2,012 research outputs found
Nonlinear Two-Dimensional Green's Function in Smectics
The problem of the strain of smectics subjected to a force distributed over a
line in the basal plane has been solved
Frictional shear cracks
We discuss crack propagation along the interface between two dissimilar
materials. The crack edge separates two states of the interface, ``stick'' and
``slip''. In the slip region we assume that the shear stress is proportional to
the sliding velocity, i.e. the linear viscous friction law. In this picture the
static friction appears as the Griffith threshold for crack propagation. We
calculate the crack velocity as a function of the applied shear stress and find
that the main dissipation comes from the macroscopic region and is mainly due
to the friction at the interface. The relevance of our results to recent
experiments,
Baumberger et al, Phys. Rev. Lett. 88, 075509 (2002), is discussed
Josephson lattice model for phase fluctuations of local pairs in copper-oxide superconductors
We derive an expression for the effective Josephson coupling from the
microscopic Hubbard model. It serves as a starting point for the description of
phase fluctuations of local Cooper pairs in -wave superconductors
in the framework of an effective model of plaquettes, the Josephson
lattice. The expression for the effective interaction is derived by means of
the local-force theorem, and it depends on local symmetry-broken correlation
functions that we obtain using the cluster dynamical mean-field theory.
Moreover, we apply the continuum limit to the Josephson lattice to obtain an
expression for the gradient term in the Ginzburg-Landau theory and compare
predicted London penetration depths and Kosterlitz-Thouless transition
temperatures with experimental data for YBaCuO.Comment: 13 pages, 13 figure
Magnon activation by hot electrons via non-quasiparticle states
We consider the situation when a femtosecond laser pulse creates a hot
electron state in half-metallic ferromagnet (e. g. ferromagnetic semiconductor)
on a picosecond timescale but do not act directly on localized spin system. We
show that the energy and magnetic moment transfer from hot itinerant electrons
to localized spins is facilitated by the so-called non-quasiparticle states,
which are the scattering states of a magnon and spin-majority electron. The
magnon distribution is described by a quantum kinetic equation that we derive
using the Keldysh diagram technique. In a typical ferromagnetic semiconductor
such as EuO magnons remain essentially in non-equilibrium on a scale of the
order of microsecond after the laser pulse.Comment: 8 pages, 2 figure
Fracture and Friction: Stick-Slip Motion
We discuss the stick-slip motion of an elastic block sliding along a rigid
substrate. We argue that for a given external shear stress this system shows a
discontinuous nonequilibrium transition from a uniform stick state to uniform
sliding at some critical stress which is nothing but the Griffith threshold for
crack propagation. An inhomogeneous mode of sliding occurs, when the driving
velocity is prescribed instead of the external stress. A transition to
homogeneous sliding occurs at a critical velocity, which is related to the
critical stress. We solve the elastic problem for a steady-state motion of a
periodic stick-slip pattern and derive equations of motion for the tip and
resticking end of the slip pulses. In the slip regions we use the linear
viscous friction law and do not assume any intrinsic instabilities even at
small sliding velocities. We find that, as in many other pattern forming
system, the steady-state analysis itself does not select uniquely all the
internal parameters of the pattern, especially the primary wavelength. Using
some plausible analogy to first order phase transitions we discuss a ``soft''
selection mechanism. This allows to estimate internal parameters such as crack
velocities, primary wavelength and relative fraction of the slip phase as
function of the driving velocity. The relevance of our results to recent
experiments is discussed.Comment: 12 pages, 7 figure
Influence of Strain on the Kinetics of Phase Transitions in Solids
We consider a sharp interface kinetic model of phase transitions accompanied
by elastic strain, together with its phase-field realization. Quantitative
results for the steady-state growth of a new phase in a strip geometry are
obtained and different pattern formation processes in this system are
investigated
Effective Heisenberg model and exchange interaction for strongly correlated systems
We consider the extended Hubbard model and introduce a corresponding
Heisenberg-like problem written in terms of spin operators. The derived
formalism is reminiscent of Anderson's idea of the effective exchange
interaction and takes into account nonlocal correlation effects. The results
for the exchange interaction and magnetic susceptibility are expressed in terms
of single-particle quantities, which can be obtained efficiently in realistic
calculations of multiband systems. In the strongly spin-polarized limit, when
the local magnetic moment is well-defined, the exchange interaction reduces to
a standard expression of the density functional theory that has been
successfully used in practical calculations of magnetic properties of real
materials.Comment: Accepted to Physical Review Letter
Superperturbation solver for quantum impurity models
We present a very efficient solver for the general Anderson impurity problem.
It is based on the perturbation around a solution obtained from exact
diagonalization using a small number of bath sites. We formulate a perturbation
theory which is valid for both weak and strong coupling and interpolates
between these limits. Good agreement with numerically exact quantum Monte-Carlo
results is found for a single bath site over a wide range of parameters. In
particular, the Kondo resonance in the intermediate coupling regime is well
reproduced for a single bath site and the lowest order correction. The method
is particularly suited for low temperatures and alleviates analytical
continuation of imaginary time data due to the absence of statistical noise
compared to quantum Monte-Carlo impurity solvers.Comment: 6 pages, 5 figure
Elastic domains in antiferromagnets
We consider periodic domain structures which appear due to the magnetoelastic
interaction if the antiferromagnetic crystal is attached to an elastic
substrate. The peculiar behavior of such structures in an external magnetic
field is discussed. In particular, we find the magnetic field dependence of the
equilibrium period and the concentrations of different domains
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