1,268 research outputs found
Random Coulomb antiferromagnets: from diluted spin liquids to Euclidean random matrices
We study a disordered classical Heisenberg magnet with uniformly
antiferromagnetic interactions which are frustrated on account of their
long-range Coulomb form, {\em i.e.} in and in . This arises naturally as the limit of the
emergent interactions between vacancy-induced degrees of freedom in a class of
diluted Coulomb spin liquids (including the classical Heisenberg
antiferromagnets on checkerboard, SCGO and pyrochlore lattices) and presents a
novel variant of a disordered long-range spin Hamiltonian. Using detailed
analytical and numerical studies we establish that this model exhibits a very
broad paramagnetic regime that extends to very large values of in both
and . In , using the lattice-Green function based finite-size
regularization of the Coulomb potential (which corresponds naturally to the
underlying low-temperature limit of the emergent interactions between
orphan-spins), we only find evidence that freezing into a glassy state occurs
in the limit of strong coupling, , while no such transition seems to
exist at all in . We also demonstrate the presence and importance of
screening for such a magnet. We analyse the spectrum of the Euclidean random
matrices describing a Gaussian version of this problem, and identify a
corresponding quantum mechanical scattering problem.Comment: two-column PRB format; 17 pages; 24 .eps figure
Argon hollow cathode
An interest in alternate propellants for ion-bombardment thrusters, together with ground applications of this technology, has prompted consideration of argon. Several variations of conventional hollow cathode designs were tried, but the bulk of the testing used a hollow tube with an internal tungsten emitter and an orifice at one end. The optimum cathode tube diameter was found to be in the range of 1.0-2.5 cm, somewhat larger than those used for cesium and mercury. Optimum orifice diameter depended on operating conditions, and varied from 0.5 to 5 mm. Biasing the internal emitter negative relative to the cathode chamber reduced the external coupling voltage and should therefore improve orifice lifetime. The expected effect of this bias on emitter lifetime was less clear. Lifetime tests were not conducted as part of this investigation, but several designs show promise of long lifetime in specific applications
Investigating teacher presence in courses using synchronous videoconferencing
This research examines teacher presence in high school distance courses that are delivered by synchronous videoconference. In rural and remote areas, many school districts are using videoconferencing as way to reach dispersed students. This collective case study uses mixed methods to unpack the notion of presence from the perspective of teachers and their students. This study reports four key findings which have implications for building presence in a videoconference course: teachersā confidence and experience aligned with higher presence; teaching videoconference and face-to-face classes simultaneously led to challenges with developing presence; immediacy behaviors correlated with higher presence; and, studentsā learning preference related to perceived teacher presence. These findings confirm many of the issues raised in the literature about technology integration but also contribute new perspectives on teaching presence in a videoconference
How periodic driving heats a disordered quantum spin chain
We study the energy absorption in real time of a disordered quantum spin chain subjected to coherent monochromatic periodic driving. We determine characteristic fingerprints of the well-known ergodic (Floquet-Eigenstate thermalization hypothesis for slow driving/weak disorder) and many-body localized (Floquet-many-body localization for fast driving/strong disorder) phases. In addition, we identify an intermediate regime, where the energy density of the system-unlike the entanglement entropy a local and bounded observable-grows logarithmically slowly over a very large time window
Complete Characterization of Quantum-Optical Processes
The technologies of quantum information and quantum control are rapidly
improving, but full exploitation of their capabilities requires complete
characterization and assessment of processes that occur within quantum devices.
We present a method for characterizing, with arbitrarily high accuracy, any
quantum optical process. Our protocol recovers complete knowledge of the
process by studying, via homodyne tomography, its effect on a set of coherent
states, i.e. classical fields produced by common laser sources. We demonstrate
the capability of our protocol by evaluating and experimentally verifying the
effect of a test process on squeezed vacuum.Comment: 5 pages, 4 figure
Excited States in Warm and Hot Dense Matter
Accurate modeling of warm and hot dense matter is challenging in part due to
the multitude of excited states that must be considered. In thermal density
functional theory, these excited states are averaged over to produce a single,
averaged, thermal ground state. Here we present a variational framework and
model that includes explicit excited states. In this framework an excited state
is defined by a set of effective one-electron occupation factors and the
corresponding energy is defined by the effective one-body energy with an
exchange and correlation term. The variational framework is applied to an
atom-in-plasma model (a generalization of the so-called average atom model).
Comparisons with a density functional theory based average atom model generally
reveal good agreement in the calculated pressure, but the new model also gives
access to the excitation energies and charge state distributions
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