629 research outputs found
Reply to the "Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy"
The "Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational
Wave Astronomy" [arXiv:1104.1223] is misleading and premised on a
misinterpretation of the main content of Phys. Rev. Lett. 106, 101101 (2011)
[arXiv:1102.3434]. The main claim in the comment - that in some exotic theories
different from general relativity (GR) there might be local degrees of freedom
even in lower dimensional spaces - is trivial. Nevertheless, the authors of the
Comment fail to come-up with a single self-consistent example. This claim,
however, has no implications for our paper, in which we make it clear we are
working within the framework of "vanishing" or "evolving" dimensions as defined
in arXiv:1003.5914.Comment: Accepted for publication in Phys. Rev. Let
Information-preserving black holes still do not preserve baryon number and other effective global quantum numbers
It has been claimed recently that the black hole information-loss paradox has
been resolved: the evolution of quantum states in the presence of a black hole
is unitary and information preserving. We point out that, contrary to some
claims in literature, information-preserving black holes still violate baryon
number and any other quantum number which follows from an effective (and thus
approximate) or anomalous symmetry.Comment: Honorable Mention on Gravity Essay Competition 2005; Published in the
special Essay issue of Int.J.Mod.Phy
A Vertex Correction in the Gap Equation for the High Temperature Superconductors
We show that the Migdal theorem is obviously violated in the high Tc cuprates
and the vertex correction should be included, in particular, in the gap
equation, in order to be consistent with the anomalously strong inelastic
scattering in the ``hot spots'', which is observed from the various normal
state experiments. The vertex correction is obtained by utilizing the
generalized Ward identity, which is shown to hold in the important scattering
channel for the pairing interaction in the high Tc cuprates. As a result, we
find a strong enhancement of Tc from the vertex correction despite of the
strong pair breaking effect due to the inelastic scattering.Comment: 5 pages, 2 figure
Toroidal equilibria in spherical coordinates
The standard Grad-Shafranov equation for axisymmetric toroidal plasma
equilibrium is customary expressed in cylindrical coordinates with toroidal
contours, and through which benchmark equilibria are solved. An alternative
approach to cast the Grad-Shafranov equation in spherical coordinates is
presented. This equation, in spherical coordinates, is examined for toroidal
solutions to describe low Solovev and high plasma equilibria in
terms of elementary functions
A Theory of the Longitudinal and Hall Conductivities of the Cuprate Superconductors
We establish the applicability to transport phenomena in the cuprate
superconductors of a nearly antiferromagnetic Fermi liquid (NAFL) description
of the magnetic interaction between planar quasiparticles by using it to obtain
the temperature dependent resistivity and Hall conductivity seen experimentally
in the normal state. Following a perturbative calculation of the anisotropic
(as one goes around the Fermi surface) quasiparticle lifetimes which are the
hallmark of a NAFL, we obtain simple approximate expressions for the
longitudinal, , and Hall, , conductivities which
reflect the magnetic crossovers seen experimentally as one varies the doping
level and temperature. We present a simple phenomenological model for the
variation in mean free path around the Fermi surface, and use this to extract
from experiments on and quasiparticle lifetimes in
the hot (strongly coupled quasiparticle) and cold (weakly coupled
quasiparticle) regions of the Fermi surface which are consistent with the
perturbation theory estimates. We improve upon the latter by carrying out
direct numerical (non-variational) solutions of the Boltzmann equation for
representative members of the YBaCuO and
LaSrCuO systems, with results for transport properties in
quantitative agreement with experiment. Using the same numerical approach we
study the influence of CuO chains on the a-b plane anisotropy and find results
in agreement with experimental findings in YBaCuO.Comment: 49 pages + 24 PostScript figure
Geostationary payload concepts for personal satellite communications
This paper reviews candidate satellite payload architectures for systems providing world-wide communication services to mobile users equipped with hand-held terminals based on large geostationary satellites. There are a number of problems related to the payload architecture, on-board routing and beamforming, and the design of the S-band Tx and L-band Rx antenna and front ends. A number of solutions are outlined, based on trade-offs with respect to the most significant performance parameters such as capacity, G/T, flexibility of routing traffic to beams and re-configuration of the spot-beam coverage, and payload mass and power. Candidate antenna and front-end configurations were studied, in particular direct radiating arrays, arrays magnified by a reflector and active focused reflectors with overlapping feed clusters for both transmit (multimax) and receive (beam synthesis). Regarding the on-board routing and beamforming sub-systems, analog techniques based on banks of SAW filters, FET or CMOS switches and cross-bar fixed and variable beamforming are compared with a hybrid analog/digital approach based on Chirp Fourier Transform (CFT) demultiplexer combined with digital beamforming or a fully digital processor implementation, also based on CFT demultiplexing
Statistics of charged solitons and formation of stripes
The 2-fold degeneracy of the ground state of a quasi-one-dimensional system
allows it to support topological excitations such as solitons. We study the
combined effects of Coulomb interactions and confinement due to interchain
coupling on the statistics of such defects. We concentrate on a 2D case which
may correspond to monolayers of polyacetylene or other charge density waves.
The theory is developped by a mapping to the 2D Ising model with long-range
4-spin interactions. The phase diagram exhibits deconfined phases for liquids
and Wigner crystals of kinks and confined ones for bikinks. Also we find
aggregated phases with either infinite domain walls of kinks or finite rods of
bikinks. Roughening effects due to both temperature and Coulomb repulsion are
observed. Applications may concern the melting of stripes in doped correlated
materials.Comment: 16 pages, 7 figure
Non-Universal Power Law of the "Hall Scattering Rate" in a Single-Layer Cuprate Bi_{2}Sr_{2-x}La_{x}CuO_{6}
In-plane resistivity \rho_{ab}, Hall coefficient, and magnetoresistance (MR)
are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals
with various carrier concentrations, from underdope to overdope. Our crystals
show the highest T_c (33 K) and the smallest residual resistivity ever reported
for Bi-2201 at optimum doping. It is found that the temperature dependence of
the Hall angle obeys a power law T^n with n systematically decreasing with
increasing doping, which questions the universality of the Fermi-liquid-like
T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of
the optimally-doped sample changes as T^{1.7}, not as T^2, while \rho_{ab}
shows a good T-linear behavior. The systematics of the MR indicates an
increasing role of spin scattering in underdoped samples.Comment: 4 pages, 5 figure
Transport Anomalies and the Role of Pseudogap in the "60-K Phase" of YBa_{2}Cu_{3}O_{7-\delta}
We report the result of our accurate measurements of the a- and b-axis
resistivity, Hall coefficient, and the a-axis thermopower in untwinned
YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping. It is found that
both the a-axis resistivity and the Hall conductivity show anomalous
dependences on the oxygen content y in the "60-K phase" below the pseudogap
temperature T^*. The complete data set enables us to narrow down the possible
pictures of the 60-K phase, with which we discuss a peculiar role of the
pseudogap in the charge transport.Comment: 4 pages, 4 figures, accepted for publication in PR
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