4,226 research outputs found
Steps and terraces at quasicrystal surfaces. Application of the 6d-polyhedral model to the analysis of STM images of i-AlPdMn
6-d polyhedral models give a periodic description of aperiodic quasicrystals.
There are powerful tools to describe their structural surface properties. Basis
of the model for icosahedral quasicrystals are given. This description is
further used to interpret high resolution STM images of the surface of i-AlPdMn
which surface preparation was followed by He diffraction. It is found that both
terrace structure and step-terrace height profiles in STM images can be
consistently interpreted by the described model
The importance of habitat quality for marine reserve fishery linkages
We model marine reserve - fishery linkages to evaluate the potential contribution of habitat-quality improvements inside a marine reserve to fish productivity and fishery catches. Data from Mombasa Marine National Park, Kenya, and the adjacent fishery are used. Marine reserves increase total fish biomass directly by providing refuge from exploitation and indirectly by improving fish habitat in the reserve. As natural mortality of the fish stock decreases in response to habitat enhancement in the reserve, catches increase by up to 2.6 tonnes (t).km(-2).year(-1) and total fish biomass by up to 36 t.km(-2). However, if habitat-quality improvement reduces the propensity of fish to move out of the reserve, catches may fall by up to 0.9 t.km(-2).year(-1). Our results indicate that habitat protection in reserves can underpin fish productivity and, depending on its effects on fish movements, augment catches
HIV-1 Evolutionary Patterns Associated with Metastatic Kaposi's Sarcoma during AIDS.
Kaposi's sarcoma (KS) in HIV-infected individuals can have a wide range of clinical outcomes, from indolent skin tumors to a life-threatening visceral cancer. KS tumors contain endothelial-related cells and inflammatory cells that may be HIV-infected. In this study we tested if HIV evolutionary patterns distinguish KS tumor relatedness and progression. Multisite autopsies from participants who died from HIV-AIDS with KS prior to the availability of antiretroviral therapy were identified at the AIDS and Cancer Specimen Resource (ACSR). Two patients (KS1 and KS2) died predominantly from non-KS-associated disease and KS3 died due to aggressive and metastatic KS within one month of diagnosis. Skin and visceral tumor and nontumor autopsy tissues were obtained (n = 12). Single genome sequencing was used to amplify HIV RNA and DNA, which was present in all tumors. Independent HIV tumor clades in phylogenies differentiated KS1 and KS2 from KS3, whose sequences were interrelated by both phylogeny and selection. HIV compartmentalization was confirmed in KS1 and KS2 tumors; however, in KS3, no compartmentalization was observed among sampled tissues. While the sample size is small, the HIV evolutionary patterns observed in all patients suggest an interplay between tumor cells and HIV-infected cells which provides a selective advantage and could promote KS progression
Water vapor radiometry research and development phase
This report describes the research and development phase for eight dual-channel water vapor radiometers constructed for the Crustal Dynamics Project at the Goddard Space Flight Center, Greenbelt, Maryland, and for the NASA Deep Space Network. These instruments were developed to demonstrate that the variable path delay imposed on microwave radio transmissions by atmospheric water vapor can be calibrated, particularly as this phenomenon affects very long baseline interferometry measurement systems. Water vapor radiometry technology can also be used in systems that involve moist air meteorology and propagation studies
Cleaved surface of i-AlPdMn quasicrystals: Influence of the local temperature elevation at the crack tip on the fracture surface roughness
Roughness of i-AlPdMn cleaved surfaces are presently analysed. From the
atomic scale to 2-3 nm, they are shown to exhibit scaling properties hiding the
cluster (0.45 nm) aperiodic structure. These properties are quantitatively
similar to those observed on various disordered materials, albeit on other
ranges of length scales. These properties are interpreted as the signature of
damage mechanisms occurring within a 2-3 nm wide zone at the crack tip. The
size of this process zone finds its origin in the local temperature elevation
at the crack tip. For the very first time, this effect is reported to be
responsible for a transition from a perfectly brittle behavior to a nanoductile
one.Comment: 8 page
Compressed sensing with l0-norm: statistical physics analysis and algorithms for signal recovery
Noiseless compressive sensing is a protocol that enables undersampling and
later recovery of a signal without loss of information. This compression is
possible because the signal is usually sufficiently sparse in a given basis.
Currently, the algorithm offering the best tradeoff between compression rate,
robustness, and speed for compressive sensing is the LASSO (l1-norm bias)
algorithm. However, many studies have pointed out the possibility that the
implementation of lp-norms biases, with p smaller than one, could give better
performance while sacrificing convexity. In this work, we focus specifically on
the extreme case of the l0-based reconstruction, a task that is complicated by
the discontinuity of the loss. In the first part of the paper, we describe via
statistical physics methods, and in particular the replica method, how the
solutions to this optimization problem are arranged in a clustered structure.
We observe two distinct regimes: one at low compression rate where the signal
can be recovered exactly, and one at high compression rate where the signal
cannot be recovered accurately. In the second part, we present two
message-passing algorithms based on our first results for the l0-norm
optimization problem. The proposed algorithms are able to recover the signal at
compression rates higher than the ones achieved by LASSO while being
computationally efficient
Unbounded quantum backflow in two dimensions
Quantum backflow refers to the counterintuitive fact that the probability can
flow in the direction opposite to the momentum of a quantum particle. This
phenomenon has been seen to be small and fragile for one-dimensional systems,
in which the maximal amount of backflow has been found to be bounded. Quantum
backflow exhibits dramatically different features in two-dimensional systems
that, contrary to the one-dimensional case, allow for degenerate energy
eigenstates. Here we investigate the case of a charged particle that is
confined to move on a finite disk punctured at the center and that is pierced
through the center, and normally to the disk, by a magnetic flux line. We
demonstrate that quantum backflow can be unbounded (in a certain sense), which
makes this system a promising physical platform regarding the
yet-to-be-performed experimental observation of this fundamental quantum
phenomenon.Comment: 11 pages, 1 figur
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