11,630 research outputs found
Far-infrared measurements of oxygen-doped polycrystalline La2CuO4.0315 superconductor under slow-cooled and fast-cooled conditions
We have studied the far-infrared (far-IR) charge dynamics of an equilibrated
pure oxygen doped La2CuO4+0.0315 under slow-cooled and fast-cooled conditions.
The superconducting transition temperature (Tc) for the slow-cooled and that
for the fast-cooled processes were respectively found to be close to the two
intrinsic Tc's: One at 30 K and the other at 15 K. Direct comparison with our
previous results and other far-IR and Raman studies on single crystalline
La2-xSrxCuO4, we conclude that the topology of the pristine electronic phases
that are responsible for the two intrinsic Tc's is holes arranged into
two-dimensional (2D) square lattices.Comment: Submitted to PR
Scientific basis for safely shutting in the Macondo Well after the April 20, 2010 Deepwater Horizon blowout
As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic pro- files, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey
We study the evolution of the number of close companions of similar
luminosities per galaxy (Nc) by choosing a volume-limited subset of the
photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The
sample contains over 157,000 objects with a moderate redshift range of 0.25 < z
< 0.8 and absolute magnitude in Rc (M_Rc) < -20. This is the largest sample
used for pair evolution analysis, providing data over 9 redshift bins with
about 17,500 galaxies in each. After applying incompleteness and projection
corrections, Nc shows a clear evolution with redshift. The Nc value for the
whole sample grows with redshift as (1+z)^m, where m = 2.83 +/- 0.33 in good
agreement with N-body simulations in a LCDM cosmology. We also separate the
sample into two different absolute magnitude bins: -25 < M_Rc < -21 and -21 <
M_Rc < -20, and find that the brighter the absolute magnitude, the smaller the
m value. Furthermore, we study the evolution of the pair fraction for different
projected separation bins and different luminosities. We find that the m value
becomes smaller for larger separation, and the pair fraction for the fainter
luminosity bin has stronger evolution. We derive the major merger remnant
fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 < M_Rc
< -20 have undergone major mergers since z = 0.8.Comment: ApJ, in pres
Atomic Interferometer with Amplitude Gratings of Light and its Applications to Atom Based Tests of the Equivalence Principle
We have developed a matter wave interferometer based on the diffraction of
atoms from effective absorption gratings of light. In a setup with cold
rubidium atoms in an atomic fountain the interferometer has been used to carry
out tests of the equivalence principle on an atomic basis. The gravitational
acceleration of the two isotopes 85Rb and 87Rb was compared, yielding a
difference Dg/g =(1.2 +-1.7)x10^{-7}. We also perform a differential free fall
measurement of atoms in two different hyperfine states, and obtained a result
of Dg/g =(0.4 +-1.2)x10^{-7}.Comment: 4 Pages, 4 figures, accepted for Physical Review Letter
Determining the crystal-field ground state in rare earth Heavy Fermion materials using soft-x-ray absorption spectroscopy
We infer that soft-x-ray absorption spectroscopy is a versatile method for
the determination of the crystal-field ground state symmetry of rare earth
Heavy Fermion systems, complementing neutron scattering. Using realistic and
universal parameters, we provide a theoretical mapping between the polarization
dependence of Ce spectra and the charge distribution of the Ce
states. The experimental resolution can be orders of magnitude larger than the
crystal field splitting itself. To demonstrate the experimental
feasibility of the method, we investigated CePdSi, thereby settling an
existing disagreement about its crystal-field ground state
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
Three dimensional topological insulators embody a newly discovered state of
matter characterized by conducting spin-momentum locked surface states that
span the bulk band gap as demonstrated via spin-resolved ARPES measurements .
This highly unusual surface environment provides a rich ground for the
discovery of novel physical phenomena. Here we present the first controlled
study of the topological insulator surfaces under strong Coulomb, magnetic and
disorder perturbations. We have used interaction of iron, with a large Coulomb
state and significant magnetic moment as a probe to \textit{systematically test
the robustness} of the topological surface states of the model topological
insulator BiSe. We observe that strong perturbation leads to the
creation of odd multiples of Dirac fermions and that magnetic interactions
break time reversal symmetry in the presence of band hybridization. We also
present a theoretical model to account for the altered surface of BiSe.
Taken collectively, these results are a critical guide in manipulating
topological surfaces for probing fundamental physics or developing device
applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with
arXiv:1009.621
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