17,614 research outputs found
A Monte Carlo study of temperature-programmed desorption spectra with attractive lateral interactions
We present results of a Monte Carlo study of temperature-programmed
desorption in a model system with attractive lateral interactions. It is shown
that even for weak interactions there are large shifts of the peak maximum
temperatures with initial coverage. The system has a transition temperature
below which the desorption has a negative order. An analytical expression for
this temperature is derived. The relation between the model and real systems is
discussed.Comment: Accepted for publication in Phys.Rev.B15, 10 pages (REVTeX), 2
figures (PostScript); discussion about Xe/Pt(111) adde
Wigner crystallization in Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10) chain compounds
We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4)
and Na(8)Cu(5)O(10), which form insulating states with commensurate charge
order. We identify these systems as one-dimensional Wigner lattices, where the
charge order is determined by long-range Coulomb interaction and the number of
holes in the d-shell of Cu. Our interpretation is supported by X-ray structure
data as well as by an analysis of magnetic susceptibility and specific heat
data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems
allow for a distinction between the (classical) Wigner lattice and the 4k_F
charge-density wave of quantum mechanical origin.Comment: 4 pages, 4 figure
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
The use of implicit evidence for relevance feedback in web retrieval
In this paper we report on the application of two contrasting types of relevance feedback for web retrieval. We compare two systems; one using explicit relevance feedback (where searchers explicitly have to mark documents relevant) and one using implicit relevance feedback (where the system endeavours to estimate relevance by mining the searcher's interaction). The feedback is used to update the display according to the user's interaction. Our research focuses on the degree to which implicit evidence of document relevance can be substituted for explicit evidence. We examine the two variations in terms of both user opinion and search effectiveness
Image Delocalisation and High Resolution Transmission Electron Microscopic Imaging with a Field Emission Gun
The high spatial and temporal coherence of a field emission gun (FEG) increases the information limit of high-resolution transmission electron microscopes (HRTEM), but has also its implications on the localisation of the high resolution information in the image. In this paper, we present the results of a combined theoretical and experimental study of delocalisation in HRTEM. First, we derive a spatial frequency analysis of the delocalisation for crystal defects. Next, the delocalisation is studied from a real-space point of view, in terms of the impulse-response function, for which an instructive asymptotic mathematical analysis has been set up. Finally, we present experimental HRTEM images of crystal defects and of an amorphous Ge film, which are recorded with a Philips CM20 FEG electron microscope, and which illustrate the delocalisation phenomena
The geomorphological setting of some of Scotland's east coast freshwater mills: a comment on Downward and Skinner (2005) βWorking rivers: the geomorphological legacy...β
Many of the water mills on Scotland's east coast streams, unlike those discussed recently by Downward and Skinner (2005 Area 37 138β47), are found in predominantly bedrock reaches immediately downstream of knickpoints (i.e. bedrock steps). Bedrock knickpoints in the lower reaches of Scottish rivers are a widespread fluvial response to the glacio-isostatic rebound of northern Britain. These steps in the river profile propagate headward over time, but for intervals of a few centuries or so they are sufficiently stable to be exploited for the elevational fall necessary to power the mill wheel. Many of these mills were apparently powered by βrun-of-the-riverβ, as are some today that formerly had mill dams. The typical lack of sediment storage along the erosional lower reaches of many Scottish rivers means that failure of mill structures in Scotland will probably have less dramatic geomorphological and management implications than those suggested by Downward and Skinner for southern English rivers
Computational science and re-discovery: open-source implementations of ellipsoidal harmonics for problems in potential theory
We present two open-source (BSD) implementations of ellipsoidal harmonic
expansions for solving problems of potential theory using separation of
variables. Ellipsoidal harmonics are used surprisingly infrequently,
considering their substantial value for problems ranging in scale from
molecules to the entire solar system. In this article, we suggest two possible
reasons for the paucity relative to spherical harmonics. The first is
essentially historical---ellipsoidal harmonics developed during the late 19th
century and early 20th, when it was found that only the lowest-order harmonics
are expressible in closed form. Each higher-order term requires the solution of
an eigenvalue problem, and tedious manual computation seems to have discouraged
applications and theoretical studies. The second explanation is practical: even
with modern computers and accurate eigenvalue algorithms, expansions in
ellipsoidal harmonics are significantly more challenging to compute than those
in Cartesian or spherical coordinates. The present implementations reduce the
"barrier to entry" by providing an easy and free way for the community to begin
using ellipsoidal harmonics in actual research. We demonstrate our
implementation using the specific and physiologically crucial problem of how
charged proteins interact with their environment, and ask: what other
analytical tools await re-discovery in an era of inexpensive computation?Comment: 25 pages, 3 figure
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