47,586 research outputs found
Recurrence Tracking Microscope
In order to probe nanostructures on a surface we present a microscope based
on the quantum recurrence phenomena. A cloud of atoms bounces off an atomic
mirror connected to a cantilever and exhibits quantum recurrences. The times at
which the recurrences occur depend on the initial height of the bouncing atoms
above the atomic mirror, and vary following the structures on the surface under
investigation. The microscope has inherent advantages over existing techniques
of scanning tunneling microscope and atomic force microscope. Presently
available experimental technology makes it possible to develop the device in
the laboratory
Heavy Meson Production in NN Collisions with Polarized Beam and Target -- A new facility for COSY
The study of near--threshold meson production in pp and pd collisions
involving polarized beams and polarized targets offers the rare opportunity to
gain insight into short--range features of the nucleon--nucleon interaction.
The Cooler Synchrotron COSY at FZ--J\"ulich is a unique environment to perform
such studies. Measurements of polarization observables require a cylindrically
symmetrical detector, capable to measure the momenta and the directions of
outgoing charged hadrons. The wide energy range of COSY leads to momenta of
outgoing protons to be detected in a single meson production reaction between
300 and 2500 MeV/c. Scattering angles of protons to be covered extend to about
in the laboratory system. An azimuthal angular coverage of the
device around 98% seems technically achievable. The required magnetic
spectrometer could consist of a superconducting toroid, providing fields around
3 T.Comment: 6 pages, 1 figure, submitted to Czechoslovak Journal of Physic
Properties of Intercalated 2H-NbSe2, 4Hb-TaS2 and 1T-TaS2
The layered compounds 2H-NbSe, 24Hb-TaS, 2and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal
A multiwavelength radial velocity search for planets around the brown dwarf LP 944-20
The nearby brown dwarf LP 944-20 has been monitored for radial velocity
variability at optical and near-infrared wavelengths using the VLT/UVES and the
Keck/NIRSPEC spectrographs, respectively. The UVES radial velocity data
obtained over 14 nights spanning a baseline of 841 days shows significant
variability with an amplitude of 3.5 km s. The periodogram analysis of
the UVES data indicates a possible period between 2.5 hours and 3.7 hours,
which is likely due to the rotation of the brown dwarf. However, the NIRSPEC
data obtained over 6 nights shows an rms dispersion of only 0.36 km s
and do not follow the periodic trend. These results indicate that the
variability seen with UVES is likely to be due to rotationally modulated
inhomogeneous surface features. We suggest that future planet searches around
very low-mass stars and brown dwarfs using radial velocities will be better
conducted in the near-infrared than in the optical.Comment: accepted by ApJ Letter
Buffalo National River Ecosystems - Part II
The priorities were established for the Buffalo National River Ecosystem Studies through meetings and correspondence with Mr. Roland Wauer and other personnel of the Office of Natural Sciences, Southwest Region of the National Park Service. These priorities were set forth in the appendix of contract no. CX 700050443 dated May 21, 1975
Europium nitride: A novel diluted magnetic semiconductor
Europium nitride is semiconducting and contains non-magnetic \3+, but
sub-stoichiometric EuN has Eu in a mix of 2+ and 3+ charge states. We show that
at \2+ ~concentrations near 15-20% EuN is ferromagnetic with a Curie
temperature as high as 120 K. The \3+ ~polarization follows that of the \2+,
confirming that the ferromagnetism is intrinsic to the EuN which is thus a
novel diluted magnetic semiconductor. Transport measurements shed light on the
likely exchange mechanisms.Comment: 5 page
Final Report: Buffalo National River Ecosystems
The objective of this study was to sample the Buffalo River on a seasonal basis for a year, in order to determine whether any potential water quality problems existed
Scanning Tunneling Microscope Operating as a Spin-diode
We theoretically investigate spin-polarized transport in a system composed of
a ferromagnetic Scanning Tunneling Microscope (STM) tip coupled to an adsorbed
atom (adatom) on a host surface. Electrons can tunnel directly from the tip to
the surface or via the adatom. Since the tip is ferromagnetic and the host
surface (metal or semiconductor) is non-magnetic we obtain a spin-diode effect
when the adatom is in the regime of single occupancy. This effect leads to an
unpolarized current for direct bias (V > 0) and polarized current for reverse
(V < 0) bias voltages, if the tip is nearby the adatom. Within the
nonequilibrium Keldysh technique we analyze the interplay between the lateral
displacement of the tip and the intra adatom Coulomb interaction on the
spindiode effect. As the tip moves away from the adatom the spin-diode effect
vanishes and the currents become polarized for both V > 0 and V < 0. We also
find an imbalance between the up and down spin populations in the adatom, which
can be tuned by the tip position and the bias. Finally, due to the presence of
the adsorbate on the surface, we observe spin-resolved Friedel oscillations in
the current, which reflects the oscillations in the calculated LDOS of the
subsystem surface+adatom.Comment: 11 pages, 4 figures. Submitte
Vorticity Budget of Weak Thermal Convection in Keplerian disks
By employing the equations of mean-square vorticity (enstrophy) fluctuations
in strong shear flows, we demonstrate that unlike energy production of
turbulent vorticity in nonrotating shear flows, the turbulent vorticity of weak
convection in Keplerian disks cannot gain energy from vortex stretching/tilting
by background shear unless the asscoiated Reynolds stresses are negative. This
is because the epicyclic motion is an energy sink of the radial component of
mean-square turbulent vorticity in Keplerian disks when Reynolds stresses are
positive. Consequently, weak convection cannot be self-sustained in Keplerian
flows. This agrees with the results implied from the equations of mean-square
velocity fluctuations in strong shear flows. Our analysis also sheds light on
the explanation of the simulation result in which positive kinetic helicity is
produced by the Balbus-Hawley instability in a vertically stratified Keplerian
disk. We also comment on the possibility of outward angular momentum transport
by strong convection based on azimuthal pressure perturbations and directions
of energy cascade.Comment: 8 pages, 1 figure, emulateapj.sty, revised version in response to
referee's comments, accepted by Ap
On the Correlated X-ray and Optical Evolution of SS Cygni
We have analyzed the variability and spectral evolution of the prototype
dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of
pointed RXTE/PCA observations allow us to trace the evolution of the X-ray
spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO
and RXTE/ASM light curves show long-term patterns. Employing a technique in
which we stack the X-ray flux over multiple outbursts, phased according to the
optical light curve, we investigate the outburst morphology. We find that the
3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen
previously, but only in a handful of cycles. The several outbursts of SS Cygni
observed with the more sensitive RXTE/PCA also show a depression of the X-rays
during optical outburst. We quantify the time lags between the optical and
X-ray outbursts, and the timescales of the X-ray recovery from outburst. The
optical light curve of SS Cygni exhibits brief anomalous outbursts. During
these events the hard X-rays and optical flux increase together. The long-term
data suggest that the X-rays decline between outburst. Our results are in
general agreement with modified disk instability models (DIM), which invoke a
two-component accretion flow consisting of a cool optically thick accretion
disk truncated at an inner radius, and a quasi-spherical hot corona-like flow
extending to the surface of the white dwarf. We discuss our results in the
framework of one such model, involving the evaporation of the inner part of the
optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister (1994).Comment: 24 pages, 8 figures, 2 tables, accepted for publication in Ap
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