13,495 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
Spectrally resolved single-shot wavefront sensing of broadband high-harmonic sources
Wavefront sensors are an important tool to characterize coherent beams of
extreme ultraviolet radiation. However, conventional Hartmann-type sensors do
not allow for independent wavefront characterization of different spectral
components that may be present in a beam, which limits their applicability for
intrinsically broadband high-harmonic generation (HHG) sources. Here we
introduce a wavefront sensor that measures the wavefronts of all the harmonics
in a HHG beam in a single camera exposure. By replacing the mask apertures with
transmission gratings at different orientations, we simultaneously detect
harmonic wavefronts and spectra, and obtain sensitivity to spatiotemporal
structure such as pulse front tilt as well. We demonstrate the capabilities of
the sensor through a parallel measurement of the wavefronts of 9 harmonics in a
wavelength range between 25 and 49 nm, with up to lambda/32 precision.Comment: 12 pages, 6 figure
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
The geomorphological setting of some of Scotland's east coast freshwater mills: a comment on Downward and Skinner (2005) ‘Working rivers: the geomorphological legacy...’
Many of the water mills on Scotland's east coast streams, unlike those discussed recently by Downward and Skinner (2005 Area 37 138–47), 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 ‘run-of-the-river’, 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
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
Spin-splitting in the quantum Hall effect of disordered GaAs layers with strong overlap of the spin subbands
With minima in the diagonal conductance G_{xx} and in the absolute value of
the derivative |dG_{xy}/dB| at the Hall conductance value G_{xy}=e^{2}/h,
spin-splitting is observed in the quantum Hall effect of heavily Si-doped GaAs
layers with low electron mobility 2000 cm^2/Vs in spite of the fact that the
spin-splitting is much smaller than the level broadening. Experimental results
can be explained in the frame of the scaling theory of the quantum Hall effect,
applied independently to each of the two spin subbands.Comment: 4 pages, 4 figure
59Co-NQR study on superconducting NaxCoO2.yH2O
Layered Co oxide NaxCoO2.yH2O with a superconducting transition temperature
Tc =4.5 K has been studied by 59Co NQR. The nuclear spin relaxation rate 1/59T1
is nearly proportional to temperature T in the normal state. In the
superconducting state, it exhibits the coherence peak and decreases with
decreasing T below ~0.8Tc. Detailed comparison of the 1/T1T values and the
magnetic susceptibilities between NaxCoO2.yH2O and NaxCoO2 implies that the
metallic state of the former system is closer to a ferromagnetic phase than
that of the latter. These experimental results impose a restriction on the
mechanism of the superconductivity.Comment: 7 pages, 5 figures. to be published in J. Phys. Soc. Jpn. 72 (2003)
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