1,310 research outputs found
Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors
We theoretically investigate intermodulation distortion in high-Tc
superconductors. We study the effect of nonmagnetic impurities on the real and
imaginary parts of nonlinear conductivity. The nonlinear conductivity is
proportional to the inverse of temperature owing to the dependence of the
damping effect on energy, which arises from the phase shift deviating from the
unitary limit. It is shown that the final-states interaction makes the real
part predominant over the imaginary part. These effects have not been included
in previous theories based on the two-fluid model, enabling a consistent
explanation for the experiments with the rf and dc fields
Wind measurement system
A system for remotely measuring vertical and horizontal winds present in discrete volumes of air at selected locations above the ground is described. A laser beam is optically focused in range by a telescope, and the output beam is conically scanned at an angle about a vertical axis. The backscatter, or reflected light, from the ambient particulates in a volume of air, the focal volume, is detected for shifts in wavelength, and from these, horizontal and vertical wind components are computed
Theory for Electron-Doped Cuprate Superconductors: d-wave symmetry order parameter
Using as a model the Hubbard Hamiltonian we determine various basic
properties of electron-doped cuprate superconductors like
and for a
spin-fluctuation-induced pairing mechanism. Most importantly we find a narrow
range of superconductivity and like for hole-doped cuprates -
symmetry for the superconducting order parameter. The superconducting
transition temperatures for various electron doping concentrations
are calculated to be much smaller than for hole-doped cuprates due to the
different Fermi surface and a flat band well below the Fermi level. Lattice
disorder may sensitively distort the symmetry via
electron-phonon interaction
Influence of gap structures to specific heat in oriented magnetic fields: Application to the orbital dependent superconductor, SrRuO
We discuss influence of modulation of gap function and anisotropy of Fermi
velocity to field angle dependences of upper critical field, , and
specific heat, , on the basis of the approximate analytic solution in the
quasiclassical formalism. Using 4-fold modulation of the gap function and the
Fermi velocity in the single-band model, we demonstrate field and temperature
dependence of oscillatory amplitude of and . We apply the method to
the effective two-band model to discuss the gap structure of SrRuO,
focusing on recent field angle-resolved experiments. It is shown that the gap
structures with the intermediate magnitude of minima in direction for
band, and tiny minima of gaps in directions for and
bands give consistent behaviors with experiments. The interplay of the
above two gaps also explains the anomalous temperature dependence of in-plane
anisotropy, where the opposite contribution from the passive
band is pronounced near .Comment: 7 pages, 11 figures in JPSJ forma
Factors affecting ammonium uptake in streams - an inter-biome perspective
The Lotic Intersite Nitrogen experiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of \u275~-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake
HST measures of Mass Accretion Rates in the Orion Nebula Cluster
The present observational understanding of the evolution of the mass
accretion rates (Macc) in pre-main sequence stars is limited by the lack of
accurate measurements of Macc over homogeneous and large statistical samples of
young stars. Such observational effort is needed to properly constrain the
theory of star formation and disk evolution. Based on HST/WFPC2 observations,
we present a study of Macc for a sample of \sim 700 sources in the Orion Nebula
Cluster, ranging from the Hydrogen-burning limit to M\ast \sim 2M\odot. We
derive Macc from both the U-band excess and the H{\alpha} luminosity
(LH{\alpha}), after determining empirically both the shape of the typical
accretion spectrum across the Balmer jump and the relation between the
accretion luminosity (Lacc) and LH{\alpha}, that is Lacc/L\odot =
(1.31\pm0.03)\cdotLH{\alpha}/L\odot + (2.63\pm 0.13). Given our large
statistical sample, we are able to accurately investigate relations between
Macc and the parameters of the central star such as mass and age. We clearly
find Macc to increase with stellar mass, and decrease over evolutionary time,
but we also find strong evidence that the decay of Macc with stellar age occurs
over longer timescales for more massive PMS stars. Our best fit relation
between these parameters is given by: log(Macc/M\odot\cdotyr)=(-5.12 \pm 0.86)
-(0.46 \pm 0.13) \cdot log(t/yr) -(5.75 \pm 1.47)\cdot log(M\ast/M\odot) +
(1.17 \pm 0.23)\cdot log(t/yr) \cdot log(M\ast/M\odot). These results also
suggest that the similarity solution model could be revised for sources with
M\ast > 0.5M\odot. Finally, we do not find a clear trend indicating
environmental effects on the accretion properties of the sources.Comment: 17 pages, 15 figures, accepted for publication in Ap
Angular dependence of the penetration depth in unconventional superconductors
We examine the Meissner state nonlinear electrodynamic effects on the field
and angular dependence of the low temperature penetration depth, , of
superconductors in several kinds of unconventional pairing states, with nodes
or deep minima (``quasinodes'') in the energy gap. Our calculations are
prompted by the fact that, for typical unconventional superconducting material
parameters, the predicted size of these effects for exceeds the
available experimental precision for this quantity by a much larger factor than
for others. We obtain expressions for the nonlinear component of the
penetration depth, , for different two- and three- dimensional
nodal or quasinodal structures. Each case has a characteristic signature as to
its dependence on the size and orientation of the applied magnetic field. This
shows that measurements can be used to elucidate the nodal or
quasinodal structure of the energy gap. For nodal lines we find that
is linear in the applied field, while the dependence is
quadratic for point nodes. For layered materials with
(YBCO) type anisotropy, our results for the
angular dependence of differ greatly from those for tetragonal
materials and are in agreement with experiment. For the two- and three-
dimensional quasinodal cases, is no longer proportional to a
power of the field and the field and angular dependences are not separable,
with a suppression of the overall signal as the node is filled in.Comment: 16 pages plus nine figure
Magneto-shear modes and a.c. dissipation in a two-dimensional Wigner crystal
The a.c. response of an unpinned and finite 2D Wigner crystal to electric
fields at an angular frequency has been calculated in the dissipative
limit, , where is the scattering rate. For
electrons screened by parallel electrodes, in zero magnetic field the
long-wavelength excitations are a diffusive longitudinal transmission line mode
and a diffusive shear mode. A magnetic field couples these modes together to
form two new magneto-shear modes. The dimensionless coupling parameter where and are the
speeds of transverse and longitudinal sound in the collisionless limit and
and are the tensor components of the
magnetoconductivity. For , both the coupled modes contribute
to the response of 2D electrons in a Corbino disk measurement of
magnetoconductivity. For , the electron crystal rotates rigidly in
a magnetic field. In general, both the amplitude and phase of the measured a.c.
currents are changed by the shear modulus. In principle, both the
magnetoconductivity and the shear modulus can be measured simultaneously.Comment: REVTeX, 7 pp., 4 eps figure
Kondo Effect of a Magnetic Ion Vibrating in a Harmonic Potential
To discuss Kondo effects of a magnetic ion vibrating in the sea of conduction
electrons, a generalized Anderson model is derived. The model includes a new
channel of hybridization associated with phonon emission or absorption. In the
simplest case of the localized electron orbital with the s-wave symmetry,
hybridization with p-waves becomes possible. Interesting interplay among the
conventional s-wave Kondo effect and the p-wave one and the Yu-Anderson type
Kondo effect is found and the ground state phase diagram is determined by using
the numerical renormalization group method. Two different types of stable fixed
points are identified and the two-channel Kondo fixed points are generically
realized on the boundary.Comment: 15 pages, 17 figures, J. Phys. Soc. Jpn. 80 (2011) No.6 to be
publishe
IPHAS and the symbiotic stars. I. Selection method and first discoveries
The study of symbiotic stars is essential to understand important aspects of
stellar evolution in interacting binaries. Their observed population in the
Galaxy is however poorly known, and is one to three orders of magnitudes
smaller than the predicted population size. IPHAS, the INT Photometric Halpha
survey of the Northern Galactic plane, gives us the opportunity to make a
systematic, complete search for symbiotic stars in a magnitude-limited volume,
and discover a significant number of new systems.
A method of selecting candidate symbiotic stars by combining IPHAS and
near-IR (2MASS) colours is presented. It allows us to distinguish symbiotic
binaries from normal stars and most of the other types of Halpha emission line
stars in the Galaxy. The only exception are T Tauri stars, which can however be
recognized because of their concentration in star forming regions. Using these
selection criteria, we discuss the classification of a list of 4338 IPHAS stars
with Halpha in emission. 1500 to 2000 of them are likely to be Be stars. Among
the remaining objects, 1183 fulfill our photometric constraints to be
considered candidate symbiotic stars. The spectroscopic confirmation of three
of these objects, which are the first new symbiotic stars discovered by IPHAS,
proves the potential of the survey and selection method.Comment: Accepted for publication on Astronomy and Astrophysics. 12 pages, 8
figure
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