61,152 research outputs found
A comparison of vertical velocities measured from specular and nonspecular echoes by a VHF radar
For a number of years, there have been doubts about the accuracy of vertical wind velocities measured with quasi-specular reflections from mesosphere-stratosphere-troposphere (MST) radar. The concern has been that the layers producing the quasi-specular reflection process this hypothetical tilt. Because of the quasi-specular reflection process, this hypothetical tilt would control the effective zenith angle of the radar antenna beam so that a small component of the horizontal velocity would be included in what was assumed to be a truly vertical beam. The purpose here is to test the hypothesis that there is an effect on the wind velocities measured on a vertical antenna beam due to a long-term tilting of the stable atmospheric layers that cause quasi-specular reflection. Gravity waves have been observed to cause short-term tilting of turbulent layers and specularly reflecting layers. In both cases, the effect was a wave-like deformation atmospheric layers with a period of a few minutes. This geometry is shown. Because of this influence of gravity waves, it was expected that there would be short-term variations in the vertical velocity
Beyond mean-field bistability in driven-dissipative lattices: bunching-antibunching transition and quantum simulation
In the present work we investigate the existence of multiple nonequilibrium
steady states in a coherently driven XY lattice of dissipative two-level
systems. A commonly used mean-field ansatz, in which spatial correlations are
neglected, predicts a bistable behavior with a sharp shift between low- and
high-density states. In contrast one-dimensional matrix product methods reveal
these effects to be artifacts of the mean-field approach, with both
disappearing once correlations are taken fully into account. Instead, a
bunching-antibunching transition emerges. This indicates that alternative
approaches should be considered for higher spatial dimensions, where classical
simulations are currently infeasible. Thus we propose a circuit QED quantum
simulator implementable with current technology to enable an experimental
investigation of the model considered
Strong Correlations in Actinide Redox Reactions
Reduction-oxidation (redox) reactions of the redox couples An(VI)/An(V),
An(V)/An(IV), and An(IV)/An(III), where An is an element in the family of early
actinides (U, Np, and Pu), as well as Am(VI)/Am(V) and Am(V)/Am(III), are
modeled by combining density functional theory with a generalized Anderson
impurity model that accounts for the strong correlations between the 5f
electrons. Diagonalization of the Anderson impurity model yields improved
estimates for the redox potentials and the propensity of the actinide complexes
to disproportionate.Comment: 17 pages, 10 figure, 3 tables. Corrections and clarifications; this
version has been accepted for publication in The Journal of Chemical Physic
Hardware for digitally controlled scanned probe microscopes
The design and implementation of a flexible and modular digital control and data acquisition system for scanned probe microscopes (SPMs) is presented. The measured performance of the system shows it to be capable of 14-bit data acquisition at a 100-kHz rate and a full 18-bit output resolution resulting in less than 0.02-Å rms position noise while maintaining a scan range in excess of 1 µm in both the X and Y dimensions. This level of performance achieves the goal of making the noise of the microscope control system an insignificant factor for most experiments. The adaptation of the system to various types of SPM experiments is discussed. Advances in audio electronics and digital signal processors have made the construction of such high performance systems possible at low cost
Analytical Rescaling of Polymer Dynamics from Mesoscale Simulations
We present a theoretical approach to scale the artificially fast dynamics of
simulated coarse-grained polymer liquids down to its realistic value. As
coarse-graining affects entropy and dissipation, two factors enter the
rescaling: inclusion of intramolecular vibrational degrees of freedom, and
rescaling of the friction coefficient. Because our approach is analytical, it
is general and transferable. Translational and rotational diffusion of
unentangled and entangled polyethylene melts, predicted from mesoscale
simulations of coarse-grained polymer melts using our rescaling procedure, are
in quantitative agreement with united atom simulations and with experiments.Comment: 6 pages, 2 figures, 2 table
The effect of low-energy ion-implantation on the electrical transport properties of Si-SiO2 MOSFETs
Using silicon MOSFETs with thin (5nm) thermally grown SiO2 gate dielectrics,
we characterize the density of electrically active traps at low-temperature
after 16keV phosphorus ion-implantation through the oxide. We find that, after
rapid thermal annealing at 1000oC for 5 seconds, each implanted P ion
contributes an additional 0.08 plus/minus 0.03 electrically active traps,
whilst no increase in the number of traps is seen for comparable silicon
implants. This result shows that the additional traps are ionized P donors, and
not damage due to the implantation process. We also find, using the room
temperature threshold voltage shift, that the electrical activation of donors
at an implant density of 2x10^12 cm^-2 is ~100%.Comment: 11 pages, 10 figure
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