23,589 research outputs found
Hidden Order in
We review current attempts to characterize the underlying nature of the
hidden order in . A wide variety of experiments point to the
existence of two order parameters: a large primary order parameter of unknown
character which co-exists with secondary antiferromagnetic order. Current
theories can be divided into two groups determined by whether or not the
primary order parameter breaks time-reversal symmetry. We propose a series of
experiments designed to test the time-reversal nature of the underlying primary
order in and to characterize its local single-ion physics
Thermodynamics of Vortices in the Plane
The thermodynamics of vortices in the critically coupled abelian Higgs model,
defined on the plane, are investigated by placing vortices in a region of
the plane with periodic boundary conditions: a torus. It is noted that the
moduli space for vortices, which is the same as that of
indistinguishable points on a torus, fibrates into a bundle over the
Jacobi manifold of the torus. The volume of the moduli space is a product of
the area of the base of this bundle and the volume of the fibre. These two
values are determined by considering two 2-surfaces in the bundle corresponding
to a rigid motion of a vortex configuration, and a motion around a fixed centre
of mass. The partition function for the vortices is proportional to the volume
of the moduli space, and the equation of state for the vortices is in the thermodynamic limit, where is the pressure, the area of
the region of the plane occupied by the vortices, and the temperature.
There is no phase transition.Comment: 17 pages, DAMTP 93-3
() spectroscopy using Cornell potential
The mass spectra and decay properties of heavy quarkonia are computed in
nonrelativistic quark-antiquark Cornell potential model. We have employed the
numerical solution of Schr\"odinger equation to obtain their mass spectra using
only four parameters namely quark mass (, ) and confinement strength
(, ). The spin hyperfine, spin-orbit and tensor
components of the one gluon exchange interaction are computed perturbatively to
determine the mass spectra of excited , , and states. Digamma,
digluon and dilepton decays of these mesons are computed using the model
parameters and numerical wave functions. The predicted spectroscopy and decay
properties for quarkonia are found to be consistent with available experimental
observations and results from other theoretical models. We also compute mass
spectra and life time of the meson without additional parameters. The
computed electromagnetic transition widths of heavy quarkonia and mesons
are in tune with available experimental data and other theoretical approaches
A thermometer technique for the determination of emissivities of finishes.
Two thermometer techniques for the determination of emissivities of surface coatings are presented. One procedure involving the attainment of steady state hear transfer relations in coated thermometers placed through the wall of a duct at right angles to the direction of air flow was found to give consistent results. Another procedure based upon the rate of change of thermometer temperature was found to give consistent results only with excellent control of air conditions. The emissivities of a variety of paints, varnishes and lacquers were determined for temperature range 90⁰ F. to 140⁰ F. using silver and black coatings for standardization. The results obtained by means of the static technique were more satisfactory
Explaining Snapshots of Network Diffusions: Structural and Hardness Results
Much research has been done on studying the diffusion of ideas or
technologies on social networks including the \textit{Influence Maximization}
problem and many of its variations. Here, we investigate a type of inverse
problem. Given a snapshot of the diffusion process, we seek to understand if
the snapshot is feasible for a given dynamic, i.e., whether there is a limited
number of nodes whose initial adoption can result in the snapshot in finite
time. While similar questions have been considered for epidemic dynamics, here,
we consider this problem for variations of the deterministic Linear Threshold
Model, which is more appropriate for modeling strategic agents. Specifically,
we consider both sequential and simultaneous dynamics when deactivations are
allowed and when they are not. Even though we show hardness results for all
variations we consider, we show that the case of sequential dynamics with
deactivations allowed is significantly harder than all others. In contrast,
sequential dynamics make the problem trivial on cliques even though it's
complexity for simultaneous dynamics is unknown. We complement our hardness
results with structural insights that can help better understand diffusions of
social networks under various dynamics.Comment: 14 pages, 3 figure
Curvature dependence of the effect of ionic functionalization on the attraction among nanoparticles in dispersion
Solubilization of nanoparticles facilitates nanomaterial processing and enables new applications. An effective method to improve dispersibility in water is provided by ionic functionalization.We explore how the necessary extent of functionalization depends on the particle geometry. Using molecular dynamics/umbrella sampling simulations, we determine the effect of the solute curvature on solventaveraged interactions among ionizing graphitic nanoparticles in aqueous dispersion. We tune the hydrophilicity of molecular-brush coated fullerenes, carbon nanotubes, and graphane platelets by gradually replacing a fraction of the methyl end groups of the alkyl coating by the ionizing –COOK or –NH3Cl groups. To assess the change in nanoparticles’ dispersibility in water, we determine the potential-of-mean-force profiles at varied degrees of ionization. When the coating comprises only propyl groups, the attraction between the hydrophobic particles intensifies from spherical to cylindrical to planar geometry. This is explained by the increasing fraction of surface groups that can be brought into contact and the reduced access to water molecules, both following the above sequence. When ionic groups are added, however, the dispersibility increases in the opposite order, with the biggest effect in the planar geometry and the smallest in the spherical geometry. These results highlight the important role of geometry in nanoparticle solubilization by ionic functionalities, with about twice higher threshold surface charge necessary to stabilize a dispersion of spherical than planar particles. At 25%–50% ionization, the potential of mean force reaches a plateau because of the counterion condensation and saturated brush hydration. Moreover, the increase in the fraction of ionic groups can weaken the repulsion through counterion correlations between adjacent nanoparticles. High degrees of ionization and concomitant ionic screening gradually reduce the differences among surface interactions in distinct geometries until an essentially curvature-independent dispersion environment is created. Insights into tuning nanoparticle interactions can guide the synthesis of a broad class of nonpolar nanoparticles, where solubility is achieved by ionic functionalization
Study of the technique of stellar occultation
The results are reported of a study of the stellar occultation technique for measuring the composition of the atmosphere. The intensity of starlight was monitored during the occultation using the Wisconsin stellar ultraviolet photometers aboard the Orbiting Astronomical Observatory (OAO-A2). A schematic diagram of an occultation is shown where the change in intensity at a given wavelength is illustrated. The vertical projection of the attenuation region is typically 60 km deep for molecular oxygen and 30 km deep for ozone. Intensity profiles obtained during various occultations were analyzed by first determining the tangential columm density of the absorbing gases, and then Abel inverting the column densities to obtain the number density profile. Errors are associated with each step in the inversion scheme and have been considered as an integral part of this study
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