79,642 research outputs found
Survival and Nonescape Probabilities for Resonant and Nonresonant Decay
In this paper we study the time evolution of the decay process for a particle
confined initially in a finite region of space, extending our analysis given
recently (Phys. Rev. Lett. 74, 337 (1995)). For this purpose, we solve exactly
the time-dependent Schroedinger equation for a finite-range potential. We
calculate and compare two quantities: (i) the survival probability S(t), i.e.,
the probability that the particle is in the initial state after a time t; and
(ii) the nonescape probability P(t), i.e., the probability that the particle
remains confined inside the potential region after a time t. We analyze in
detail the resonant and nonresonant decay. In the former case, after a very
short time, S(t) and P(t) decay exponentially, but for very long times they
decay as a power law, albeit with different exponents. For the nonresonant case
we obtain that both quantities differ initially. However, independently of the
resonant and nonresonant character of the initial state we always find a
transition to the ground state of the system which indicates a process of
``loss of memory'' in the decay.Comment: 26 pages, RevTex file, figures available upon request from
[email protected] (To be published in Annals of Physics
Bosonic versus fermionic pairs of topological spin defects in monolayered high-T_c superconductors
The energy associated with bosonic and fermionic pairs of topological spin
defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated
within a resonating valence bond scenario, as described by a t-t'-J-like model
Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional
screening of the electrostatic repulsion within the bosonic pairs. For
parameters appropriate for monolayered high-T_c superconductors, both fermionic
and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping
such that the energy of the bosonic pairs goes below twice the energy of two
fermionic pairs at their Fermi level. This finding could be related to the
onset of high-T_c superconductivity.Comment: 10 pages, 6 figures. To be published in Phys. Rev.
Rotational apparent mass by electrical analogy
Electrical analogy technique for determining rotational apparent masses of body in two- dimensional fluid flo
Absorbing-state phase transitions with extremal dynamics
Extremal dynamics represents a path to self-organized criticality in which
the order parameter is tuned to a value of zero. The order parameter is
associated with a phase transition to an absorbing state. Given a process that
exhibits a phase transition to an absorbing state, we define an ``extremal
absorbing" process, providing the link to the associated extremal
(nonabsorbing) process. Stationary properties of the latter correspond to those
at the absorbing-state phase transition in the former. Studying the absorbing
version of an extremal dynamics model allows to determine certain critical
exponents that are not otherwise accessible. In the case of the Bak-Sneppen
(BS) model, the absorbing version is closely related to the "-avalanche"
introduced by Paczuski, Maslov and Bak [Phys. Rev. E {\bf 53}, 414 (1996)], or,
in spreading simulations to the "BS branching process" also studied by these
authors. The corresponding nonextremal process belongs to the directed
percolation universality class. We revisit the absorbing BS model, obtaining
refined estimates for the threshold and critical exponents in one dimension. We
also study an extremal version of the usual contact process, using mean-field
theory and simulation. The extremal condition slows the spread of activity and
modifies the critical behavior radically, defining an ``extremal directed
percolation" universality class of absorbing-state phase transitions.
Asymmetric updating is a relevant perturbation for this class, even though it
is irrelevant for the corresponding nonextremal class.Comment: 24 pages, 11 figure
Simultaneous analysis of elastic scattering and transfer/breakup channels for the 6He+208Pb reaction at energies near the Coulomb barrier
The elastic and alpha-production channels for the 6He+208Pb reaction are
investigated at energies around the Coulomb barrier (E_{lab}=14, 16, 18, 22,
and 27 MeV). The effect of the two-neutron transfer channels on the elastic
scattering has been studied within the Coupled-Reaction-Channels (CRC) method.
We find that the explicit inclusion of these channels allows a simultaneous
description of the elastic data and the inclusive alpha cross sections at
backward angles. Three-body Continuum-Discretized Coupled-Channels (CDCC)
calculations are found to reproduce the elastic data, but not the
transfer/breakup data. The trivially-equivalent local polarization potential
(TELP) derived from the CRC and CDCC calculations are found to explain the
features found in previous phenomenological optical model calculations for this
system.Comment: 7 pages, 6 figures (replaced with updated version
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