688 research outputs found
Anisotropic conductivity of Nd_{1.85}Ce_{0.15}CuO_{4-\delta} films at submillimeter wavelengths
The anisotropic conductivity of thin NdCeCuO
films was measured in the frequency range 8 cm 40 cm and
for temperatures 4 K K. A tilted sample geometry allowed to extract
both, in-plane and c-axis properties. The in-plane quasiparticle scattering
rate remains unchanged as the sample becomes superconducting. The temperature
dependence of the in-plane conductivity is reasonably well described using the
Born limit for a d-wave superconductor. Below T_{{\rm C}%} the c-axis
dielectric constant changes sign at the screened c-axis plasma
frequency. The temperature dependence of the c-axis conductivity closely
follows the linear in T behavior within the plane.Comment: 4 pages, 4 figure
Optical Properties of Layered Superconductors near the Josephson Plasma Resonance
We study the optical properties of crystals with spatial dispersion and show
that the usual Fresnel approach becomes invalid near frequencies where the
group velocity of the wave packets inside the crystal vanishes. Near these
special frequencies the reflectivity depends on the atomic structure of the
crystal provided that disorder and dissipation are very low. This is
demonstrated explicitly by a detailed study of layered superconductors with
identical or two different alternating junctions in the frequency range near
the Josephson plasma resonance. Accounting for both inductive and charge
coupling of the intrinsic junctions, we show that multiple modes are excited
inside the crystal by the incident light, determine their relative amplitude by
the microscopic calculation of the additional boundary conditions and finally
obtain the reflectivity.
Spatial dispersion also provides a novel method to stop light pulses, which
has possible applications for quantum information processing and the artificial
creation of event horizons in a solid.Comment: 25 pages, 20 figures, submitted to Phys. Rev.
Effective Actions and Phase Fluctuations in d-wave Superconductors
We study effective actions for order parameter fluctuations at low
temperature in layered d-wave superconductors such as the cuprates. The order
parameter lives on the bonds of a square lattice and has two amplitude and two
phase modes associated with it. The low frequency spectral weights for
amplitude and relative phase fluctuations is determined and found to be
subdominant to quasiparticle contributions. The Goldstone phase mode and its
coupling to density fluctuations in charged systems is treated in a
gauge-invariant manner. The Gaussian phase action is used to study both the
-axis Josephson plasmon and the more conventional in-plane plasmon in the
cuprates. We go beyond the Gaussian theory by deriving a coarse-grained quantum
XY model, which incorporates important cutoff effects overlooked in previous
studies. A variational analysis of this effective model shows that in the
cuprates, quantum effects of phase fluctuations are important in reducing the
zero temperature superfluid stiffness, but thermal effects are small for .Comment: Some numerical estimates corrected and figures changed. to appear in
PRB, Sept.1 (2000
Experimental implications of quantum phase fluctuations in layered superconductors
I study the effect of quantum and thermal phase fluctuations on the in-plane
and c-axis superfluid stiffness of layered d-wave superconductors. First, I
show that quantum phase fluctuations in the superconductor can be damped in the
presence of external screening of Coulomb interactions, and suggest an
experiment to test the importance of these fluctuations, by placing a metal in
close proximity to the superconductor to induce such screening. Second, I show
that a combination of quantum phase fluctuations and the linear temperature
dependence of the in-plane superfluid stiffness leads to a linear temperature
dependence of the c-axis penetration depth, below a temperature scale
determined by the magnitude of in-plane dissipation.Comment: 6 pgs, 1 figure, minor changes in comparison with c-axis expt, final
published versio
The dependence of the EIT wave velocity on the magnetic field strength
"EIT waves" are a wavelike phenomenon propagating in the corona, which were
initially observed in the extreme ultraviolet (EUV) wavelength by the EUV
Imaging Telescope (EIT). Their nature is still elusive, with the debate between
fast-mode wave model and non-wave model. In order to distinguish between these
models, we investigate the relation between the EIT wave velocity and the local
magnetic field in the corona. It is found that the two parameters show
significant negative correlation in most of the EIT wave fronts, {\it i.e.},
EIT wave propagates more slowly in the regions of stronger magnetic field. Such
a result poses a big challenge to the fast-mode wave model, which would predict
a strong positive correlation between the two parameters. However, it is
demonstrated that such a result can be explained by the fieldline stretching
model, \emph{i.e.,} that "EIT waves" are apparently-propagating brightenings,
which are generated by successive stretching of closed magnetic field lines
pushed by the erupting flux rope during coronal mass ejections (CMEs).Comment: 11 pages, 8 figures, accepted for publication in Solar Phy
Coronal Shock Waves, EUV waves, and Their Relation to CMEs. I. Reconciliation of "EIT waves", Type II Radio Bursts, and Leading Edges of CMEs
We show examples of excitation of coronal waves by flare-related abrupt
eruptions of magnetic rope structures. The waves presumably rapidly steepened
into shocks and freely propagated afterwards like decelerating blast waves that
showed up as Moreton waves and EUV waves. We propose a simple quantitative
description for such shock waves to reconcile their observed propagation with
drift rates of metric type II bursts and kinematics of leading edges of coronal
mass ejections (CMEs). Taking account of different plasma density falloffs for
propagation of a wave up and along the solar surface, we demonstrate a close
correspondence between drift rates of type II bursts and speeds of EUV waves,
Moreton waves, and CMEs observed in a few known events.Comment: 30 pages, 15 figures. Solar Physics, published online. The final
publication is available at http://www.springerlink.co
Numerical study of the thermoelectric power factor in ultra-thin Si nanowires
Low dimensional structures have demonstrated improved thermoelectric (TE)
performance because of a drastic reduction in their thermal conductivity,
{\kappa}l. This has been observed for a variety of materials, even for
traditionally poor thermoelectrics such as silicon. Other than the reduction in
{\kappa}l, further improvements in the TE figure of merit ZT could potentially
originate from the thermoelectric power factor. In this work, we couple the
ballistic (Landauer) and diffusive linearized Boltzmann electron transport
theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB)
electronic structure model. We calculate the room temperature electrical
conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires
(NWs). We describe the numerical formulation of coupling TB to those transport
formalisms, the approximations involved, and explain the differences in the
conclusions obtained from each model. We investigate the effects of cross
section size, transport orientation and confinement orientation, and the
influence of the different scattering mechanisms. We show that such methodology
can provide robust results for structures including thousands of atoms in the
simulation domain and extending to length scales beyond 10nm, and point towards
insightful design directions using the length scale and geometry as a design
degree of freedom. We find that the effect of low dimensionality on the
thermoelectric power factor of Si NWs can be observed at diameters below ~7nm,
and that quantum confinement and different transport orientations offer the
possibility for power factor optimization.Comment: 42 pages, 14 figures; Journal of Computational Electronics, 201
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
One of the major discoveries of the Extreme ultraviolet Imaging Telescope
(EIT) on SOHO were intensity enhancements propagating over a large fraction of
the solar surface. The physical origin(s) of the so-called `EIT' waves is still
strongly debated. They are considered to be either wave (primarily fast-mode
MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in
understanding the nature of EUV waves lies with the limitations of the EIT
observations which have been used almost exclusively for their study. Their
limitations are largely overcome by the SECCHI/EUVI observations on-board the
STEREO mission. The EUVI telescopes provide high cadence, simultaneous
multi-temperature coverage, and two well-separated viewpoints. We present here
the first detailed analysis of an EUV wave observed by the EUVI disk imagers on
December 07, 2007 when the STEREO spacecraft separation was .
Both a small flare and a CME were associated with the wave cadence, and single
temperature and viewpoint coverage. These limitations are largely overcome by
the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes
provide high cadence, simultaneous multi-temperature coverage, and two
well-separated viewpoints. Our findings give significant support for a
fast-mode interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.Comment: Solar Physics, 2009, Special STEREO Issue, in pres
Large-scale Bright Fronts in the Solar Corona: A Review of "EIT waves"
``EIT waves" are large-scale coronal bright fronts (CBFs) that were first
observed in 195 \AA\ images obtained using the Extreme-ultraviolet Imaging
Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}.
Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that
propagate pseudo-radially across the solar disk at velocities of 100--700 km
s with front widths of 50-100 Mm. As their speed is greater than the
quiet coronal sound speed (200 km s) and comparable to the
local Alfv\'{e}n speed (1000 km s), they were initially
interpreted as fast-mode magnetoacoustic waves ().
Their propagation is now known to be modified by regions where the magnetosonic
sound speed varies, such as active regions and coronal holes, but there is also
evidence for stationary CBFs at coronal hole boundaries. The latter has led to
the suggestion that they may be a manifestation of a processes such as Joule
heating or magnetic reconnection, rather than a wave-related phenomena. While
the general morphological and kinematic properties of CBFs and their
association with coronal mass ejections have now been well described, there are
many questions regarding their excitation and propagation. In particular, the
theoretical interpretation of these enigmatic events as magnetohydrodynamic
waves or due to changes in magnetic topology remains the topic of much debate.Comment: 34 pages, 19 figure
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