1,210 research outputs found
Quantum Correction to Conductivity Close to Ferromagnetic Quantum Critical Point in Two Dimensions
We study the temperature dependence of the conductivity due to quantum
interference processes for a two-dimensional disordered itinerant electron
system close to a ferromagnetic quantum critical point. Near the quantum
critical point, the cross-over between diffusive and ballistic regimes of
quantum interference effects occurs at a temperature , where is the parameter associated with the Landau
damping of the spin fluctuations, is the impurity scattering time, and
is the Fermi energy. For a generic choice of parameters, is
smaller than the nominal crossover scale . In the ballistic quantum
critical regime, the conductivity behaves as .Comment: 5 pages, 1 figur
Initial Conditions for Semiclassical Field Theory
Semiclassical approximation based on extracting a c-number classical
component from quantum field is widely used in the quantum field theory.
Semiclassical states are considered then as Gaussian wave packets in the
functional Schrodinger representation and as Gaussian vectors in the Fock
representation. We consider the problem of divergences and renormalization in
the semiclassical field theory in the Hamiltonian formulation. Although
divergences in quantum field theory are usually associated with loop Feynman
graphs, divergences in the Hamiltonian approach may arise even at the tree
level. For example, formally calculated probability of pair creation in the
leading order of the semiclassical expansion may be divergent. This observation
was interpretted as an argumentation for considering non-unitary evolution
transformations, as well as non-equivalent representations of canonical
commutation relations at different time moments. However, we show that this
difficulty can be overcomed without the assumption about non-unitary evolution.
We consider first the Schrodinger equation for the regularized field theory
with ultraviolet and infrared cutoffs. We study the problem of making a limit
to the local theory. To consider such a limit, one should impose not only the
requirement on the counterterms entering to the quantum Hamiltonian but also
the requirement on the initial state in the theory with cutoffs. We find such a
requirement in the leading order of the semiclassical expansion and show that
it is invariant under time evolution. This requirement is also presented as a
condition on the quadratic form entering to the Gaussian state.Comment: 20 pages, Plain TeX, one postscript figur
In-plane current-voltage characteristics and oscillatory Josephson-vortex flow resistance in La-free BiSrCuO single crystals in high magnetic fields
We have investigated the in-plane characteristics and the Josephson
vortex flow resistance in high-quality La-free
BiSrCuO (Bi2201) single crystals in parallel and
tilted magnetic fields at temperatures down to 40 mK. For parallel magnetic
fields below the resistive upper critical field , the
characteristic obey a power-law with a smooth change with increasing
magnetic-field of the exponent from above 5 down to 1. In contrast to the
double-layer cuprate Bi2212, the observed smooth change suggests that there is
no change in the mechanism of dissipation (no Kosterlitz-Thouless transition)
over the range of temperatures investigated. At small angles between the
applied field and the -plane, prominent current steps in the
characteristics and periodic oscillations of Josephson-vortex flow resistance
are observed. While the current steps are periodic in the voltage at constant
fields, the voltage position of the steps, together with the flux-flow voltage,
increases nonlinearly with magnetic field. The -flow resistance oscillates
as a function of field with a constant period over a wide range of magnetic
fields and temperatures. The current steps in the characteristics and
the flow resistance oscillations can be linked to the motion of Josephson
vortices across layers
Profiling and variation of laser pulse parameters as a way to preserve the stability of self-injected bunches during excitation of a wakefield in plasma
The paper considers the excitation of a wakefield in a metal-density plasma
using a chain of x-ray laser pulses. The profiling parameters and the necessary
parameters of laser pulses for obtaining stable high-quality bunches are found.
An essential problem is the destruction of self-injected bunches in the course
of their motion. The results of the study are one of the ways to solve the
problem of transverse betatron oscillations, which lead to the destruction of
bunches.Comment: 5 pages, 12 figure
Syntactic Complexity of R- and J-Trivial Regular Languages
The syntactic complexity of a regular language is the cardinality of its
syntactic semigroup. The syntactic complexity of a subclass of the class of
regular languages is the maximal syntactic complexity of languages in that
class, taken as a function of the state complexity n of these languages. We
study the syntactic complexity of R- and J-trivial regular languages, and prove
that n! and floor of [e(n-1)!] are tight upper bounds for these languages,
respectively. We also prove that 2^{n-1} is the tight upper bound on the state
complexity of reversal of J-trivial regular languages.Comment: 17 pages, 5 figures, 1 tabl
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