3,838 research outputs found
Berry phases in superconducting transitions
I generalize the concept of Berry's geometrical phase for quasicyclic
Hamiltonians to the case in which the ground state evolves adiabatically to an
excited state after one cycle, but returns to the ground state after an integer
number of cycles. This allows to extend the charge Berry phase gamma_c related
to the macroscopic polarization, to many-body systems with fractional number of
particles per site. Under certain conditions, gamma_c and the spin Berry phase
gamma_s jump in pi at the boundary of superconducting phases. In the extended
Hubbard chain with on-site attraction U and nearest-neighbor interaction V at
quarter filling, the transitions detected agree very well with exact results in
two limits solved by the Bethe ansatz, and with previous numerical studies. In
chains with spin SU(2) symmetry, gamma_s jumps when a spin gap opens.Comment: 5 pages, 3 figures, accepted in Europhys. Let
A spatial scan statistic for zero-inflated Poisson process
The scan statistic is widely used in spatial cluster detection applications
of inhomogeneous Poisson processes. However, real data may present substantial
departure from the underlying Poisson process. One of the possible departures
has to do with zero excess. Some studies point out that when applied to data
with excess zeros, the spatial scan statistic may produce biased inferences. In
this work, we develop a closed-form scan statistic for cluster detection of
spatial zero-inflated count data. We apply our methodology to simulated and
real data. Our simulations revealed that the Scan-Poisson statistic steadily
deteriorates as the number of zeros increases, producing biased inferences. On
the other hand, our proposed Scan-ZIP and Scan-ZIP+EM statistics are, most of
the time, either superior or comparable to the Scan-Poisson statistic
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Linkage disequilibrium in natural populations of Trypanosoma cruzi (flagellate), the agent of Chagas's disease
We have studied linkage disequilibrium in natural populations of Trypanosoma cruzi, the agent of Chagas' disease, by analyzing (i) a set of 524 stocks from the whole geographical range of the parasite, characterized at four gene loci coding for enzymes; (ii) a subsample of 121 stocks characterized at 12 enzyme loci; and (iii) a subset of 386 stocks from six locations in Bolivia, characterized by four enzyme loci. Our results show that the linkage disequilibrium reaches the maximum possible value, given the observed allelic frequencies, for almost all the locus pairs. This result is most consistent with the hypothesis that genetic recombination is absent or very rare in T. cruzi natural populations. Partition of the linkage disequilibrium variance for the six Bolivian populations shows that both inter- and intrapopulation components are substantial and that the relationships among the components are D2IS less than D2ST, and D'2IS less than D'2ST. These inequalities are interpreted as the result of an interplay between genetic drift, rare or absent mating, and clonal selection in generating linkage disequilibrium in T. cruzi populations
Temperature in nonequilibrium systems with conserved energy
We study a class of nonequilibrium lattice models which describe local
redistributions of a globally conserved energy. A particular subclass can be
solved analytically, allowing to define a temperature T_{th} along the same
lines as in the equilibrium microcanonical ensemble. The
fluctuation-dissipation relation is explicitely found to be linear, but its
slope differs from the inverse temperature T_{th}^{-1}. A numerical
renormalization group procedure suggests that, at a coarse-grained level, all
models behave similarly, leading to a two-parameter description of their
macroscopic properties.Comment: 4 pages, 1 figure, final versio
Targeting Oxidative Stress and Aberrant Critical Period Plasticity in the Developmental Trajectory to Schizophrenia
Schizophrenia is a neurodevelopmental disorder reflecting a convergence of genetic risk and early life stress. The slow progression to first psychotic episode represents both a window of vulnerability as well as opportunity for therapeutic intervention. Here, we consider recent neurobiological insight into the cellular and molecular components of developmental critical periods and their vulnerability to redox dysregulation. In particular, the consistent loss of parvalbumin-positive interneuron (PVI) function and their surrounding perineuronal nets (PNNs) as well as myelination in patient brains is consistent with a delayed or extended period of circuit instability. This linkage to critical period triggers (PVI) and brakes (PNN, myelin) implicates mistimed trajectories of brain development in mental illness. Strategically introduced antioxidant treatment or later reinforcement of molecular brakes may then offer a novel prophylactic psychiatr
Computation of dynamical correlation functions of Heisenberg chains in a field
We compute the momentum- and frequency-dependent longitudinal spin structure
factor for the one-dimensional spin-1/2 Heisenberg spin chain in a
magnetic field, using exact determinant representations for form factors on the
lattice. Multiparticle contributions are computed numerically throughout the
Brillouin zone, yielding saturation of the sum rule to high precision.Comment: 4 pages, 14 figure
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