5,474 research outputs found
Impurity scattering in unconventional density waves: non-crossing approximation for arbitrary scattering rate
We present a detailed theoretical study on the thermodynamic properties of
impure quasi-one dimensional unconventional charge-, and spin-density waves in
the framework of mean-field theory. The impurities are of the ordinary
non-magnetic type. Making use of the full self-energy that takes into account
all ladder-, and rainbow-type diagrams, we are able to calculate the relevant
low temperature quantities for arbitrary impurity concentration and scattering
rates. These are the density of states, specific heat and the shift in the
chemical potential. Our results therefore cover the whole parameter space: they
include both the self-consistent Born and the resonant unitary limits, and most
importantly give exact results in between.Comment: 11 pages, 8 figure
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
Enhanced Coherence of Antinodal Quasiparticles in a Dirty d-wave Superconductor
Recent ARPES experiments show a narrow quasiparticle peak at the gap edge
along the antinodal [1,0]-direction for the overdoped cuprate superconductors.
We show that within weak coupling BCS theory for a d-wave superconductor the
s-wave single-impurity scattering cross section vanishes for energies of the
gap edge. This coherence effect occurs through multiple scattering off the
impurity. For small impurity concentrations the spectral function has a
pronounced increase of the (scattering) lifetime for antinodal quasiparticles
but shows a very broad peak in the nodal direction, in qualitative agreement
with experiment and in strong contrast to the behavior observed in underdoped
cuprates.Comment: 4 pages, 3 figures, submitte
Pairbreaking Without Magnetic Impurities in Disordered Superconductors
We study analytically the effects of inhomogeneous pairing interactions in
short coherence length superconductors, using a spatially varying
Bogoliubov-deGennes model. Within the Born approximation, it reproduces all of
the standard Abrikosov-Gor'kov pairbreaking and gaplessness effects, even in
the absence of actual magnetic impurities. For pairing disorder on a single
site, the T-matrix gives rise to bound states within the
BCS gap. Our results are compared with recent scanning tunneling microscopy
measurements on BiSrCaCuO with Zn or Ni impurities.Comment: 4 pages, 2 figures, submitted to PR
Control structures for high speed processors
A special processor was designed to function as a Reed Solomon decoder with throughput data rate in the Mhz range. This data rate is significantly greater than is possible with conventional digital architectures. To achieve this rate, the processor design includes sequential, pipelined, distributed, and parallel processing. The processor was designed using a high level language register transfer language. The RTL can be used to describe how the different processes are implemented by the hardware. One problem of special interest was the development of dependent processes which are analogous to software subroutines. For greater flexibility, the RTL control structure was implemented in ROM. The special purpose hardware required approximately 1000 SSI and MSI components. The data rate throughput is 2.5 megabits/second. This data rate is achieved through the use of pipelined and distributed processing. This data rate can be compared with 800 kilobits/second in a recently proposed very large scale integration design of a Reed Solomon encoder
Phytoplankton Community and Algal Toxicity at a Recurring Bloom in Sullivan Bay, Kabetogama Lake, Minnesota, USA
Kabetogama Lake in Voyageurs National Park, Minnesota, USA suffers from recurring late summer algal blooms that often contain toxin-producing cyanobacteria. Previous research identified the toxin microcystin in blooms, but we wanted to better understand how the algal and cyanobacterial community changed throughout an open water season and how changes in community structure were related to toxin production. Therefore, we sampled one recurring bloom location throughout the entire open water season. The uniqueness of this study is the absence of urban and agricultural nutrient sources, the remote location, and the collection of samples before any visible blooms were present. Through quantitative polymerase chain reaction (qPCR), we discovered that toxin-forming cyanobacteria were present before visible blooms and toxins not previously detected in this region (anatoxin-a and saxitoxin) were present, indicating that sampling for additional toxins and sampling earlier in the season may be necessary to assess ecosystems and human health risk
Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas
We theoretically investigate a possible idea to introduce magnetic impurities
to a superfluid Fermi gas. In the presence of population imbalance
(, where is the number of Fermi atoms with
pseudospin ), we show that nonmagnetic potential
scatterers embedded in the system are magnetized in the sense that some of
excess -spin atoms are localized around them. They destroy the
superfluid order parameter around them, as in the case of magnetic impurity
effect discussed in the superconductivity literature. This pair-breaking effect
naturally leads to localized excited states below the superfluid excitation
gap. To confirm our idea in a simply manner, we treat an attractive Fermi
Hubbard model within the mean-field theory at T=0. We self-consistently
determine superfluid properties around a nonmagnetic impurity, such as the
superfluid order parameter, local population imbalance, as well as
single-particle density of states, in the presence of population imbalance.
Since the competition between superconductivity and magnetism is one of the
most fundamental problems in condensed matter physics, our results would be
useful for the study of this important issue in cold Fermi gases.Comment: 27 pages, 14 figure
Engineering out the risk for infection with urinary catheters.
Catheter-associated urinary tract infection (CAUTI) is the most common nosocomial infection. Each year, more than 1 million patients in U.S. acute-care hospitals and extended-care facilities acquire such an infection; the risk with short-term catheterization is 5% per day. CAUTI is the second most common cause of nosocomial bloodstream infection, and studies suggest that patients with CAUTI have an increased institutional death rate, unrelated to the development of urosepsis. Novel urinary catheters impregnated with nitrofurazone or minocycline and rifampin or coated with a silver alloy-hydrogel exhibit antiinfective surface activity that significantly reduces the risk of CAUTI for short-term catheterizations not exceeding 2-3 weeks
Sound propagation in density wave conductors and the effect of long-range Coulomb interaction
We study theoretically the sound propagation in charge- and spin-density
waves in the hydrodynamic regime. First, making use of the method of comoving
frame, we construct the stress tensor appropriate for quasi-one dimensional
systems within tight-binding approximation. Taking into account the screening
effect of the long-range Coulomb interaction, we find that the increase of the
sound velocity below the critical temperature is about two orders of magnitude
less for longitudinal sound than for transverse one. It is shown that only the
transverse sound wave with displacement vector parallel to the chain direction
couples to the phason of the density wave, therefore we expect significant
electromechanical effect only in this case.Comment: revtex, 14 pages (in preprint form), submitted to PR
Interplay of paramagnetic, orbital and impurity effects on the phase transition of a normal metal to superconducting state
We derive the generalized Ginzburg-Landau free energy functional for
conventional and unconventional singlet superconductors in the presence of
paramagnetic, orbital and impurity effects. Within the mean field theory, we
determine the criterion for appearence of the non uniform
(Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state, with vortex lattice
structure and additional modulation along the magnetic field. We also discuss
the possible change of the order of transition from normal to superconducting
state. We find that the superconducting phase diagram is very sensitive to
geometrical effects such as the nature of the order parameter and the shape of
the Fermi surface. In particular, we obtain the qualitative phase diagrams for
three-dimensional isotropic s-wave superconductors and in quasi two-dimensional
d-wave superconductors under magnetic field perpendicular to the conducting
layers.
In addition, we determine the criterion for instability toward non uniform
superconducting state in s-wave superconductors in the dirty limit.Comment: 15 pages, 4 figure
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