168,547 research outputs found
Ginzburg-Landau Polynomials and the Asymptotic Behavior of the Magnetization Near Critical and Tricritical Points
For the mean-field version of an important lattice-spin model due to Blume
and Capel, we prove unexpected connections among the asymptotic behavior of the
magnetization, the structure of the phase transitions, and a class of
polynomials that we call the Ginzburg-Landau polynomials. The model depends on
the parameters n, beta, and K, which represent, respectively, the number of
spins, the inverse temperature, and the interaction strength. Our main focus is
on the asymptotic behavior of the magnetization m(beta_n,K_n) for appropriate
sequences (beta_n,K_n) that converge to a second-order point or to the
tricritical point of the model and that lie inside various subsets of the
phase-coexistence region. The main result states that as (beta_n,K_n) converges
to one of these points (beta,K), m(beta_n,K_n) ~ c |beta - beta_n|^gamma --> 0.
In this formula gamma is a positive constant, and c is the unique positive,
global minimum point of a certain polynomial g that we call the Ginzburg-Landau
polynomial. This polynomial arises as a limit of appropriately scaled
free-energy functionals, the global minimum points of which define the
phase-transition structure of the model. For each sequence (beta_n,K_n) under
study, the structure of the global minimum points of the associated
Ginzburg-Landau polynomial mirrors the structure of the global minimum points
of the free-energy functional in the region through which (beta_n,K_n) passes
and thus reflects the phase-transition structure of the model in that region.
The properties of the Ginzburg-Landau polynomials make rigorous the predictions
of the Ginzburg-Landau phenomenology of critical phenomena, and the asymptotic
formula for m(beta_n,K_n) makes rigorous the heuristic scaling theory of the
tricritical point.Comment: 70 pages, 8 figure
Quickest Sequence Phase Detection
A phase detection sequence is a length- cyclic sequence, such that the
location of any length- contiguous subsequence can be determined from a
noisy observation of that subsequence. In this paper, we derive bounds on the
minimal possible in the limit of , and describe some sequence
constructions. We further consider multiple phase detection sequences, where
the location of any length- contiguous subsequence of each sequence can be
determined simultaneously from a noisy mixture of those subsequences. We study
the optimal trade-offs between the lengths of the sequences, and describe some
sequence constructions. We compare these phase detection problems to their
natural channel coding counterparts, and show a strict separation between the
fundamental limits in the multiple sequence case. Both adversarial and
probabilistic noise models are addressed.Comment: To appear in the IEEE Transactions on Information Theor
Unified perspective on proteins: A physics approach
We study a physical system which, while devoid of the complexity one usually
associates with proteins, nevertheless displays a remarkable array of
protein-like properties. The constructive hypothesis that this striking
resemblance is not accidental leads not only to a unified framework for
understanding protein folding, amyloid formation and protein interactions but
also has implications for natural selection.Comment: 26 pages, 15 figures, to appear on Phys. Rev.
Temperature phase transition and an effective expansion parameter in the O(N)-model
The temperature phase transition in the N-component scalar field theory with
spontaneous symmetry breaking is investigated in the perturbative approach. The
second Legendre transform is used together with the consideration of the gap
equations in the extrema of the free energy. Resummations are performed on the
super daisy level and beyond. The phase transition turns out to be weakly of
first order. The diagrams beyond the super daisy ones which are calculated
correspond to next-to-next-to-leading order in 1/N. It is shown that these
diagrams do not alter the phase transition qualitatively. In the limit N goes
to infinity the phase transition becomes second order. A comparison with other
approaches is done.Comment: 28 pages, 5 figures, corrected for some misprints, unnecessary
section remove
Oscillatory convective modes in red giants: a possible explanation of the long secondary periods
We discuss properties of oscillatory convective modes in low-mass red giants,
and compare them with observed properties of the long secondary periods (LSPs)
of semi-regular red giant variables. Oscillatory convective modes are very
nonadiabatic g modes and they are present in luminous stars, such as red
giants with \log L/{\rm L}_\odot \ga 3. Finite amplitudes for these modes are
confined to the outermost nonadiabatic layers, where the radiative energy flux
is more important than the convective energy flux. The periods of oscillatory
convection modes increase with luminosity, and the growth times are comparable
to the oscillation periods. The LSPs of red giants in the Large Magellanic
Cloud (LMC) are observed to lie on a distinct period-luminosity sequence called
sequence D. This sequence D period-luminosity relation is roughly consistent
with the predictions for dipole oscillatory convective modes in AGB models if
we adopt a mixing length of 1.2 pressure scale height ().
However, the effective temperature of the red-giant sequence of the LMC is
consistent to models with , which predict periods too short by a
factor of two.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Differential Phase Estimation with the SeaMARC II Bathymetric Sidescan Sonar System
A maximum-likelihood estimator is used to extract differential phase measurements from noisy seafloor echoes received at pairs of transducers mounted on either side of the SeaMARC II bathymetricsidescan sonar system. Carrier frequencies for each side are about 1 kHz apart, and echoes from a transmitted pulse 2 ms long are analyzed. For each side, phase difference sequences are derived from the full complex data consisting of base-banded and digitized quadrature components of the received echoes. With less bias and a lower variance, this method is shown to be more efficient than a uniform mean estimator. It also does not exhibit the angular or time ambiguities commonly found in the histogram method used in the SeaMARC II system. A figure for the estimation uncertainty of the phasedifference is presented, and results are obtained for both real and simulated data. Based on this error estimate and an empirical verification derived through coherent ping stacking, a single filter length of 100 ms is chosen for data processing application
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