10,284 research outputs found
Stress relief as the driving force for self-assembled Bi nanolines
Stress resulting from mismatch between a substrate and an adsorbed material
has often been thought to be the driving force for the self-assembly of
nanoscale structures. Bi nanolines self-assemble on Si(001), and are remarkable
for their straightness and length -- they are often more than 400 nm long, and
a kink in a nanoline has never been observed. Through electronic structure
calculations, we have found an energetically favourable structure for these
nanolines that agrees with our scanning tunneling microscopy and photoemission
experiments; the structure has an extremely unusual subsurface structure,
comprising a double core of 7-membered rings of silicon. Our proposed structure
explains all the observed features of the nanolines, and shows that surface
stress resulting from the mismatch between the Bi and the Si substrate are
responsible for their self-assembly. This has wider implications for the
controlled growth of nanostructures on semiconductor surfaces.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Inverse Ising inference using all the data
We show that a method based on logistic regression, using all the data,
solves the inverse Ising problem far better than mean-field calculations
relying only on sample pairwise correlation functions, while still
computationally feasible for hundreds of nodes. The largest improvement in
reconstruction occurs for strong interactions. Using two examples, a diluted
Sherrington-Kirkpatrick model and a two-dimensional lattice, we also show that
interaction topologies can be recovered from few samples with good accuracy and
that the use of -regularization is beneficial in this process, pushing
inference abilities further into low-temperature regimes.Comment: 5 pages, 2 figures. Accepted versio
Topological Chern-Simons Sigma Model
We consider topological twisting of recently constructed Chern-Simons-matter
theories in three dimensions with N=4 or higher supersymmetry. We enumerate
physically inequivalent twistings for each N, and find two different twistings
for N=4, one for N=5,6, and four for N=8. We construct the two types of N=4
topological theories, which we call A/B-models, in full detail. The A-model has
been recently studied by Kapustin and Saulina. The B-model is new and it
consists solely of a Chern-Simons term of a complex gauge field up to
BRST-exact terms. We also compare the new theories with topological Yang-Mills
theories and find some interesting connections. In particular, the A-model
seems to offer a new perspective on Casson invariant and its relation to
Rozansky-Witten theory.Comment: 31 pages, no figure; v2. references adde
Influence of operating parameters on the biodegradation of steroid estrogens and nonylphenolic compounds during biological wastewater treatment processes
This document is the unedited author's version of a Submitted Work that was subsequently accepted for
publication in Environmental Science & Technology, copyright © American Chemical Society after peer
review. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/es901612v.This study investigated operational factors influencing the removal of steroid estrogens and nonylphenolic compounds in two sewage treatment works, one a nitrifying/denitrifying activated sludge plant and the other a nitrifying/denitrifying activated sludge plant with phosphorus removal. Removal efficiencies of >90% for steroid estrogens and for longer chain nonylphenol ethoxylates (NP4−12EO) were observed at both works, which had equal sludge ages of 13 days. However, the biological activity in terms of milligrams of estrogen removed per day per tonne of biomass was found to be 50−60% more efficient in the nitrifying/denitrifying activated sludge works compared to the works which additionally incorporated phosphorus removal. A temperature reduction of 6 °C had no impact on the removal of free estrogens, but removal of the conjugated estrone-3-sulfate was reduced by 20%. The apparent biomass sorption (LogKp) values were greater in the nitrifying/denitrifying works than those in the nitrifying/denitrifying works with phosphorus removal for both steroid estrogens and nonylphenolic compounds possibly indicating a different cell surface structure and therefore microbial population. The difference in biological activity (mg tonne−1 d−1) identified in this study, of up to seven times, suggests that there is the potential for enhancing the removal of estrogens and nonylphenols if more detailed knowledge of the factors responsible for these differences can be identified and maximized, thus potentially improving the quality of receiving waters.Public Utilities Board (Singapore), Anglian Water Ltd, Severn Trent Water Ltd, Thames Water Utilities Ltd, United Utilities 393 Plc and Yorkshire Water Services
Anisotropic Dirac fermions in a Bi square net of SrMnBi2
We report the highly anisotropic Dirac fermions in a Bi square net of
SrMnBi2, based on a first principle calculation, angle resolved photoemission
spectroscopy, and quantum oscillations for high-quality single crystals. We
found that the Dirac dispersion is generally induced in the (SrBi)+ layer
containing a double-sized Bi square net. In contrast to the commonly observed
isotropic Dirac cone, the Dirac cone in SrMnBi2 is highly anisotropic with a
large momentum-dependent disparity of Fermi velocities of ~ 8. These findings
demonstrate that a Bi square net, a common building block of various layered
pnictides, provide a new platform that hosts highly anisotropic Dirac fermions.Comment: 5 pages, 4 figure
Statistical Basis for Predicting Technological Progress
Forecasting technological progress is of great interest to engineers, policy
makers, and private investors. Several models have been proposed for predicting
technological improvement, but how well do these models perform? An early
hypothesis made by Theodore Wright in 1936 is that cost decreases as a power
law of cumulative production. An alternative hypothesis is Moore's law, which
can be generalized to say that technologies improve exponentially with time.
Other alternatives were proposed by Goddard, Sinclair et al., and Nordhaus.
These hypotheses have not previously been rigorously tested. Using a new
database on the cost and production of 62 different technologies, which is the
most expansive of its kind, we test the ability of six different postulated
laws to predict future costs. Our approach involves hindcasting and developing
a statistical model to rank the performance of the postulated laws. Wright's
law produces the best forecasts, but Moore's law is not far behind. We discover
a previously unobserved regularity that production tends to increase
exponentially. A combination of an exponential decrease in cost and an
exponential increase in production would make Moore's law and Wright's law
indistinguishable, as originally pointed out by Sahal. We show for the first
time that these regularities are observed in data to such a degree that the
performance of these two laws is nearly tied. Our results show that
technological progress is forecastable, with the square root of the logarithmic
error growing linearly with the forecasting horizon at a typical rate of 2.5%
per year. These results have implications for theories of technological change,
and assessments of candidate technologies and policies for climate change
mitigation
Electronic Structure of Electron-doped Sm1.86Ce0.14CuO4: Strong `Pseudo-Gap' Effects, Nodeless Gap and Signatures of Short Range Order
Angle resolved photoemission (ARPES) data from the electron doped cuprate
superconductor SmCeCuO shows a much stronger pseudo-gap
or "hot-spot" effect than that observed in other optimally doped -type
cuprates. Importantly, these effects are strong enough to drive the
zone-diagonal states below the chemical potential, implying that d-wave
superconductivity in this compound would be of a novel "nodeless" gap variety.
The gross features of the Fermi surface topology and low energy electronic
structure are found to be well described by reconstruction of bands by a
order. Comparison of the ARPES and optical data from
the sample shows that the pseudo-gap energy observed in optical data is
consistent with the inter-band transition energy of the model, allowing us to
have a unified picture of pseudo-gap effects. However, the high energy
electronic structure is found to be inconsistent with such a scenario. We show
that a number of these model inconsistencies can be resolved by considering a
short range ordering or inhomogeneous state.Comment: 5 pages, 4 figure
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