetization measurement. The T_{FOT}(H) line in the
H-T phase diagram becomes steeper with increasing doping. In the higher-field
region where the FOT is not observed, the local ac response still shows a
broadened but distinct feature, which can be interpreted to mark the growth of
a short-range order in the vortex system.Comment: 4 pages, including 5 eps figure
Survey of the ISM in Early-Type Galaxies. IV. The Hot Dust Component
We present mid-IR photometric properties for a sample of 28 early-type
galaxies observed at 6.75, 9.63 and 15 um with the ISOCAM instrument on board
the ISO satellite. We find total mid-IR luminosities in the range 3-48x10^8
L_sun. The spectral energy distribution (SED) of the galaxies were derived
using the mid-IR data together with previously published UV, optical and
near-IR data. These SEDs clearly show a mid-IR emission coming from dust heated
at T ~ 260 K. Dust grains properties are inferred from the mid-IR colors. The
masses of the hot dust component are in the range 10-400 M_sun. The
relationship between the masses derived from mid-IR observations and those
derived from visual extinction are discussed. The possible common heating
source for the gas and dust is investigated through the correlations between Ha
and mid-IR luminosities.Comment: 10 pages, LaTeX (aa.cls), 11 figures (f. 2-4 are colour plates).
Accepted for publication in Astronomy & Astrophysic
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
Patterned Irradiation of YBa_2Cu_3O_(7-x) Thin Films
We present a new experiment on YBa_2Cu_3O_{7-x} (YBCO) thin films using
spatially resolved heavy ion irradiation. Structures consisting of a periodic
array of strong and weak pinning channels were created with the help of metal
masks. The channels formed an angle of +/-45 Deg with respect to the symmetry
axis of the photolithographically patterned structures. Investigations of the
anisotropic transport properties of these structures were performed. We found
striking resemblance to guided vortex motion as it was observed in YBCO single
crystals containing an array of unidirected twin boundaries. The use of two
additional test bridges allowed to determine in parallel the resistivities of
the irradiated and unirradiated parts as well as the respective current-voltage
characteristics. These measurements provided the input parameters for a
numerical simulation of the potential distribution of the Hall patterning. In
contrast to the unidirected twin boundaries in our experiment both strong and
weak pinning regions are spatially extended. The interfaces between
unirradiated and irradiated regions therefore form a Bose-glass contact. The
experimentally observed magnetic field dependence of the transverse voltage
vanishes faster than expected from the numerical simulation and we interpret
this as a hydrodynamical interaction between a Bose-glass phase and a vortex
liquid.Comment: 7 pages, 8 Eps figures included. Submitted to PR
Kinematics of Interstellar Gas in Nearby UV-Selected Galaxies Measured with HST/STIS Spectroscopy
We measure Doppler shifts of interstellar absorption lines in HST/STIS
spectra of individual star clusters in nearby UV-selected galaxies. Values for
systemic velocities, which are needed to quantify outflow speeds, are taken
from the literature, and verified with stellar lines. We detect outflowing gas
in eight of 17 galaxies via low-ionization lines (e.g., CII, SiII, AlII), which
trace cold and/or warm gas. The starbursts in our sample are intermediate in
luminosity (and mass) to dwarf galaxies and luminous infrared galaxies (LIRGs),
and we confirm that their outflow speeds (ranging from -100 km/s to nearly -520
km/s with an accuracy of ~80 km/s) are intermediate to those previously
measured in dwarf starbursts and LIRGs. We do not detect the outflow in
high-ionization lines (such as CIV or SiIV); higher quality data will be needed
to empirically establish how velocities vary with the ionization state of the
outflow. We do verify that the low-ionization UV lines and optical NaI doublet
give roughly consistent outflow velocities solidifying an important link
between studies of galactic winds at low and high redshift. To obtain higher
signal-to-noise, we create a local average composite spectrum, and compare it
to the high-z Lyman Break composite spectrum. Surprisingly, the low-ionization
lines show similar outflow velocities in the two samples. We attribute this to
a combination of weighting towards higher luminosities in the local composite,
as well as both samples being on average brighter than the ``turnover''
luminosity in the v-SFR relation.Comment: 41 pages, 14 figures, accepted for publication in The Astrophysical
Journa
Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays
We study the dependence of the transport properties of square Josephson
Junctions arrays with the direction of the applied dc current, both
experimentally and numerically. We present computational simulations of
current-voltage curves at finite temperatures for a single vortex in the array
(), and experimental measurements in
arrays under a low magnetic field corresponding to . We find that
the transverse voltage vanishes only in the directions of maximum symmetry of
the square lattice: the [10] and [01] direction (parallel bias) and the [11]
direction (diagonal bias). For orientations different than the symmetry
directions, we find a finite transverse voltage which depends strongly on the
angle of the current. We find that vortex motion is pinned in the [10]
direction (), meaning that the voltage response is insensitive to small
changes in the orientation of the current near . We call this
phenomenon orientational pinning. This leads to a finite transverse critical
current for a bias at and to a transverse voltage for a bias at
. On the other hand, for diagonal bias in the [11] direction the
behavior is highly unstable against small variations of , leading to a
rapid change from zero transverse voltage to a large transverse voltage within
a few degrees. This last behavior is in good agreement with our measurements in
arrays with a quasi-diagonal current drive.Comment: 9 pages, 9 figure
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