56 research outputs found
Two-Dimensional Wigner Crystal in Anisotropic Semiconductor
We investigate the effect of mass anisotropy on the Wigner crystallization
transition in a two-dimensional (2D) electron gas. The static and dynamical
properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais
lattices in the presence of anisotropic mass, as may be obtainable in Si
MOSFETs with (110) surface. By studying the stability of all possible lattices,
we find significant change in the crystal structure and melting density of the
electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure
London equation studies of thin-film superconductors with a triangular antidot lattice
We report on a study of vortex pinning in nanoscale antidot defect arrays in
the context of the London Theory. Using a wire network model, we discretize the
array with a fine mesh, thereby providing a detailed treatment of pinning
phenomena. The use of a fine grid has enabled us to examine both circular and
elongated defects, patterned in the form of a rhombus. The latter display
pinning characteristics superior to circular defects constructed with the
similar area. We calculate pinning potentials for defects containing zero and
single quanta, and we obtain a pinning phase diagram for the second matching
field, .Comment: 10 pages and 14 figure
An experimental study to discriminate between the validity of diffraction theories for off-Bragg replay
We show that experiments clearly verify the assumptions made by the
first-order two-wave coupling theory for one dimensional lossless unslanted
planar volume holographic gratings using the beta-value method rather than
Kogelnik's K-vector closure method. Apart from the fact that the diffraction
process is elastic, a much more striking difference between the theories
becomes apparent particularly in the direction of the diffracted beam in
off-Bragg replay. We therefore monitored the direction of the diffracted beam
as a function of the off-Bragg replay angle in two distinct cases: [a] the
diffracted beam lies in the plane of incidence and [b] the sample surface
normal, the grating vector and the incoming beam do not form a plane which
calls for the vectorial theory and results in conical scattering.Comment: Corrected Eqs. (3) & (6); 14 pages, 8 figure
Universality in the Screening Cloud of Dislocations Surrounding a Disclination
A detailed analytical and numerical analysis for the dislocation cloud
surrounding a disclination is presented. The analytical results show that the
combined system behaves as a single disclination with an effective fractional
charge which can be computed from the properties of the grain boundaries
forming the dislocation cloud. Expressions are also given when the crystal is
subjected to an external two-dimensional pressure. The analytical results are
generalized to a scaling form for the energy which up to core energies is given
by the Young modulus of the crystal times a universal function. The accuracy of
the universality hypothesis is numerically checked to high accuracy. The
numerical approach, based on a generalization from previous work by S. Seung
and D.R. Nelson ({\em Phys. Rev A 38:1005 (1988)}), is interesting on its own
and allows to compute the energy for an {\em arbitrary} distribution of
defects, on an {\em arbitrary geometry} with an arbitrary elastic {\em energy}
with very minor additional computational effort. Some implications for recent
experimental, computational and theoretical work are also discussed.Comment: 35 pages, 21 eps file
Stability of the lattice formed in first-order phase transitions to matter containing strangeness in protoneutron stars
Well into the deleptonization phase of a core collapse supernova, a
first-order phase transition to matter with macroscopic strangeness content is
assumed to occur and lead to a structured lattice defined by negatively charged
strange droplets. The lattice is shown to crystallize for expected droplet
charges and separations at temperatures typically obtained during the
protoneutronstar evolution. The melting curve of the lattice for small
spherical droplets is presented. The one-component plasma model proves to be an
adequate description for the lattice in its solid phase with deformation modes
freezing out around the melting temperature. The mechanical stability against
shear stresses is such that velocities predicted for convective phenomena and
differential rotation during the Kelvin-Helmholtz cooling phase might prevent
the crystallization of the phase transition lattice. A solid lattice might be
fractured by transient convection, which could result in anisotropic neutrino
transport. The melting curve of the lattice is relevant for the mechanical
evolution of the protoneutronstar and therefore should be included in future
hydrodynamics simulations.Comment: accepted for publication in Physical Review
Skymrion lattice melting in the quantum Hall system
The melting and magnetic disordering of the skyrmion lattice in the quantum
Hall system at filling factor are studied. A
Berezinskii-Kosterlitz-Thouless renormalization group theory is employed to
describe the coupled magnetic and translational degrees of freedom. The
non-trivial magnetic properties of the skyrmion system stem from the in-plane
components of the non-collinear magnetization in the vicinity of skyrmions,
which are described by an antiferromagnetic XY model. In a Coulomb gas
formulation the `particles' are the topological defects of the XY model
(vortices) and of the lattice (dislocations and disclinations). The latter
frustrate the antiferromagnetic order and acquire fractional vorticity in order
to minimize their energy. We find a number of melting/disordering scenarios for
various lattice types. While these results do not depend on a particular model,
we also consider a simple classical model for the skyrmion system. It results
in a rich T=0 phase diagram. We propose that the triangular and square skyrmion
lattices are generically separated by a centered rectangular phase in the
quantum Hall system.Comment: 15 pages with 5 figures. Minor revisions. Important reference to M.
Rao, S. Sengupta, and R. Shankar, Phys. Rev. Lett. 79, 3998 (1997) adde
Critical Dynamics of Magnets
We review our current understanding of the critical dynamics of magnets above
and below the transition temperature with focus on the effects due to the
dipole--dipole interaction present in all real magnets. Significant progress in
our understanding of real ferromagnets in the vicinity of the critical point
has been made in the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin interactions.
We start our review with a discussion of the theoretical results for the
critical dynamics based on recent renormalization group, mode coupling and spin
wave theories. A detailed comparison is made of the theory with experimental
results obtained by different measuring techniques, such as neutron scattering,
hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and
magnetic relaxation, in various materials. Furthermore we discuss the effects
of dipolar interaction on the critical dynamics of three--dimensional isotropic
antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the existence of
magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and
antiferromagnets. We close our review with a formulation of critical dynamics
in terms of nonlinear Langevin equations.Comment: Review article (154 pages, figures included
Atom-optics hologram in the time domain
The temporal evolution of an atomic wave packet interacting with object and
reference electromagnetic waves is investigated beyond the weak perturbation of
the initial state. It is shown that the diffraction of an ultracold atomic beam
by the inhomogeneous laser field can be interpreted as if the beam passes
through a three-dimensional hologram, whose thickness is proportional to the
interaction time. It is found that the diffraction efficiency of such a
hologram may reach 100% and is determined by the duration of laser pulses. On
this basis a method for reconstruction of the object image with matter waves is
offered.Comment: RevTeX, 13 pages, 8 figures; minor grammatical change
Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12
We examine quantitatively the temperature dependence of the linewidths and
line shifts in electron paramagnetic resonance experiments on single crystals
of the single-molecule magnets Fe and Mn, at fixed frequency, with
an applied magnetic field along the easy axis. We include inter-molecular
spin-spin interactions (dipolar and exchange) and distributions in both the
uniaxial anisotropy parameter and the Land\'{e} -factor. The temperature
dependence of the linewidths and the line shifts are mainly caused by the
spin-spin interactions. For Fe and Mn, the temperature dependence of
the calculated line shifts and linewidths agrees well with the trends of the
experimental data. The linewidths for Fe reveal a stronger temperature
dependence than those for Mn, because for Mn a much wider
distribution in overshadows the temperature dependence of the spin-spin
interactions. For Fe, the line-shift analysis suggests two competing
interactions: a weak ferromagnetic exchange coupling between neighboring
molecules and a longer-ranged dipolar interaction. This result could have
implications for ordering in Fe at low temperatures.Comment: published versio
N-body simulations of gravitational dynamics
We describe the astrophysical and numerical basis of N-body simulations, both
of collisional stellar systems (dense star clusters and galactic centres) and
collisionless stellar dynamics (galaxies and large-scale structure). We explain
and discuss the state-of-the-art algorithms used for these quite different
regimes, attempt to give a fair critique, and point out possible directions of
future improvement and development. We briefly touch upon the history of N-body
simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu
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