4,370 research outputs found
Dynamics of polymers: classic results and recent developments
In this chapter we review concepts and theories of polymer dynamics. We think
of it as an introduction to the topic for scientists specializing in other
subfields of statistical mechanics and condensed matter theory, so, for the
readers reference, we start with a short review of the equilibrium static
properties of polymer systems. Most attention is paid to the dynamics of
unentangled polymer systems, where apart from classical Rouse and Zimm models
we review some recent scaling and analytical generalizations. The dynamics of
systems with entanglements is also briefly reviewed. Special attention is paid
to the discussion of comparatively weakly understood topological states of
polymer systems and possible approaches to the description of their dynamics
Optical chaos in nonlinear photonic crystals
We examine a spatial evolution of lightwaves in a nonlinear photonic crystal
with a quadratic nonlinearity when simultaneously a second harmonic and a
sum-frequency generation are quasi-phase-matched. We find the conditions of a
transition to Hamiltonian chaos for different amplitudes of lightwaves at the
boundary of the crystal.Comment: LaTEX2e, 5 pages, 4 figure
Direct current generation due to harmonic mixing: From bulk semiconductors to semiconductor superlattices
We discuss an effect of dc current and dc voltage (stopping bias) generation
in a semiconductor superlattice subjected by an ac electric field and its
phase-shifted n-th harmonic. In the low field limit, we find a simple
dependence of dc voltage on a strength, frequency, and relative phase of mixing
harmonics for an arbitrary even value of n.
We show that the generated dc voltage has a maximum when a frequency of ac
field is of the order of a scattering constant of electrons in a superlattice.
This means that for typical semiconductor superlattices at room temperature
operating in the THz frequency domain the effect is really observable.
We also made a comparison of a recent paper describing an effect of a
directed current generation in a semiconductor superlattice subjected by ac
field and its second harmonic (n=2) [K.Seeger, Appl.Phys.Lett. 76(2000)82] with
our earlier findings describing the same effect [K.Alekseev et al., Europhys.
Lett. 47(1999)595; cond-mat/9903092 ].
For the mixing of an ac field and its n-th harmonic with n>=4, we found that
additionally to the phase-shift controlling of the dc current, there is a
frequency control. This frequency controlling of the dc current direction is
absent in the case of n=2. The found effect is that, both the dc current
suppression and the dc current reversals exist for some particular values of ac
field frequency. For typical semiconductor superlattices such an interesting
behavior of the dc current should be observable also in the THz domain.
Finally, we briefly review the history of the problem of the dc current
generation at mixing of harmonics in semiconductors and semiconductor
microstructures.Comment: 9 pages, 1 figure, RevTEX, EPS
Quantization of Diffeomorphism-Invariant Theories with Fermions
We extend ideas developed for the loop representation of quantum gravity to
diffeomorphism-invariant gauge theories coupled to fermions. Let P -> Sigma be
a principal G-bundle over space and let F be a vector bundle associated to P
whose fiber is a sum of continuous unitary irreducible representations of the
compact connected gauge group G, each representation appearing together with
its dual. We consider theories whose classical configuration space is A x F,
where A is the space of connections on P and F is the space of sections of F,
regarded as a collection of Grassmann-valued fermionic fields. We construct the
`quantum configuration space a x f as a completion of A x F. Using this we
construct a Hilbert space L^2(a x f) for the quantum theory on which all
automorphisms of P act as unitary operators, and determine an explicit `spin
network basis' of the subspace L^2((a x f)/G) consisting of gauge-invariant
states. We represent observables constructed from holonomies of the connection
along paths together with fermionic fields and their conjugate momenta as
operators on L^2((a x f)/G). We also construct a Hilbert space H_diff of
diffeomorphism-invariant states using the group averaging procedure of
Ashtekar, Lewandowski, Marolf, Mourao and Thiemann.Comment: 28 pages, latex, 7 ps-files (included) are needed to process the
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