1,903 research outputs found
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Low Resolution Limits and Inaccurate Algorithms Decrease Significantly the Value of Late Loss in Current Drug-Eluting Stent Trials
Quantitative coronary and vascular angiography (QCA resp., QVA) remains the current gold standard for evaluation of restenosis. Late loss as one of the most commonly accepted parameters to highlight efficacy of the various devices has shown high correlation to clinical parameters but, surprisingly, has no impact on the evaluation of the remaining amount of restenostic tissue. The current clinical practice leads to unrealistic late loss calculations. Smaller late loss differences are usually not greater than the inherited resolution limits of QCA, which is especially the case in small differences between the various stents in the drug-eluting stent era. Late loss include additional systematic and random errors, due to the fact that measurements were taken at two different time points including the inherited resolution and calibration limits of QCA on two occasions. Due to the limited value of late loss in discriminating the small differences between the one and other DES, late lumen area loss and clearly defined calculation algorithms (e.g., MLD-relocation) should be used in future DES studies also to fulfill the more stringent regulatory requirements
Influence of Fermi surface topology on the quasiparticle spectrum in the vortex state
We study the influence of Fermi surface topology on the quasiparticle density
of states in the vortex state of type II superconductors. We observe that the
field dependence and the shape of the momentum and spatially averaged density
of states is affected significantly by the topology of the Fermi surface. We
show that this behavior can be understood in terms of characteristic Fermi
surface functions and that an important role is played by the number of points
on the Fermi surface at which the Fermi velocity is directed parallel to the
magnetic field. A critical comparison is made with a broadened BCS type density
of states, that has been used frequently in analysis of tunneling data. We
suggest a new formula as a replacement for the broadened BCS model for the
special case of a cylindrical Fermi surface. We apply our results to the two
gap superconductor MgB and show that in this particular case the field
dependence of the partial densities of states of the two gaps behaves very
differently due to the different topologies of the corresponding Fermi
surfaces, in qualitative agreement with recent tunneling experiments.Comment: 12 pages 12 figure
Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO
We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle
tunneling spectra of thin film grain-boundary Josephson junctions made of the
electron doped cuprate superconductor LaCeCuO. An applied magnetic field
reduces the spectral weight around zero energy and shifts it non-linearly to
higher energies consistent with a Doppler shift of the Andreev bound states
(ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the
onset of superconductivity. These observations strongly suggest that the ZBCP
results from the formation of ABS at the junction interfaces, and,
consequently, that there is a sign change in the symmetry of the
superconducting order parameter of this compound consistent with a d-wave
symmetry.Comment: 9 pages, 7 figures; December 2004, accepted for publication in Phys.
Rev.
Anomalous microwave conductivity coherence peak in c-axis MgB2 thin film
The temperature dependence of the real part of the microwave complex
conductivity at 17.9 GHz obtained from surface impedance measurements of two
c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature
around 0.6 times the critical temperature. Calculations in the frame of a
two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that
this maximum corresponds to an anomalous coherence peak resembling the two-gap
nature of MgB2. Our model assumes there is no interband impurity scattering and
a weak interband pairing interaction, as suggested by bandstructure
calculations. In addition, the observation of a coherence peak indicates that
the pi-band is in the dirty limit and dominates the total conductivity of our
filmsComment: 10 pages, 4 figures, to be published in Phys. Rev. Let
Four-dimensional laser induced fluorescence study of the structure of molecular mixing in turbulent flows
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77075/1/AIAA-1994-820-515.pd
Direct, high resolution, fourâdimensional measurements of the fine scale structure of Scâ«1 molecular mixing in turbulent flows
Results from highly resolved, fourâdimensional measurements of the fine structure of the fully spaceâ and timeâvarying Scâ«1 conserved scalar field and the associated scalar energy dissipation rate field in a turbulent flow are presented. The resolution achieved in all three spatial dimensions and in time reaches down to the local strainâlimited molecular diffusion scale in the flow, allowing all three components of the instantaneous scalar gradient vector field âζ(x,t) and their time evolution at every point in the data space to be directly evaluated. Results are presented in the form of fine structure maps of the instantaneous dissipation field logeââζâ
âζ(x,t) in several spatially adjacent data planes within an individual threeâdimensional spatial data volume, as well as in several temporally successive data planes from a sequence of such threeâdimensional data volumes. The degree of anisotopy in the underlying scalar gradient field is characterized in terms of the joint distribution ÎČ(Ï,Ï) of spherical orientation angles. The probability density of true scalar energy dissipation rates is presented and compared with the distributions that would result from lowerâdimensional measurements of the scalar gradient vector. From this the ââspottinessââ of the scalar dissipation field is directly quantified by determining the true fraction of the total dissipation that occurs in any given volume fraction of the flow.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70449/2/PFADEB-3-5-1115-1.pd
A scalar imaging velocimetry technique for fully resolved fourâdimensional vector velocity field measurements in turbulent flows
This paper presents an experimental technique for obtaining fully resolved measurements of the vector velocity field u(x,t) throughout a fourâdimensional spatiotemporal region in a turbulent flow. The method uses fully resolved fourâdimensional scalar field imaging measurements in turbulent flows [Phys. Fluids A 3, 1115 (1991)] to extract the underlying velocity field from the exact conserved scalar transport equation. A procedure for accomplishing this is described, and results from a series of test cases are presented. These involve synthetically generated scalar fields as well as actual measured turbulent flow scalar fields advected numerically by various imposed flow fields. The imposed velocity fields are exactly known, allowing a careful validation of the technique and its potential accuracy. Results obtained from a zeroth iteration of the technique are found to be very close to the exact underlying vector velocity field. Further results show that successive iterations bring the velocity field from the zeroth iteration even closer to the exact result. It is also shown that the comparatively dense velocity field information that this technique provides is well suited for accurate extraction of the more dynamically insightful strain rate and vorticity fields Ï”(x,t) and Ï(x,t).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69930/2/PFADEB-4-10-2191-1.pd
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
Radiation linewidth of a long Josephson junction in the flux-flow regime
Theoretical model for the radiation linewidth in a multi-fluxon state of a
long Josephson junction is presented. Starting from the perturbed sine-Gordon
model with the temperature dependent noise term, we develop a collective
coordinate approach which allows to calculate the finite radiation linewidth
due to excitation of the internal degrees of freedom in the moving fluxon
chain. At low fluxon density, the radiation linewidth is expected to be
substantially larger than that of a lumped Josephson oscillator. With
increasing the fluxon density, a crossover to a much smaller linewidth
corresponding to the lumped oscillator limit is predicted.Comment: 11 pages LaTeX, to appear in Phys Rev
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