151,977 research outputs found
Barium release system
A chemical system is described for releasing a good yield of free barium neutral atoms and barium ions in the upper atmosphere and interplanetary space for the study of the geophysical properties of the medium. The barium is released in the vapor phase so that it can be ionized by solar radiation and also be excited to emit resonance radiation in the visible range. The ionized luminous cloud of barium becomes a visible indication of magnetic and electrical characteristics in space and allows determination of these properties over relatively large areas at a given time
Computer program to determine pressure distributions and forces on blunt bodies of revolution
Program was written to include integration of surface pressure in order to obtain axial-force, normal-force, and pitching-moment coefficients. Program was written in CDC FORTRAN for the CDC-6600 computer system
Rocket having barium release system to create ion clouds in the upper atmosphere
A chemical system for releasing a good yield of free barium atoms and barium ions to create ion clouds in the upper atmosphere and interplanetary space for the study of the geophysical properties of the medium is presented
Topological phases in a two-dimensional lattice: Magnetic field versus spin-orbit coupling
In this work, we explore the rich variety of topological states that arise in
two-dimensional systems, by considering the competing effects of spin-orbit
couplings and a perpendicular magnetic field on a honeycomb lattice. Unlike
earlier approaches, we investigate minimal models in order to clarify the
effects of the intrinsic and Rashba spin-orbit couplings, and also of the
Zeeman splitting, on the quantum Hall states generated by the magnetic field.
In this sense, our work provides an interesting path connecting quantum Hall
and quantum spin Hall physics. First, we consider the properties of each term
individually and we analyze their similarities and differences. Secondly, we
investigate the subtle competitions that arise when these effects are combined.
We finally explore the various possible experimental realizations of our model.Comment: 19 pages, 15 figure
Genesis of the Floquet Hofstadter butterfly
We investigate theoretically the spectrum of a graphene-like sample
(honeycomb lattice) subjected to a perpendicular magnetic field and irradiated
by circularly polarized light. This system is studied using the Floquet
formalism, and the resulting Hofstadter spectrum is analyzed for different
regimes of the driving frequency. For lower frequencies, resonances of various
copies of the spectrum lead to intricate formations of topological gaps. In the
Landau-level regime, new wing-like gaps emerge upon reducing the driving
frequency, thus revealing the possibility of dynamically tuning the formation
of the Hofstadter butterfly. In this regime, an effective model may be
analytically derived, which allows us to retrace the energy levels that exhibit
avoided crossings and ultimately lead to gap structures with a wing-like shape.
At high frequencies, we find that gaps open for various fluxes at , and
upon increasing the amplitude of the driving, gaps also close and reopen at
other energies. The topological invariants of these gaps are calculated and the
resulting spectrum is elucidated. We suggest opportunities for experimental
realization and discuss similarities with Landau-level structures in non-driven
systems.Comment: 8 pages, 4 figure
Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point
The quasi-linear bands in the topologically trivial skutterudite insulator
CoSb are studied under adiabatic, symmetry-conserving displacement of the
Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a
transition from trivial insulator to topological point Fermi surface system
occurs through a critical point in which massless (Dirac) bands are {\it
degenerate} with massive bands. Spin-orbit coupling does not alter the
character of the transition. The mineral skutterudite (CoSb) is very near
the critical point in its natural state.Comment: 5 pages, 3 figure
MHD‐driven kinetic dissipation in the solar wind and corona
Mechanisms for the deposition of heat in the lower coronal plasma are discussed, emphasizing recent attempts to reconcile the fluid and kinetic perspectives. Structures at magnetohydrodynamic (MHD) scales may drive a nonlinear cascade, preferentially exciting high perpendicular wavenumber fluctuations. Relevant dissipative kinetic processes must be identified that can absorb the associated energy flux. The relationship between the MHD cascade and direct cyclotron absorption, including cyclotron sweep, is discussed. We conclude that for coronal and solar wind parameters the perpendicular cascade cannot be neglected and may be more rapid than cyclotron sweep. Solar wind observational evidence suggests the relevance of the ion inertial scale, which is associated with current sheet thickness during reconnection. We conclude that a significant fraction of dissipation in the corona and solar wind likely proceeds through a perpendicular cascade and small-scale reconnection, coupled to kinetic processes that act at oblique wavevectors
Phase-field crystal study of grain-boundary premelting
We study the phenomenon of grain-boundary premelting for temperatures below
the melting point in the phase-field crystal model of a pure material with
hexagonal ordering in two dimensions. We investigate the structures of
symmetric tilt boundaries as a function of misorientation for two different
inclinations and compute in the grand canonical ensemble the disjoining
potential V(w) that governs the fundamental interaction between crystal-melt
interfaces as a function of the premelted layer width w. The results reveal
qualitatively different behaviors for high-angle grain boundaries that are
uniformly wetted, with w diverging logarithmically as the melting point is
approached from below, and low-angle boundaries that are punctuated by liquid
pools surrounding dislocations, separated by solid bridges. This qualitative
difference between high and low angle boundaries is reflected in the
w-dependence of the disjoining potential that is purely repulsive (V'(w)<0 for
all w) above a critical misorientation, but switches from repulsive at small w
to attractive at large w for low angles. In the latter case, V(w) has a minimum
that corresponds to a premelted boundary of finite width at the melting point.
Furthermore, we find that the standard wetting condition (the grain boundary
energy is equal to twice the solid-liquid free energy) gives a much too low
estimate of the critical misorientation when a low-temperature value of the
grain boundary energy is used. In contrast, a reasonable estimate is obtained
if the grain boundary energy is extrapolated to the melting point, taking into
account both the elastic softening of the material at high temperature and
local melting around dislocations.Comment: 24 pages, 13 figures, some figure files with reduced resolution
because of submission size limitations. In the 2nd version, some parts (and
figures) have been modified, especially in Sec. V (discussion
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