3,966 research outputs found
Study of 'ratio' automatically assembled structures final report, 15 jun. 1963 - 15 jun. 1964
Structural analysis of RATIO automatically assembled panel sections for application to spaceborne paraboloidal antenn
Biostratigraphic and magnetostratigraphic synthesis of the Celebes and Sulu Seas, Leg 124
During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 holes
drilled at five sites in the Celebes and Sulu basins. Paleomagnetic measurements and biostratigraphic studies using
calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, and diatoms were completed and
summarized here. Two Neogene sediment sections recovered in the Sulu Basin yielded excellent core recoveries
and magnetic reversal records, allowing direct magnetobiostratigraphic correlations for the Pliocene and Quaternary
at Site 768 and for the middle Miocene to Quaternary at Site 769. The interpolated ages of biohorizons are not
consistent between sites and only a few of them are in good agreement with previous calibrations. The differences
may be the results of redeposition by turbidity currents and selective dissolution of key fossils
Destroying coherence in high temperature superconductors with current flow
The loss of single-particle coherence going from the superconducting state to
the normal state in underdoped cuprates is a dramatic effect that has yet to be
understood. Here, we address this issue by performing angle resolved
photoemission spectroscopy (ARPES) measurements in the presence of a transport
current. We find that the loss of coherence is associated with the development
of an onset in the resistance, in that well before the midpoint of the
transition is reached, the sharp peaks in the ARPES spectra are completely
suppressed. Since the resistance onset is a signature of phase fluctuations,
this implies that the loss of single-particle coherence is connected with the
loss of long-range phase coherence.Comment: 7 pages, 7 figure
Orang-utans rely on orientation cues and egocentric rules when judging others' perspectives in a competitive food task
Adopting the paradigm of a study conducted with chimpanzees, Pan troglodytes (Melis et al. 2006, Journal of Comparative Psychology, 120, 154–162), we investigated orang-utans', Pongo pygmaeus, understanding of others' visual perspectives. More specifically, we examined whether orang-utans would adjust their behaviour in a way that prevents a human competitor from seeing them steal a piece of food. In the task, subjects had to reach through one of two opposing Plexiglas tunnels in order to retrieve a food reward. Both rewards were also physically accessible to a human competitor sitting opposite the subject. Subjects always had the possibility of reaching one piece of food that was outside the human's line of sight. This was because either the human was oriented to one, but not the other, reward or because one tunnel was covered by an opaque barrier and the other remained transparent. In the situation in which the human was oriented towards one reward, the orang-utans successfully avoided the tunnel that the competitor was facing. If one tunnel was covered, they marginally preferred to reach through the opaque versus the transparent tunnel. However, they did so frequently after initially inspecting the transparent tunnel (then switching to the opaque one). Considering only the subjects' initial inspections, they chose randomly between the opaque and transparent tunnel, indicating that their final decision to reach was probably driven by a more egocentric behavioural rule. Overall the results suggest that orang-utans have a limited understanding of others' perspectives, relying mainly on cues from facial and bodily orientation and egocentric rules when making such judgements
The Fermi surface of Bi2Sr2CaCu2O8
We study the Fermi surface of Bi2Sr2CaCu2O8 (Bi2212) using angle resolved
photoemission (ARPES) with a momentum resolution of ~ 0.01 of the Brillouin
zone. We show that, contrary to recent suggestions, the Fermi surface is a
large hole barrel centered at (pi,pi), independent of the incident photon
energy.Comment: 4 pages (revtex), 4 figures (eps, 2 color
Inelastic electron relaxation rates caused by Spin M/2 Kondo Impurities
We study a spin S=M/2--Kondo system coupled to electrons in an arbitrary
nonequilibrium situation above Kondo temperature. Coupling to hot electrons
leads to an increased inverse lifetime of pseudo particles, related to the
Korringa width. This in turn is responsible for the increased inelastic
relaxation rates of the electronic system. The rates are related to spin--spin
correlation functions which are determined using a projection operator
formalism. The results generalize recent findings for S=1/2--Kondo impurities
which have been used to describe energy relaxation experiments in disordered
mesoscopic wires.Comment: Brief Report, 4 page
Global versus Local Ferromagnetism in a Model for Diluted Magnetic Semiconductors Studied with Monte Carlo Techniques
A model recently introduced for diluted magnetic semiconductors by Berciu and
Bhatt (PRL 87, 107203 (2001)) is studied with a Monte Carlo technique, and the
results are compared to Hartree-Fock calculations. For doping rates close to
the experimentally observed metal-insulator transition, a picture dominated by
ferromagnetic droplets formed below a T* scale emerges. The moments of these
droplets align as the temperature is lowered below a critical value Tc<T*. Our
Monte Carlo investigations provide critical temperatures considerably smaller
than Hartree-Fock predictions. Disorder does not seem to enhance ferromagnetism
substantially. The inhomogeneous droplet state should be strongly susceptible
to changes in doping and external fields.Comment: 4 pages, 4 figure
The change of Fermi surface topology in Bi2Sr2CaCu2O8 with doping
We report the observation of a change in Fermi surface topology of
Bi2Sr2CaCu2O8 with doping. By collecting high statistics ARPES data from
moderately and highly overdoped samples and dividing the data by the Fermi
function, we answer a long standing question about the Fermi surface shape of
Bi2Sr2CaCu2O8 close to the (pi,0) point. For moderately overdoped samples
(Tc=80K) we find that both the bonding and antibonding sheets of the Fermi
surface are hole-like. However for a doping level corresponding to Tc=55K we
find that the antibonding sheet becomes electron-like. This change does not
directly affect the critical temperature and therefore the superconductivity.
However, since similar observations of the change of the topology of the Fermi
surface were observed in LSCO and Bi2Sr2Cu2O6, it appears to be a generic
feature of hole-doped superconductors. Because of bilayer splitting, though,
this doping value is considerably lower than that for the single layer
materials, which again argues that it is unrelated to Tc
Dual character of the electronic structure in YBa2Cu4O8: conduction bands of CuO2 planes and CuO chains
We use microprobe Angle-Resolved Photoemission Spectroscopy (muARPES) to
separately investigate the electronic properties of CuO2 planes and CuO chains
in the high temperature superconductor, YBa2Cu4O8. In the CuO2 planes, a two
dimensional (2D) electronic structure with nearly momentum independent bilayer
splitting is observed. The splitting energy is 150 meV at (pi,0), almost 50%
larger than in Bi2Sr2CaCu2O(8+d) and the electron scattering at the Fermi level
in the bonding band is about 1.5 times stronger than in the antibonding band.
The CuO chains have a quasi one dimensional (1D) electronic structure. We
observe two 1D bands separated by ~ 550meV: a conducting band and an insulating
band with an energy gap of ~ 240meV. We find that the conduction electrons are
well confined within the planes and chains with a non-trivial hybridization.Comment: 4 pages, 4 figure
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