6,644 research outputs found
Thermal conductivity in superconducting borocarbides LuNi2B2C and YNi2B2C
We have recently proposed the s+g wave model for superconducting
borocarbides. In spite of a substantial s-wave component, this order parameter
exhibits the H^1/2 dependent specific heat and a thermal conductivity linear in
H in the vortex state. This is characteristic for nodal superconductors when T,
\Gamma << \Delta where \Gamma is the quasiparticle scattering rate and \Delta
the maximum superconducting gap. Here we investigate the thermal conductivity
parallel to the c- and a- axis in a magnetic field tilted by \theta from the c-
axis and rotating within the a-b plane.Comment: SCES 2002 conference contribution, 2 pages and 2 figure
Gapped optical excitations from gapless phases: imperfect nesting in unconventional density waves
We consider the effect of imperfect nesting in quasi-one-dimensional
unconventional density waves in the case, when the imperfect nesting and the
gap depends on the same wavevector component.
The phase diagram is very similar to that in a conventional density wave. The
density of states is highly asymmetric with respect to the Fermi energy.
The optical conductivity at T=0 remains unchanged for small deviations from
perfect nesting. For higher imperfect nesting parameter, an optical gap opens,
and considerable amount of spectral weight is transferred to higher
frequencies. This makes the optical response of our system very similar to that
of a conventional density wave. Qualitatively similar results are expected in
d-density waves.Comment: 8 pages, 7 figure
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
Detecting z > 10 objects through carbon, nitrogen and oxygen emission lines
By redshift of 10, star formation in the first objects should have produced
considerable amounts of Carbon, Nitrogen and Oxygen. The submillimeter lines of
C, N and O redshift into the millimeter and centimeter bands (0.5 mm -- 1.2
cm), where they may be detectable. High spectral resolution observations could
potentially detect inhomogeneities in C, N and O emission, and see the first
objects forming at high redshift. We calculate expected intensity fluctuations
and discuss frequency and angular resolution required to detect them. For CII
emission, we estimate the intensity using two independent methods: the line
emission coefficient argument and the luminosity density argument. We find they
are in good agreement. At 1+z \sim 10, the typical protogalaxy has a velocity
dispersion of 30 km s^{-1} and angular size of 1 arcsecond. If CII is the
dominant coolant, then we estimate a characteristic line strength of \sim 0.1 K
km s^{-1}. We also discuss other atomic lines and estimate their signal.
Observations with angular resolution of 10^{-3} can detect moderately nonlinear
fluctuations of amplitude 2 \cdot 10^{-5} times the microwave background. If
the intensity fluctuations are detected, they will probe matter density
inhomogeneity, chemical evolution and ionization history at high redshifts.Comment: 15 pages, 1 postscript figures included; Uses aaspp4.sty (AASTeX
v4.0); Submitted to The Astrophysical Journa
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
Gap Symmetry an Thermal Conductivity in Nodal Superconductors
There are now many nodal superconductors in heavy fermion (HF) systems,
charge conjugated organic metals, high Tc cuprates and ruthenates. On the other
hand only few of them have a well established gap function. We present here a
study of the angular dependent thermal conductivity in the vortex state of some
of the nodal superconductors. We hope it will help to identify the nodal
directions in the gap function of UPd_2Al_3, UNi_2Al_3, UBe_13 and URu_2Si_2.Comment: 4 pages, 5 figure
Rumour Processes on N
We study four discrete time stochastic systems on \bbN modeling processes
of rumour spreading. The involved individuals can either have an active or a
passive role, speaking up or asking for the rumour. The appetite in spreading
or hearing the rumour is represented by a set of random variables whose
distributions may depend on the individuals. Our goal is to understand - based
on those random variables distribution - whether the probability of having an
infinite set of individuals knowing the rumour is positive or not
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