4,814 research outputs found
Fluctuations, Higher Order Anharmonicities, and Landau Expansion for Barium Titanate
Correct phenomenological description of ferroelectric phase transitions in
barium titanate requires accounting for eighth-order terms in the free energy
expansion, in addition to the conventional sixth-order contributions. Another
unusual feature of BaTiO_3 crystal is that the coefficients B_1 and B_2 of the
terms P_x^4 and P_x^2*P_y^2 in the Landau expansion depend on the temperature.
It is shown that the temperature dependence of B_1 and B_2 may be caused by
thermal fluctuations of the polarization, provided the fourth-order
anharmonicity is anomalously small, i. e. the nonlinearity of P^4 type and
higher-order ones play comparable roles. Non-singular (non-critical)
fluctuation contributions to B_1 and B_2 are calculated in the first
approximation in sixth-order and eighth-order anharmonic constants. Both
contributions increase with the temperature, which is in agreement with
available experimental data. Moreover, the theory makes it possible to
estimate, without any additional assumptions, the ratio of fluctuation
(temperature dependent) contributions to coefficients B_1 and B_2. Theoretical
value of B_1/B_2 appears to be close to that given by experiments.Comment: 5 pages, 1 figur
First passage time of N excluded volume particles on a line
Motivated by recent single molecule studies of proteins sliding on a DNA
molecule, we explore the targeting dynamics of N particles ("proteins") sliding
diffusively along a line ("DNA") in search of their target site (specific
target sequence). At lower particle densities, one observes an expected
reduction of the mean first passage time proportional to 1/N**2, with
corrections at higher concentrations. We explicitly take adsorption and
desorption effects, to and from the DNA, into account. For this general case,
we also consider finite size effects, when the continuum approximation based on
the number density of particles, breaks down. Moreover, we address the first
passage time problem of a tagged particle diffusing among other particles.Comment: 9 pages, REVTeX, 6 eps figure
Quantum Resonances of Kicked Rotor and SU(q) group
The quantum kicked rotor (QKR) map is embedded into a continuous unitary
transformation generated by a time-independent quasi-Hamiltonian. In some
vicinity of a quantum resonance of order , we relate the problem to the {\it
regular} motion along a circle in a -component inhomogeneous
"magnetic" field of a quantum particle with intrinsic degrees of freedom
described by the group. This motion is in parallel with the classical
phase oscillations near a non-linear resonance.Comment: RevTeX, 4 pages, 3 figure
Intersubband plasmons in quasi-one-dimensional electron systems on a liquid helium surface
The collective excitation spectra are studied for a multisubband
quasi-one-dimensional electron gas on the surface of liquid helium. Different
intersubband plasmon modes are identified by calculating the spectral weight
function of the electron gas within a 12 subband model. Strong intersubband
coupling and depolarization shifts are found. When the plasmon energy is close
to the energy differences between two subbands, Landau damping in this finite
temperature system leads to plasmon gaps at small wavevectors.Comment: To be published as a Rapid Communication in Phys. Rev.
Towards deterministic equations for Levy walks: the fractional material derivative
Levy walks are random processes with an underlying spatiotemporal coupling.
This coupling penalizes long jumps, and therefore Levy walks give a proper
stochastic description for a particle's motion with broad jump length
distribution. We derive a generalized dynamical formulation for Levy walks in
which the fractional equivalent of the material derivative occurs. Our approach
will be useful for the dynamical formulation of Levy walks in an external force
field or in phase space for which the description in terms of the continuous
time random walk or its corresponding generalized master equation are less well
suited
Understanding Anomalous Transport in Intermittent Maps: From Continuous Time Random Walks to Fractals
We show that the generalized diffusion coefficient of a subdiffusive
intermittent map is a fractal function of control parameters. A modified
continuous time random walk theory yields its coarse functional form and
correctly describes a dynamical phase transition from normal to anomalous
diffusion marked by strong suppression of diffusion. Similarly, the probability
density of moving particles is governed by a time-fractional diffusion equation
on coarse scales while exhibiting a specific fine structure. Approximations
beyond stochastic theory are derived from a generalized Taylor-Green-Kubo
formula.Comment: 4 pages, 3 eps figure
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