3,299 research outputs found
Turing degrees of limit sets of cellular automata
Cellular automata are discrete dynamical systems and a model of computation.
The limit set of a cellular automaton consists of the configurations having an
infinite sequence of preimages. It is well known that these always contain a
computable point and that any non-trivial property on them is undecidable. We
go one step further in this article by giving a full characterization of the
sets of Turing degrees of cellular automata: they are the same as the sets of
Turing degrees of effectively closed sets containing a computable point
Optimized tracking of RF carriers with phase noise, including Pioneer 10 results
The ability to track very weak signals from distant spacecraft is limited by the phase instabilities of the received signal and of the local oscillator employed by the receiver. These instabilities ultimately limit the minimum loop bandwidth that can be used in a phase-coherent receiver, and hence limit the ratio of received carrier power to noise spectral density which can be tracked phase coherently. A method is presented for near real time estimation of the received carrier phase and additive noise spectrum, and optimization of the phase locked loop bandwidth. The method was used with the breadboard Deep Space Network (DSN) Advanced Receiver to optimize tracking of very weak signals from the Pioneer 10 spacecraft, which is now more distant that the edge of the solar system. Tracking with bandwidths of 0.1 Hz to 1.0 Hz reduces tracking signal threshold and increases carrier loop signal to noise ratio (SNR) by 5 dB to 15 dB compared to the 3 Hz bandwidth of the receivers now used operationally in the DSN. This will enable the DSN to track Pioneer 10 until its power sources fails near the end of the century
Resonant tunneling through a small quantum dot coupled to superconducting leads
We address the problem of non-linear transport through discrete electronic
levels in a small quantum dot coupled to superconducting electrodes. In our
approach the low temperature I-V characteristics can be calculated including
all multiple quasi-particle and Andreev processes. The limit of very weak
coupling to the leads and large charging energies is briefly analyzed comparing
the calculated lineshapes of the I-V curves with recent experimental results.
When the coupling to the leads increases and Coulomb blockade effects can be
neglected, the combination of multiple Andreev processes and resonant
transmission gives rise to a rich subgap structure which largely differs from
the one found in the more studied S-N-S systems. We show how multiple processes
can be included within a simple sequential tunneling picture qualitatively
explaining the subgap structure. We suggest an experimental set-up where the
predicted effects could be observed.Comment: 11 pages, 4 postscript figures, to be published in Phys. Rev. B
(rapid communications
Superconducting d-wave junctions: The disappearance of the odd ac components
We study voltage-biased superconducting planar d-wave junctions for arbitrary
transmission and arbitrary orientation of the order parameters of the
superconductors. For a certain orientation of the superconductors the odd ac
components disappear, resulting in a doubling of the Josephson frequency. We
study the sensitivity of this disappearance to orientation and compare with
experiments on grain boundary junctions. We also discuss the possibility of a
current flow parallel to the junction.Comment: 5 pages, 3 figure
Anomalous Fermi Liquid Behavior of Overdoped High-Tc Superconductors
According to a generic temperature vs. carrier-doping (T-p) phase diagram of
high-temperature superconductors it has been proposed that as doping increases
to the overdoped region they approach gradually a conventional (canonical)
Fermi Liquid. However, Hall effect measurements in several systems reported by
different authors show a still strong \emph{T}-dependence in overdoped samples.
We report here electrical transport measurements of
Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-delta} thin films presenting a temperature
dependence of the Hall constant, R_H, which does not present a gradual
transition towards the T-independent behavior of a canonical Fermi Liquid.
Instead, the T-dependence passes by a minimum near optimal doping and then
increases again in the overdoped region. We discuss the theoretical predictions
from two representative Fermi Liquid models and show that they can not give a
satisfactory explanation to our data. We conclude that this region of the phase
diagram in YBCO, as in most HTSC, is not a canonical Fermi Liquid, therefore we
call it Anomalous Fermi Liquid.Comment: 9 pages, 12 figures, to be published in Phys. Rev.
Attractions between charged colloids at water interfaces
The effective potential between charged colloids trapped at water interfaces
is analyzed. It consists of a repulsive electrostatic and an attractive
capillary part which asymptotically both show dipole--like behavior. For
sufficiently large colloid charges, the capillary attraction dominates at large
separations.
The total effective potential exhibits a minimum at intermediate separations
if the Debye screening length of water and the colloid radius are of comparable
size.Comment: 8 pages, 1 figure, revised version (one paragraph added) accepted in
JPC
Orbital magnetization and its effects in spin-chiral ferromagnetic Kagome lattice
Recently, Berry phase in the semiclassical dynamical of Bloch electrons has
been found to make a correction to the phase-space density of states and a
general multi-band formula for finite-temperature orbital magnetization has
been given [Phys. Rev. Lett. \textbf{97}, 026603 (2006)], where the orbital
magnetization consists of two parts, i.e., the conventional part
and the Berry-phase correction part . Using this general
formula, we theoretically investigate the orbital magnetization and its effects
on thermoelectric transport and magnetic susceptibility properties of the
two-dimensional \textit{kagom\'{e}} lattice with spin anisotropies included.
The study in this paper is highly interesting by the occurrence of nonzero
Chern number in the lattice. The spin chirality parameter (see text)
results in profound effects on the orbital magnetization properties. It is
found that the two parts in orbital magnetization opposite each other. In
particular, we show that and yield the paramagnetic and
diamagnetic responses, respectively. It is further shown that the orbital
magnetization displays fully different behavior in the metallic and insulating
regions, which is due to the different roles and play in
these two regions. The anomalous Nernst conductivity is also calculated, which
displays a peak-valley structure as a function of the electron Fermi energy.Comment: 9 pages, 7 figure
Running coupling expansion for the renormalized -trajectory from renormalization invariance
We formulate a renormalized running coupling expansion for the
--function and the potential of the renormalized --trajectory on
four dimensional Euclidean space-time. Renormalization invariance is used as a
first principle. No reference is made to bare quantities. The expansion is
proved to be finite to all orders of perturbation theory. The proof includes a
large momentum bound on the connected free propagator amputated vertices.Comment: 14 pages LaTeX2e, typos and references correcte
Berry's phase contribution to the anomalous Hall effect of gadolinium
When conduction electrons are forced to follow the local spin texture, the
resulting Berry phase can induce an anomalous Hall effect (AHE). In gadolinium,
as in double-exchange magnets, the exchange interaction is mediated by the
conduction electrons and the AHE may therefore resemble that of chromium
dioxide and other metallic double-exchange ferromagnets. The Hall resistivity,
magnetoresistance, and magnetization of single crystal gadolinium were measured
in fields up to 30 T. Measurements between 2 K and 400 K are consistent with
previously reported data. A scaling analysis for the Hall resistivity as a
function of the magnetization suggests the presence of a Berry's-phase
contribution to the anomalous Hall effect.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
A precursor state to unconventional superconductivity in CeIrIn
We present sensitive measurements of the Hall effect and magnetoresistance in
CeIrIn down to temperatures of 50 mK and magnetic fields up to 15 T. The
presence of a low temperature coherent Kondo state is established. Deviations
from Kohler's rule and a quadratic temperature dependence of the cotangent of
the Hall angle are reminiscent of properties observed in the high temperature
superconducting cuprates. The most striking observation pertains to the
presence of a \textit{precursor} state--characterized by a change in the Hall
mobility--that appears to precede the superconductivity in this material, in
similarity to the pseudogap in the cuprate high superconductors.Comment: 4 figure
- …