455 research outputs found
Marginal Pinning of Quenched Random Polymers
An elastic string embedded in 3D space and subject to a short-range
correlated random potential exhibits marginal pinning at high temperatures,
with the pinning length becoming exponentially sensitive to
temperature. Using a functional renormalization group (FRG) approach we find
, with the
depinning temperature. A slow decay of disorder correlations as it appears in
the problem of flux line pinning in superconductors modifies this result, .Comment: 4 pages, RevTeX, 1 figure inserte
Thermally activated Hall creep of flux lines from a columnar defect
We analyse the thermally activated depinning of an elastic string (line
tension ) governed by Hall dynamics from a columnar defect modelled
as a cylindrical potential well of depth for the case of a small
external force An effective 1D field Hamiltonian is derived in order to
describe the 2D string motion. At high temperatures the decay rate is
proportional to with a constant of order of the
critical force and U(F) \sim{\left ({\epsilon V_{0}})}^{{1}/{2}}{V_{0}/{F}}
the activation energy. The results are applied to vortices pinned by columnar
defects in superclean superconductors.Comment: 12 pages, RevTeX, 2 figures inserte
Quantum depinning of a pancake-vortex from a columnar defect
We consider the problem of the depinning of a weakly driven ()
pancake vortex from a columnar defect in a Josephson-coupled superconductor,
where denotes the force acting on the vortex ( is the critical
force).
The dynamics of the vortex is supposed to be of the Hall type. The Euclidean
action is calculated in the entire temperature range; the result
is universal and does not depend on the detailed form of the pinning potential.
We show that the transition from quantum to classical behavior is second-order
like with the temperature of the transition scaling like
Special attention is paid to the regime of applicability of our results, in
particular, the influence of the large vortex mass appearing in the superclean
limit is discussed.Comment: 11 pages, RevTeX, 4 figures inserte
The competency-based approach in education: issues and options
The paper discusses the systems problems of competency-based approach in education and solutions of them. The main problem of the division of competency into the independent components is systems complexity. Simulation modeling allows to overcome the systems complexity of the object of study by identifying cause-and-effect relationships in the simulation experimentВ работе обсуждаются системные проблемы компетентностного подхода в образовании и пути их решения. Главной проблемой разделения профессиональных качеств на независимые компетенции является их системная сложность. Применение имитационного моделирования позволяет преодолевать системную сложность объекта исследования путем выявления причинно-следственных связей в ходе имитационного эксперимент
Correlation Functions for an Elastic String in a Random Potential: Instanton Approach
We develop an instanton technique for calculations of correlation functions
characterizing statistical behavior of the elastic string in disordered media
and apply the proposed approach to correlations of string free energies
corresponding to different low-lying metastable positions. We find high-energy
tails of correlation functions for the case of long-range disorder (the
disorder correlation length well exceeds the characteristic distance between
the sequential string positions) and short-range disorder with the correlation
length much smaller then the characteristic string displacements. The former
case refers to energy distributions and correlations on the distances below the
Larkin correlation length, while the latter describes correlations on the large
spatial scales relevant for the creep dynamics.Comment: 5 pages; 1 .eps figure include
Quantum Collective Creep: a Quasiclassical Langevin Equation Approach
The dynamics of an elastic medium driven through a random medium by a small
applied force is investigated in the low-temperature limit where quantum
fluctuations dominate. The motion proceeds via tunneling of segments of the
manifold through barriers whose size grows with decreasing driving force .
In the limit of small drive, at zero-temperature the average velocity has the
form . For strongly
dissipative dynamics, there is a wide range of forces where the dissipation
dominates and the velocity--force characteristics takes the form
, with the
action for a typical tunneling event, the force dependence being determined by
the roughness exponent of the -dimensional manifold. This result
agrees with the one obtained via simple scaling considerations. Surprisingly,
for asymptotically low forces or for the case when the massive dynamics is
dominant, the resulting quantum creep law is {\it not} of the usual form with a
rate proportional to ; rather we find corresponding to and , with the naive scaling exponent for massive
dynamics. Our analysis is based on the quasi-classical Langevin approximation
with a noise obeying the quantum fluctuation--dissipation theorem. The many
space and time scales involved in the dynamics are treated via a functional
renormalization group analysis related to that used previously to treat the
classical dynamics of such systems. Various potential difficulties with these
approaches to the multi-scale dynamics -- both classical and quantum -- are
raised and questions about the validity of the results are discussed.Comment: RevTeX, 30 pages, 8 figures inserte
Diffusion and Creep of a Particle in a Random Potential
We investigate the diffusive motion of an overdamped classical particle in a
1D random potential using the mean first-passage time formalism and demonstrate
the efficiency of this method in the investigation of the large-time dynamics
of the particle. We determine the -time diffusion {<{<
x^{2}(t)>}_{th}>}_{dis}=A\ln^{\beta} \left ({t}/{t_{r}}) and relate the
prefactor the relaxation time and the exponent to the
details of the (generally non-gaussian) long-range correlated potential.
Calculating the moments {}_{th}>}_{dis} of the first-passage time
distribution we reconstruct the large time distribution function itself
and draw attention to the phenomenon of intermittency. The results can be
easily interpreted in terms of the decay of metastable trapped states. In
addition, we present a simple derivation of the mean velocity of a particle
moving in a random potential in the presence of a constant external force.Comment: 6 page
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