10,104 research outputs found
A quasilocal calculation of tidal heating
We present a method for computing the flux of energy through a closed surface
containing a gravitating system. This method, which is based on the quasilocal
formalism of Brown and York, is illustrated by two applications: a calculation
of (i) the energy flux, via gravitational waves, through a surface near
infinity and (ii) the tidal heating in the local asymptotic frame of a body
interacting with an external tidal field. The second application represents the
first use of the quasilocal formalism to study a non-stationary spacetime and
shows how such methods can be used to study tidal effects in isolated
gravitating systems.Comment: REVTex, 4 pages, 1 typo fixed, standard sign convention adopted for
the Newtonian potential, a couple of lines added to the discussion of gauge
dependent term
Quantifying structural damage from self-irradiation in a plutonium superconductor
The 18.5 K superconductor PuCoGa5 has many unusual properties, including
those due to damage induced by self-irradiation. The superconducting transition
temperature decreases sharply with time, suggesting a radiation-induced Frenkel
defect concentration much larger than predicted by current radiation damage
theories. Extended x-ray absorption fine-structure measurements demonstrate
that while the local crystal structure in fresh material is well ordered, aged
material is disordered much more strongly than expected from simple defects,
consistent with strong disorder throughout the damage cascade region. These
data highlight the potential impact of local lattice distortions relative to
defects on the properties of irradiated materials and underscore the need for
more atomic-resolution structural comparisons between radiation damage
experiments and theory.Comment: 7 pages, 5 figures, to be published in PR
Making graphene visible
Microfabrication of graphene devices used in many experimental studies
currently relies on the fact that graphene crystallites can be visualized using
optical microscopy if prepared on top of silicon wafers with a certain
thickness of silicon dioxide. We study graphene's visibility and show that it
depends strongly on both thickness of silicon dioxide and light wavelength. We
have found that by using monochromatic illumination, graphene can be isolated
for any silicon dioxide thickness, albeit 300 nm (the current standard) and,
especially, approx. 100 nm are most suitable for its visual detection. By using
a Fresnel-law-based model, we quantitatively describe the experimental data
without any fitting parameters.Comment: Since v1: minor changes to text and figures to improve clarity;
references added. Submitted to Applied Physics Letters, 30-Apr-07. 3 pages, 3
figure
X-ray Absorption Fine Structure in Embedded Atoms
Oscillatory structure is found in the atomic background absorption in
x-ray-absorption fine structure (XAFS). This atomic-XAFS or AXAFS arises from
scattering within an embedded atom, and is analogous to the Ramsauer-Townsend
effect. Calculations and measurements confirm the existence of AXAFS and show
that it can dominate contributions such as multi-electron excitations. The
structure is sensitive to chemical effects and thus provides a new probe of
bonding and exchange effects on the scattering potential.Comment: 4 pages plus 2 postscript figures, REVTEX 3.
Local lattice disorder in the geometrically-frustrated spin glass pyrochlore Y2Mo2O7
The geometrically-frustrated spin glass Y2Mo2O7 has been considered widely to
be crystallographically ordered with a unique nearest neighbor magnetic
exchange interaction, J. To test this assertion, we present x-ray-absorption
fine-structure results for the Mo and Y K edges as a function of temperature
and compare them to results from a well-ordered pyrochlore, Tl2Mn2O7. We find
that the Mo-Mo pair distances are significantly disordered at approximately
right angles to the Y-Mo pairs. These results strongly suggest that lattice
disorder nucleates the spin-glass phase in this material.Comment: 9 pages, 2 Postscript figures, Phys. Rev. B: Rapid, in pres
Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies
The combined local structure techniques, extended x-ray absorption fine
structure (EXAFS) and neutron pair distribution function analysis, have been
used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT)
distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co
sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co
sites exist.Comment: 5 page
Looking for the S-Z Effect towards Distant ROSAT Clusters of Galaxies
We report on observations of the Sunyaev-Zeldovich effect towards X-ray ROSAT
clusters taken with a double channel (1.2 and 2 mm) photometer installed at the
focus of the 15m SEST antenna in Chile. This paper describes the first results
obtained for the high-z clusters S1077, A2744 and S295. Marginal detections
were found for A2744 and at 1 mm for S1077. We discuss these data in terms of
contamination of sources along the line of sight and give a constraint on the
amplitude of the kinematic effect.Comment: 17 pg Latex file (using aasms4.sty) gzip'd tar'd uuencoded file
including 1 ps figure, ApJ Letter in pres
Hall Effect of La2/3(Ca,Pb)1/3MnO3 Single Crystals near the Critical Temperature
The Hall resistivity rho_{xy} of a La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal
has been measured as a function of temperature and field. The overall behavior
is similar to that observed previously in thin-films. At 5 K, rho_{xy} is
positive and linear in field, indicating that the anomalous contribution
is negligible. However, the effective carrier density in a free electron model
is n_{eff}=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for
thin-films and far larger than the 0.33 holes/Mn expected from the doping
level. As temperature increases, a strong, negative contribution to rho_{xy}
appears, that we ascribe to R_S. Using detailed magnetization data, we separate
the ordinary (\propto B) and anomalous (\propto M) contributions. Below T_C,
R_S \propto rho_{xx}, indicating that magnetic skew scattering is the dominant
mechanism in the metallic ferromagnetic regime. At and above the
resistivity-peak temperature, we find that rho_{xy}/rho_{xx}M is a constant,
independent of temperature and field. This implies that the anomalous Hall
coefficient is proportional to the magnetoresistance. A different explanation
based on two fluid model is also presented.Comment: revtex, 11 pages, 4 figure
Magnetic-field-induced collapse of charge-ordered nanoclusters and the Colossal Magnetoresistance effect in Nd(0.3)Sr(0.3)MnO(3)
We report synchrotron x-ray scattering studies of charge/orbitally ordered
(COO) nanoclusters in NdSrMnO. We find that the COO
nanoclusters are strongly suppressed in an applied magnetic field, and that
their decreasing concentration follows the field-induced decrease of the sample
electrical resistivity. The COO nanoclusters, however, do not completely
disappear in the conducting state, suggesting that this state is inhomogeneous
and contains an admixture of an insulating phase. Similar results were also
obtained for the zero-field insulator-metal transition that occurs as
temperature is reduced. These observations suggest that these correlated
lattice distortions play a key role in the Colossal Magnetoresistance effect in
this prototypical manganite.Comment: 5 pages, 3 embedded eps figures; to appear in PRB Rapid
Commumication
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