7,422 research outputs found
Effective Hamiltonians for some highly frustrated magnets
In prior work, the authors developed a method of degenerate perturbation
theory about the Ising limit to derive an effective Hamiltonian describing
quantum fluctuations in a half-polarized magnetization plateau on the
pyrochlore lattice. Here, we extend this formulation to an arbitrary lattice of
corner sharing simplexes of sites, at a fraction of the
saturation magnetization, with . We present explicit effective
Hamiltonians for the examples of the checkerboard, kagome, and pyrochlore
lattices. The consequent ground states in these cases for are also
discussed.Comment: 10 pages, 2 figures,. Conference proceedings for Highly Frustrated
Magnetism 200
Bulk metals with helical surface states
In the flurry of experiments looking for topological insulator materials, it
has been recently discovered that some bulk metals very close to topological
insulator electronic states, support the same topological surface states that
are the defining characteristic of the topological insulator. First observed in
spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the
helical surface states in the metallic systems appear to be robust to at least
mild disorder. We present here a theoretical investigation of the nature of
these "helical metals" - bulk metals with helical surface states. We explore
how the surface and bulk states can mix, in both clean and disordered systems.
Using the Fano model, we discover that in a clean system, the helical surface
states are \emph{not} simply absorbed by hybridization with a non-topological
parasitic metallic band. Instead, they are pushed away from overlapping in
momentum and energy with the bulk states, leaving behind a finite-lifetime
surface resonance in the bulk energy band. Furthermore, the hybridization may
lead in some cases to multiplied surface state bands, in all cases retaining
the helical characteristic. Weak disorder leads to very similar effects -
surface states are pushed away from the energy bandwidth of the bulk, leaving
behind a finite-lifetime surface resonance in place of the original surface
states
Comment on "Optical Response of Strongly Coupled Nanopraticles in Dimer Arrays" (Phys. Rev. B 71(4), 045404, 2005)
I have re-calculated the extinction spectra of aggregates of two silver
nanospheres shown in Figs.~2 and 3 of Ref.~8. I have used the approximate
method of images according to Ref.~8 and an exact numerical technique. I have
found that the three sets of data (those I have obtained by the method of
images, the numerical results, and the results published in Ref.~8) do not
coincide. In this Comment, I discuss the reasons for these discrepancies and
the general applicability of the method of images to the quasi-static
electromagnetic problem of two interacting nanospheres.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Space station integrated propulsion and fluid systems study
This Databook addresses the integration of fluid systems of the Space Station program. It includes a catalog of components required for the Space Station elements fluid systems and information on potential hardware commonality. The components catalog is in four parts. The first part lists the components defined for all the fluid systems identified in EP 2.1, Space Station Program Fluid Systems Configuration Databook. The components are cross-referenced in three sections. Section 2.1 lists the components by the fluid system in which they are used. Section 2.2 lists the components by type. Section 2.3 lists by the type of fluid media handled by the component. The next part of the catalog provides a description of the individual component. This section (2.4) is made up of data retrieved from Martin Marietta Denver Aerospace component data base. The third part is an assessment of propulsion hardware technology requirements. Section 2.5 lists components identified during the study as requiring development prior to flight qualification. Finally, Section 2.6 presents the results of the evaluation of commonality between components. The specific requirements of each component have been reviewed and duplication eliminated
Space station integrated propulsion and fluid system study: Fluid systems configuration databook
This databook contains fluid system requirements and system descriptions for Space Station program elements including the United States and International modules, integrated fluid systems, attached payloads, fluid servicers and vehicle accommodation facilities. Separate sections are devoted to each of the program elements and include a discussion of the overall system requirements, specific fluid systems requirements and systems descriptions. The systems descriptions contain configurations, fluid inventory data and component lists. In addition, a list of information sources is referenced at the end of each section
Scalable Parallel Numerical Constraint Solver Using Global Load Balancing
We present a scalable parallel solver for numerical constraint satisfaction
problems (NCSPs). Our parallelization scheme consists of homogeneous worker
solvers, each of which runs on an available core and communicates with others
via the global load balancing (GLB) method. The parallel solver is implemented
with X10 that provides an implementation of GLB as a library. In experiments,
several NCSPs from the literature were solved and attained up to 516-fold
speedup using 600 cores of the TSUBAME2.5 supercomputer.Comment: To be presented at X10'15 Worksho
Ordering in a frustrated pyrochlore antiferromagnet proximate to a spin liquid
We perform a general study of spin ordering on the pyrochlore lattice with a
3:1 proportionality of two spin polarizations. Equivalently, this describes
valence bond solid conformations of a quantum dimer model on the diamond
lattice. We determine the set of likely low temperature ordered phases, on the
assumption that the ordering is weak, i.e the system is close to a ``U(1)''
quantum spin liquid in which the 3:1 proportionality is maintained but the
spins are strongly fluctuating. The nature of the 9 ordered states we find is
determined by a ``projective symmetry'' analysis. All the phases exhibit
translational and rotational symmetry breaking, with an enlarged unit cell
containing 4 to 64 primitive cells of the underlying pyrochlore. The simplest
of the 9 phases is the same ``R'' state found earlier in a theoretical study of
the ordering on the magnetization plateau in the materials \cdaf and
\hgaf. We suggest that the spin/dimer model proposed therein undergoes a direct
transition from the spin liquid to the R state, and describe a field theory for
the universal properties of this critical point, at zero and non-zero
temperatures
New Method to Calculate Electrical Forces Acting on a Sphere in an Electrorheological Fluid
We describe a method to calculate the electrical force acting on a sphere in
a suspension of dielectric spheres in a host with a different dielectric
constant, under the assumption that a spatially uniform electric field is
applied. The method uses a spectral representation for the total electrostatic
energy of the composite. The force is expressed as a certain gradient of this
energy, which can be expressed in a closed analytic form rather than evaluated
as a numerical derivative. The method is applicable even when both the spheres
and the host have frequency-dependent dielectric functions and nonzero
conductivities, provided the system is in the quasistatic regime. In principle,
it includes all multipolar contributions to the force, and it can be used to
calculate multi-body as well as pairwise forces. We also present several
numerical examples, including host fluids with finite conductivities. The force
between spheres approaches the dipole-dipole limit, as expected, at large
separations, but departs drastically from that limit when the spheres are
nearly in contact. The force may also change sign as a function of frequency
when the host is a slightly conducting fluid.Comment: 29 pages, 8 figures, Accepted for Publication in Physical Review
High volumetric capacitance near insulator-metal percolation transition
A new type of a capacitor with a very high volumetric capacitance is
proposed. It is based upon the known phenomenon of a sharp increase of the
dielectric constant of the metal-insulator composite in the vicinity of the
percolation threshold, but still on the insulator side. The optimization
suggests that the metallic particles should be of nanoscale and that the
distance between planar electrodes should be somewhat larger than the
correlation length of the percolation theory and 10 to 20 times larger than the
size of the particles while the area of the electrodes might be unlimited. The
random electric field in the capacitors is found to be larger than the average
field corresponding to the potential difference of electrodes. This random
field is potentially responsible for dielectric breakdown. The estimated
breakdown voltage of the new capacitor shows that the stored energy density
might be significantly larger than that of electrolytic capacitors while the
volumetric capacitances might be comparable. The charging and discharging times
should be significantly smaller than corresponding times of batteries and even
electrolytic capacitors.Comment: 10 pages 1 EPS figur
Generalized Paraxial Ray Trace Procedure Derived from Geodesic Deviation
Paraxial ray tracing procedures have become widely accepted techniques for
acoustic models in seismology and underwater acoustics. To date a generic form
of these procedures including fluid motion and time dependence has not appeared
in the literature. A detailed investigation of the characteristic curves of the
equations of hydrodynamics allows for an immediate generalization of the
procedure to be extracted from the equation form geodesic deviation. The
general paraxial ray trace equations serve as an ideal supplement to ordinary
ray tracing in predicting the deformation of acoustic beams in random
environments. The general procedure is derived in terms of affine
parameterization and in a coordinate time parameterization ideal for
application to physical acoustic ray propagation. The formalism is applied to
layered media, where the deviation equation reduces to a second order
differential equation for a single field with a general solution in terms of a
depth integral along the ray path. Some features are illustrated through
special cases which lead to exact solutions in terms of either ordinary or
special functions.Comment: Original; 40 pages (double spaced), 1 figure Replaced version; 36
pages single spaced, 7 figures. Expanded content; Complete derivation of the
equations from the equations of hydrodynamics, introduction of an auxiliary
basis for three dimensional wave-front modeling. Typos in text and equations
correcte
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