17,327 research outputs found
A new bound and algorithm for star 16-QAM carrier phase estimation
Copyright © 2003 IEEEThe true Cramer-Rao lower bound (CRLB) is derived and evaluated for the estimation of carrier phase of Star 16-quadrature amplitude modulation (QAM) and can be simply applied to carrier frequency estimation. Different geometries are investigated by varying the ring ratio (RR). For signal-to-noise ratios (SNRs) between 6-15 dB, the CRLB with RR=3 is lower than that of Square 16-QAM. A modified phase estimator is presented, which closely follows the new CRLB. Investigation of symbol error performance in short packet length reveals Star 16-QAM to be superior to Square 16-QAM for SNR<13 dB, which is a reasonable operating range for a coded system. Although Square 16-QAM and Star RR=1.8 are optimum for a perfect receiver, when the effect of phase estimation is considered, we find Star RR=3 to be better for SNR below 10 dB.Feng Rice, Mark Rice, and Bill Cowle
Superconductivity generated by coupling to a Cooperon in a 2-dimensional array of 4-leg Hubbard ladders
Starting from an array of four-leg Hubbard ladders weakly doped away from
half-filling and weakly coupled by inter-ladder tunneling, we derive an
effective low energy model which contains a partially truncated Fermi surface
and a well defined Cooperon excitation formed by a bound pair of holes. An
attractive interaction in the Cooper channel is generated on the Fermi surface
through virtual scattering into the Cooperon state. Although the model is
derived in the weak coupling limit of a four-leg ladder array, an examination
of exact results on finite clusters for the strong coupling t-J model suggests
the essential features are also present for a strong coupling Hubbard model on
a square lattice near half-filling.Comment: 20 pages, 4 figure
The role of the energy equation in the fragmentation of protostellar discs during stellar encounters
In this paper, we use high-resolution smoothed particle hydrodynamics (SPH)
simulations to investigate the response of a marginally stable self-gravitating
protostellar disc to a close parabolic encounter with a companion discless
star. Our main aim is to test whether close brown dwarfs or massive planets can
form out of the fragmentation of such discs. We follow the thermal evolution of
the disc by including the effects of heating due to compression and shocks and
a simple prescription for cooling and find results that contrast with previous
isothermal simulations. In the present case we find that fragmentation is
inhibited by the interaction, due to the strong effect of tidal heating, which
results in a strong stabilization of the disc. A similar behaviour was also
previously observed in other simulations involving discs in binary systems. As
in the case of isolated discs, it appears that the condition for fragmentation
ultimately depends on the cooling rate.Comment: 9 pages, 10 figures, accepted in MNRA
Quantum Oscillations in Magnetic Field Induced Antiferromagnetic Phase of Underdoped Cuprates : Application to Ortho-II YBa2Cu3O6.5
Magnetic field induced antiferromagnetic phase of the underdoped cuprates is
studied within the t-t'-J model. A magnetic field suppresses the pairing
amplitude, which in turn may induce antiferromagnetism. We apply our theory to
interpret the recently reported quantum oscillations in high magnetic field in
ortho-II YBa2Cu3O6.5 and propose that the total hole density abstracted from
the oscillation period is reduced by 50% due to the antiferromagnetism.Comment: 5 pages, 3 figure
Interlayer couplings and the coexistence of antiferromagnetic and d-wave pairing order in multilayer cuprates
A more extended low density region of coexisting uniform antiferromagnetism
and d-wave superconductivity has been reported in multilayer cuprates, when
compared to single or bilayer cuprates. This coexistence could be due to the
enhanced screening of random potential modulations in inner layers or to the
interlayer Heisenberg and Josephson couplings. A theoretical analysis using a
renormalized mean field theory, favors the former explanation. The potential
for an improved determination of the antiferromagnetic and superconducting
order parameters in an ideal single layer from zero field NMR and infrared
Josephson plasma resonances in multilayer cuprates is discussed.Comment: 6 pages, 2 figure
Random wave functions and percolation
Recently it was conjectured that nodal domains of random wave functions are
adequately described by critical percolation theory. In this paper we
strengthen this conjecture in two respects. First, we show that, though wave
function correlations decay slowly, a careful use of Harris' criterion confirms
that these correlations are unessential and nodal domains of random wave
functions belong to the same universality class as non critical percolation.
Second, we argue that level domains of random wave functions are described by
the non-critical percolation model.Comment: 13 page
Placing Confidence Limits on Polarization Measurements
The determination of the true source polarization given a set of measurements
is complicated by the requirement that the polarization always be positive.
This positive bias also hinders construction of upper limits, uncertainties,
and confidence regions, especially at low signal-to-noise levels. We generate
the likelihood function for linear polarization measurements and use it to
create confidence regions and upper limits. This is accomplished by integrating
the likelihood function over the true polarization (parameter space), rather
than the measured polarization (data space). These regions are valid for both
low and high signal-to-noise measurements.Comment: 8 pages, 3 figures, 1 table, submitted to PAS
Coherent open-loop optimal control of light-harvesting dynamics
We apply theoretically open-loop quantum optimal control techniques to
provide methods for the verification of various quantum coherent transport
mechanisms in natural and artificial light-harvesting complexes under realistic
experimental constraints. We demonstrate that optimally shaped laser pulses
allow to faithfully prepare the photosystem in specified initial states (such
as localized excitation or coherent superposition, i.e. propagating and
non-propagating states) and to probe efficiently the dynamics. These results
provide a path towards the discrimination of the different transport pathways
and to the characterization of environmental properties, enhancing our
understanding of the role that coherent processes may play in biological
complexes.Comment: 12 pages, 15 figure
Rotational Symmetry Breaking in Sodium Doped Cuprates
For reasonable parameters a hole bound to a Na^{+} acceptor in
Ca_{2-x}Na_{x}CuO_{2}Cl_{2} has a doubly degenerate ground state whose
components can be represented as states with even (odd) reflection symmetry
around the x(y) -axes. The conductance pattern for one state is anisotropic as
the tip of a tunneling microscope scans above the Cu-O-Cu bonds along the
x(y)-axes. This anisotropy is pronounced at lower voltages but is reduced at
higher voltages. Qualitative agreement with recent experiments leads us to
propose this effect as an explanation of the broken local rotational symmetry.Comment: 10 pages, 4 figure
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