1,250 research outputs found
Random Tensors and Planted Cliques
The r-parity tensor of a graph is a generalization of the adjacency matrix,
where the tensor's entries denote the parity of the number of edges in
subgraphs induced by r distinct vertices. For r=2, it is the adjacency matrix
with 1's for edges and -1's for nonedges. It is well-known that the 2-norm of
the adjacency matrix of a random graph is O(\sqrt{n}). Here we show that the
2-norm of the r-parity tensor is at most f(r)\sqrt{n}\log^{O(r)}n, answering a
question of Frieze and Kannan who proved this for r=3. As a consequence, we get
a tight connection between the planted clique problem and the problem of
finding a vector that approximates the 2-norm of the r-parity tensor of a
random graph. Our proof method is based on an inductive application of
concentration of measure
On the structure and evolution of a polar crown prominence/filament system
Polar crown prominences are made of chromospheric plasma partially circling
the Suns poles between 60 and 70 degree latitude. We aim to diagnose the 3D
dynamics of a polar crown prominence using high cadence EUV images from the
Solar Dynamics Observatory (SDO)/AIA at 304 and 171A and the Ahead spacecraft
of the Solar Terrestrial Relations Observatory (STEREO-A)/EUVI at 195A. Using
time series across specific structures we compare flows across the disk in 195A
with the prominence dynamics seen on the limb. The densest prominence material
forms vertical columns which are separated by many tens of Mm and connected by
dynamic bridges of plasma that are clearly visible in 304/171A two-color
images. We also observe intermittent but repetitious flows with velocity 15
km/s in the prominence that appear to be associated with EUV bright points on
the solar disk. The boundary between the prominence and the overlying cavity
appears as a sharp edge. We discuss the structure of the coronal cavity seen
both above and around the prominence. SDO/HMI and GONG magnetograms are used to
infer the underlying magnetic topology. The evolution and structure of the
prominence with respect to the magnetic field seems to agree with the filament
linkage model.Comment: 24 pages, 14 figures, Accepted for publication in Solar Physics
Journal, Movies can be found at http://www2.mps.mpg.de/data/outgoing/panesar
Absence of Scaling in the Integer Quantum Hall Effect
We have studied the conductivity peak in the transition region between the
two lowest integer Quantum Hall states using transmission measurements of edge
magnetoplasmons. The width of the transition region is found to increase
linearly with frequency but remains finite when extrapolated to zero frequency
and temperature. Contrary to prevalent theoretical pictures, our data does not
show the scaling characteristics of critical phenomena.These results suggest
that a different mechanism governs the transition in our experiment.Comment: Minor changes and new references include
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Edge state transmission, duality relation and its implication to measurements
The duality in the Chalker-Coddington network model is examined. We are able
to write down a duality relation for the edge state transmission coefficient,
but only for a specific symmetric Hall geometry. Looking for broader
implication of the duality, we calculate the transmission coefficient in
terms of the conductivity and in the diffusive
limit. The edge state scattering problem is reduced to solving the diffusion
equation with two boundary conditions
and
.
We find that the resistances in the geometry considered are not necessarily
measures of the resistivity and () holds only
when is quantized. We conclude that duality alone is not sufficient
to explain the experimental findings of Shahar et al and that Landauer-Buttiker
argument does not render the additional condition, contrary to previous
expectation.Comment: 16 pages, 3 figures, to appear in Phys. Rev.
Solar city indicator: a methodology to predict city level PV installed capacity by combining physical capacity and socio-economic factors
Shifting to renewable sources of electricity is imperative in achieving global reductions in carbon emissions and ensuring future energy security. One technology, solar photovoltaics (PV), has begun to generate a noticeable contribution to the electricity mix in numerous countries. However, the upper limits of this contribution have not been explored in a way that combines both building-by-building solar resource appraisals with the city-scale socio-economic contexts that dictate PV uptake. This paper presents such a method, whereby a ‘Solar City Indicator’ is calculated and used to rank cities by their capacity to generate electricity from roof-mounted PV. Seven major UK cities were chosen for analysis based on available data; Dundee, Derby, Edinburgh, Glasgow, Leicester, Nottingham and Sheffield. The physical capacity of each city was established using a GIS-based methodology, exploiting digital surface models and LiDAR data, with distinct methodologies for large and small properties. Socio-economic factors (income, education, environmental consciousness, building stock and ownership) were chosen based on existing literature and correlation with current levels of PV installations. These factors were enumerated using data that was readily available across each city. Results show that Derby has the greatest potential of all the cities analysed, as it offers both good physical and socio-economic potential. In terms of physical capacity it was seen that over a 15 year payback period there are two plateaus, showing a marked difference in viability between small and large PV arrays. It was found that both the physical and socio-economic potential of a city are strongly influenced by the nature of the local building stock. This study also identifies areas where policy needs to be focused in order to encourage uptake and highlights factors limiting maximum PV uptake. While this methodology has been demonstrated using UK cities, it is equally applicable to any country where city data is available
Mesoscopic conductance and its fluctuations at non-zero Hall angle
We consider the bilocal conductivity tensor, the two-probe conductance and
its fluctuations for a disordered phase-coherent two-dimensional system of
non-interacting electrons in the presence of a magnetic field, including
correctly the edge effects. Analytical results are obtained by perturbation
theory in the limit . For mesoscopic systems the conduction
process is dominated by diffusion but we show that, due to the lack of
time-reversal symmetry, the boundary condition for diffusion is altered at the
reflecting edges. Instead of the usual condition, that the derivative along the
direction normal to the wall of the diffusing variable vanishes, the derivative
at the Hall angle to the normal vanishes. We demonstrate the origin of this
boundary condition from different starting points, using (i) a simplified
Chalker-Coddington network model, (ii) the standard diagrammatic perturbation
expansion, and (iii) the nonlinear sigma-model with the topological term, thus
establishing connections between the different approaches. Further boundary
effects are found in quantum interference phenomena. We evaluate the mean
bilocal conductivity tensor , and the mean and variance
of the conductance, to leading order in and to order
, and find that the variance of the conductance
increases with the Hall ratio. Thus the conductance fluctuations are no longer
simply described by the unitary universality class of the case,
but instead there is a one-parameter family of probability distributions. In
the quasi-one-dimensional limit, the usual universal result for the conductance
fluctuations of the unitary ensemble is recovered, in contrast to results of
previous authors. Also, a long discussion of current conservation.Comment: Latex, uses RevTex, 58 pages, 5 figures available on request at
[email protected]. Submitted to Phys. Rev.
Bipolar spintronics: From spin injection to spin-controlled logic
An impressive success of spintronic applications has been typically realized
in metal-based structures which utilize magnetoresistive effects for
substantial improvements in the performance of computer hard drives and
magnetic random access memories. Correspondingly, the theoretical understanding
of spin-polarized transport is usually limited to a metallic regime in a linear
response, which, while providing a good description for data storage and
magnetic memory devices, is not sufficient for signal processing and digital
logic. In contrast, much less is known about possible applications of
semiconductor-based spintronics and spin-polarized transport in related
structures which could utilize strong intrinsic nonlinearities in
current-voltage characteristics to implement spin-based logic. Here we discuss
the challenges for realizing a particular class of structures in semiconductor
spintronics: our proposal for bipolar spintronic devices in which carriers of
both polarities (electrons and holes) contribute to spin-charge coupling. We
formulate the theoretical framework for bipolar spin-polarized transport, and
describe several novel effects in two- and three-terminal structures which
arise from the interplay between nonequilibrium spin and equilibrium
magnetization.Comment: 16 pages, 7 figure
Use of Anti-Inflammatory Drugs and Lower Esophageal Sphincter-Relaxing Drugs and Risk of Esophageal and Gastric Cancers
Background & Aims: The incidence of esophageal and gastric cardia adenocarcinoma has increased in Western countries in recent decades for largely unknown reasons. We investigated whether use of LES-relaxing drugs was related to an increased risk of esophageal and gastric cardia adenocarcinoma, and whether use of NSAIDs was related to a reduced risk of esophageal and gastric cancers. Methods: We examined these associations by using administrative databases in a case-control study in 2 integrated health care delivery systems. Cases were incident esophageal adenocarcinomas (n = 163) and squamous cell carcinomas (n = 114) and gastric cardia (n = 176) and non-cardia adenocarcinomas (n = 320), diagnosed between 1980-2002 in one health system and between 1993-2002 in the other. Matched controls (n = 3996) were selected. Complete prescription information was available for the study period. Results: Prescription of corticosteroids was associated with a decreased risk of esophageal adenocarcinoma (odds ratio [OR], 0.6; 95% confidence interval [CI], 0.4-0.9), esophageal squamous cell carcinoma (OR, 0.4; 95% CI, 0.2-0.6), and gastric non-cardia carcinoma (OR, 0.4, 95% CI, 0.3-0.6). Ever use of pharmacy-purchased aspirin was associated with 30%-60% decreased risks of the studied cancers. As a group, LES-relaxing drugs showed little evidence of association with increased risk of any esophageal or gastric cancer. Conclusions: Corticosteroid and aspirin use were associated with significantly decreased risks of esophageal and gastric cancer. LES-relaxing drugs as a group did not affect these risks, although we had limited power to assess individual drugs. The possibility that corticosteroids and aspirin might reduce esophageal cancer risk warrants further consideration
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
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