25,866 research outputs found
Simulation of the Gravsat/Geopause mission
A simulation of the proposed low Gravsat and high Geopause satellite mission is presented. This mission promises fundamental improvements in the accuracy of low order geopotential coefficients by using satellite-to-satellite tracking technology coupled with a global sampling of the gravity field. Ten days of data from six stations are assumed. A drag compensation system for the low satellite is also postulated. The results show a one to two order of magnitude improvement in the accuracy of the low order coefficients through degree 8 and order 6. These results are easily adjusted to reflect a different data accuracy level and low satellite altitude
Long and short arc altitude determination for GEOS-C
The accuracy with which the GEOS-C altitude may be estimated over long (7 day) and short (40 minute) orbital arcs is investigated. Over the long are excellent agreement was attained between a simulation of the orbit determination process and a covariance analysis. Both approaches yielded RMS altitude errors of about 1.5 meters over the Caribbean calibration area and approximately 7.5 meters overall. The geopotential was identified as the largest error source. For the short arc, the covariance analysis revealed that the propagated altitude error is linearly dependent upon station survey component errors which are also the largest source of altitude errors. An Appendix contains the mathematics of covariance analysis as applied to orbit determination
The 'gated-diode' configuration in MOSFET's, a sensitive tool for characterizing hot-carrier degradation
This paper describes a new measurement technique, the forward gated-diode current characterized at low drain voltages to be applied in MOSFET's for investigating hot-carrier stress-induced defects at high spatial resolution. The generation/recombination current in the drain-to-substrate diode as a function of gate voltage, combined with two-dimensional numerical simulation, provides a sensitive tool for detecting the spatial distribution and density of interface defects. In the case of strong accumulation, additional information is obtained from interband tunneling processes occurring via interface defects. The various mechanisms for generating interface defects and fixed charges at variable stress conditions are discussed, showing that information complementary to that available from other methods is obtaine
Coarse-grained interaction potentials for polyaromatic hydrocarbons
Using Kohn-Sham density functional theory (KS-DFT), we have studied the
interaction between various polyaromatic hydrocarbon molecules. The systems
range from mono-cyclic benzene up to hexabenzocoronene (hbc). For several
conventional exchange-correlation functionals potential energy curves of
interaction of the - stacking hbc dimer are reported. It is found
that all pure local density or generalized gradient approximated functionals
yield qualitatively incorrect predictions regarding structure and interaction.
Inclusion of a non-local, atom-centered correction to the KS-Hamiltonian
enables quantitative predictions. The computed potential energy surfaces of
interaction yield parameters for a coarse-grained potential, which can be
employed to study discotic liquid-crystalline mesophases of derived
polyaromatic macromolecules
Slow synaptic transmission in frog sympathetic ganglia
Bullfrog ganglia contain two classes of neurone, B and C cells, which receive different inputs and exhibit different slow synaptic potentials. B cells, to which most effort has been directed, possess slow and late slow EPSPs. The sEPSP reflects a muscarinic action of acetylcholine released from boutons on B cells, whereas the late sEPSP is caused by a peptide (similar to teleost LHRH) released from boutons on C cells. During either sEPSP there is a selective reduction in two slow potassium conductances, designated 'M' and 'AHP'. The M conductance is voltage dependent and the AHP conductance is calcium dependent. Normally they act synergistically to prevent repetitive firing of action potentials during maintained stimuli. Computer stimulation of the interactions of these conductances with the other five voltage-dependent conductances present in the membrane allows a complete reconstruction of the effects of slow synaptic transmission on electrical behaviour
Influence of firing mechanisms on gain modulation
We studied the impact of a dynamical threshold on the f-I curve-the
relationship between the input and the firing rate of a neuron-in the presence
of background synaptic inputs. First, we found that, while the leaky
integrate-and-fire model cannot reproduce the f-I curve of a cortical neuron,
the leaky integrate-and-fire model with dynamical threshold can reproduce it
very well. Second, we found that the dynamical threshold modulates the onset
and the asymptotic behavior of the f-I curve. These results suggest that a
cortical neuron has an adaptation mechanism and that the dynamical threshold
has some significance for the computational properties of a neuron.Comment: 7 pages, 4 figures, conference proceeding
Efficient one- and two-qubit pulsed gates for an oscillator stabilized Josephson qubit
We present theoretical schemes for performing high-fidelity one- and
two-qubit pulsed gates for a superconducting flux qubit. The "IBM qubit"
consists of three Josephson junctions, three loops, and a superconducting
transmission line. Assuming a fixed inductive qubit-qubit coupling, we show
that the effective qubit-qubit interaction is tunable by changing the applied
fluxes, and can be made negligible, allowing one to perform high fidelity
single qubit gates. Our schemes are tailored to alleviate errors due to 1/f
noise; we find gates with only 1% loss of fidelity due to this source, for
pulse times in the range of 20-30ns for one-qubit gates (Z rotations,
Hadamard), and 60ns for a two-qubit gate (controlled-Z). Our relaxation and
dephasing time estimates indicate a comparable loss of fidelity from this
source. The control of leakage plays an important role in the design of our
shaped pulses, preventing shorter pulse times. However, we have found that
imprecision in the control of the quantum phase plays the major role in the
limitation of the fidelity of our gates.Comment: Published version. Added references. Corrected minor typos. Added
discussion on how the influence of 1/f noise is modeled. 36 pages, 11 figure
A Two-Parameter Recursion Formula For Scalar Field Theory
We present a two-parameter family of recursion formulas for scalar field
theory. The first parameter is the dimension . The second parameter
() allows one to continuously extrapolate between Wilson's approximate
recursion formula and the recursion formula of Dyson's hierarchical model. We
show numerically that at fixed , the critical exponent depends
continuously on . We suggest the use of the independence as a
guide to construct improved recursion formulas.Comment: 7 pages, uses Revtex, one Postcript figur
The Quantum Speed Limit of Optimal Controlled Phasegates for Trapped Neutral Atoms
We study controlled phasegates for ultracold atoms in an optical potential. A
shaped laser pulse drives transitions between the ground and electronically
excited states where the atoms are subject to a long-range 1/R^3 interaction.
We fully account for this interaction and use optimal control theory to
calculate the pulse shapes. This allows us to determine the minimum pulse
duration, respectively, gate time T that is required to obtain high fidelity.
We accurately analyze the speed limiting factors, and we find the gate time to
be limited either by the interaction strength in the excited state or by the
ground state vibrational motion in the trap. The latter needs to be resolved by
the pulses in order to fully restore the motional state of the atoms at the end
of the gate.Comment: 11 pages, 10 figures, 1 tabl
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