271 research outputs found
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In search of Early Bronze Age potters at Tell es-Safi/Gath: A new perspective on vessel manufacture for discriminating chaines operatoires
Using acoustic waves to induce high-frequency current oscillations in superlattices
We show that GHz acoustic waves in semiconductor superlattices can induce THz
electron dynamics that depend critically on the wave amplitude. Below a
threshold amplitude, the acoustic wave drags electrons through the superlattice
with a peak drift velocity overshooting that produced by a static electric
field. In this regime, single electrons perform drifting orbits with THz
frequency components. When the wave amplitude exceeds the critical threshold,
an abrupt onset of Bloch-like oscillations causes negative differential
velocity. The acoustic wave also affects the collective behavior of the
electrons by causing the formation of localised electron accumulation and
depletion regions, which propagate through the superlattice, thereby producing
self-sustained current oscillations even for very small wave amplitudes. We
show that the underlying single-electron dynamics, in particular the transition
between the acoustic wave dragging and Bloch oscillation regimes, strongly
influence the spatial distribution of the electrons and the form of the current
oscillations. In particular, the amplitude of the current oscillations depends
non-monotonically on the strength of the acoustic wave, reflecting the
variation of the single-electron drift velocity.Comment: 10 pages, 8 figure
Recommended from our members
Trichinella Surveillance in Black Bears (Ursus Americanus) from Oregon, USA
We used serology and muscle digestion to test black bears (Ursus americanus) from western Oregon, USA, for Trichinella. Results indicate black bears in Oregon are not part of a sylvatic cycle for Trichinella, and risk of human exposure to Trichinella larvae from eating black bear meat from Oregon appears low
The most creative organization in the world? The BBC, 'creativity' and managerial style
The managerial styles of two BBC directors-general, John Birt and Greg Dyke, have often been contrasted but not so far analysed from the perspective of their different views of 'creative management'. This article first addresses the orthodox reading of 'Birtism'; second, it locates Dyke's 'creative' turn in the wider context of fashionable neo-management theory and UK government creative industries policy; third, it details Dyke's drive to change the BBC's culture; and finally, it concludes with some reflections on the uncertainties inherent in managing a creative organisation
Carbon clusters near the crossover to fullerene stability
The thermodynamic stability of structural isomers of ,
, and , including
fullerenes, is studied using density functional and quantum Monte Carlo
methods. The energetic ordering of the different isomers depends sensitively on
the treatment of electron correlation. Fixed-node diffusion quantum Monte Carlo
calculations predict that a isomer is the smallest stable
graphitic fragment and that the smallest stable fullerenes are the
and clusters with and
symmetry, respectively. These results support proposals that a
solid could be synthesized by cluster deposition.Comment: 4 pages, includes 4 figures. For additional graphics, online paper
and related information see http://www.tcm.phy.cam.ac.uk/~prck
L-Edge Spectroscopy of Dilute, Radiation-Sensitive Systems Using a Transition-Edge-Sensor Array
We present X-ray absorption spectroscopy and resonant inelastic X-ray
scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous
ferricyanide. These measurements demonstrate the ability of high-throughput
transition-edge-sensor (TES) spectrometers to access the rich soft X-ray
(100-2000eV) spectroscopy regime for dilute and radiation-sensitive samples.
Our low-concentration data are in agreement with high-concentration
measurements recorded by conventional grating-based spectrometers. These
results show that soft X-ray RIXS spectroscopy acquired by high-throughput TES
spectrometers can be used to study the local electronic structure of dilute
metal-centered complexes relevant to biology, chemistry and catalysis. In
particular, TES spectrometers have a unique ability to characterize frozen
solutions of radiation- and temperature-sensitive samples.Comment: 19 pages, 4 figure
Results from the Atacama B-mode Search (ABS) Experiment
The Atacama B-mode Search (ABS) is an experiment designed to measure cosmic
microwave background (CMB) polarization at large angular scales (). It
operated from the ACT site at 5190~m elevation in northern Chile at 145 GHz
with a net sensitivity (NEQ) of 41 K. It employed an
ambient-temperature sapphire half-wave plate rotating at 2.55 Hz to modulate
the incident polarization signal and reduce systematic effects. We report here
on the analysis of data from a 2400 deg patch of sky centered at
declination and right ascension . We perform a blind
analysis. After unblinding, we find agreement with the Planck TE and EE
measurements on the same region of sky. We marginally detect polarized dust
emission and give an upper limit on the tensor-to-scalar ratio of (95%
cl) with the equivalent of 100 on-sky days of observation. We also present a
new measurement of the polarization of Tau A and introduce new methods
associated with HWP-based observations.Comment: 38 pages, 11 figure
Evidence for dark energy from the cosmic microwave background alone using the Atacama Cosmology Telescope lensing measurements
For the first time, measurements of the cosmic microwave background radiation
(CMB) alone favor cosmologies with dark energy over models without dark
energy at a 3.2-sigma level. We demonstrate this by combining the CMB lensing
deflection power spectrum from the Atacama Cosmology Telescope with temperature
and polarization power spectra from the Wilkinson Microwave Anisotropy Probe.
The lensing data break the geometric degeneracy of different cosmological
models with similar CMB temperature power spectra. Our CMB-only measurement of
the dark energy density confirms other measurements from
supernovae, galaxy clusters and baryon acoustic oscillations, and demonstrates
the power of CMB lensing as a new cosmological tool.Comment: 4 pages, 3 figures; replaced with version accepted by Physical Review
Letters, added sentence on models with non-standard primordial power spectr
The Atacama Cosmology Telescope: Sunyaev-Zel'dovich Selected Galaxy Clusters at 148 GHz from Three Seasons of Data
[Abridged] We present a catalog of 68 galaxy clusters, of which 19 are new
discoveries, detected via the Sunyaev-Zel'dovich effect (SZ) at 148 GHz in the
Atacama Cosmology Telescope (ACT) survey of 504 square degrees on the celestial
equator. A subsample of 48 clusters within the 270 square degree region
overlapping SDSS Stripe 82 is estimated to be 90% complete for M_500c > 4.5e14
Msun and 0.15 < z < 0.8. While matched filters are used to detect the clusters,
the sample is studied further through a "Profile Based Amplitude Analysis"
using a single filter at a fixed \theta_500 = 5.9' angular scale. This new
approach takes advantage of the "Universal Pressure Profile" (UPP) to fix the
relationship between the cluster characteristic size (R_500) and the integrated
Compton parameter (Y_500). The UPP scalings are found to be nearly identical to
an adiabatic model, while a model incorporating non-thermal pressure better
matches dynamical mass measurements and masses from the South Pole Telescope. A
high signal to noise ratio subsample of 15 ACT clusters is used to obtain
cosmological constraints. We first confirm that constraints from SZ data are
limited by uncertainty in the scaling relation parameters rather than sample
size or measurement uncertainty. We next add in seven clusters from the ACT
Southern survey, including their dynamical mass measurements based on galaxy
velocity dispersions. In combination with WMAP7 these data simultaneously
constrain the scaling relation and cosmological parameters, yielding \sigma_8 =
0.829 \pm 0.024 and \Omega_m = 0.292 \pm 0.025. The results include
marginalization over a 15% bias in dynamical mass relative to the true halo
mass. In an extension to LCDM that incorporates non-zero neutrino mass density,
we combine our data with WMAP7+BAO+Hubble constant measurements to constrain
\Sigma m_\nu < 0.29 eV (95% C. L.).Comment: 32 pages, 21 figures To appear in J. Cosmology and Astroparticle
Physic
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