11,629 research outputs found
Results of literature search on dielectric properties and electron interaction phenomena related to spacecraft charging
The objective of the literature search was to determine the required material properties and electron interaction parameters needed for modeling charge buildup and breakdown in insulators. A brief overview of the results of the literature search is given. A partial list of the references covered is included in a bibliography. Although inorganic insulators were also considered in the search, coverage is limited to the organics, primarily Kapton and Teflon
Investigations for the improvement of space shuttle main engine electron beam welding equipment
Progress made in the testing, evaluation, and correction of MSFC's 7.5 kW electron beam welder in support of space shuttle main engine component welding is summarized. The objective of this project was to locate and correct the deficiencies in the welder. Some 17 areas were deficient in the 7.5 kW ERI welding system and the associated corrective action was taken to improve its operational performance. An overall improvement of 20 times the original reliability was obtained at full rated capacity after the modifications were made
Structural characterization of YBa(2)Cu(3)O(7)/Y(2)O(3) composite films
Using 4-circle x-ray diffraction and transmission electron microscopy we have
studied the microstructure and in-plane orientation of the phases present in
thin film composite mixtures of YBa(2)Cu(3)O(7) and Y(2)O(3). We see a high
degree of in-plane orientation and have verified a previous prediction for the
in-plane order of Y(2)BaCuO(5) on (110) MgO. Transmission electron microscopy
shows the composite films to be a mixture of two phases, with YBCO grain sizes
of 1 micron. We have also compared our observations of the in-plane order to
the predictions of a modified near coincidence site lattice model.Comment: To be published in Journal of Materials Research, (4 pages, 4 jpeg
figures
Quantum magnetism with ultracold molecules
This article gives an introduction to the realization of effective quantum
magnetism with ultracold molecules in an optical lattice, reviews experimental
and theoretical progress, and highlights future opportunities opened up by
ongoing experiments. Ultracold molecules offer capabilities that are otherwise
difficult or impossible to achieve in other effective spin systems, such as
long-ranged spin-spin interactions with controllable degrees of spatial and
spin anisotropy and favorable energy scales. Realizing quantum magnetism with
ultracold molecules provides access to rich many-body behaviors, including many
exotic phases of matter and interesting excitations and dynamics.
Far-from-equilibrium dynamics plays a key role in our exposition, just as it
did in recent ultracold molecule experiments realizing effective quantum
magnetism. In particular, we show that dynamical probes allow the observation
of correlated many-body spin physics, even in polar molecule gases that are not
quantum degenerate. After describing how quantum magnetism arises in ultracold
molecules and discussing recent observations of quantum magnetism with polar
molecules, we survey prospects for the future, ranging from immediate goals to
long-term visions.Comment: 21 pages, 6 figures, 1 table. Review articl
Automated weld torch guidance control system
A device for automatically controlling the movement of a welding torch while welding an elongated joint is described. A charge injection television camera is carried on a movable support. The camera includes a matrix of individual light sensing video elements which generate voltages responsive to light reflected off of the joint and surrounding areas of the work piece. The voltages produced by the pixels are converted to digital words which are fed to a microprocessor for generating an error signal. This error signal is fed to a digital motor which is used to drive a movable support upon which the television camera is carried
Solar cell radiation response near the interface of different atomic number materials
The response of cobalt 60 irradiated N/P silicon solar cells was measured as a function of the atomic number of the medium adjacent to the cell and the direction of the gamma ray beam. The interpositioning of various thicknesses of aluminum between the adjacent material and the cell had the effect of moving the cell to various locations in an approximate monatomic numbered medium. Using this technique the solar cell response was determined at various distances from the interface for gold and beryllium. The results were compared with predictions based upon ionization chamber measurements of dose perturbations in aluminum and found to agree within five percent. Ionization chamber data was then used to estimate the influence of various base contact materials
Cooling Fermions in an Optical Lattice by Adiabatic Demagnetization
The Fermi-Hubbard model describes ultracold fermions in an optical lattice
and exhibits antiferromagnetic long-ranged order below the N\'{e}el
temperature. However, reaching this temperature in the lab has remained an
elusive goal. In other atomic systems, such as trapped ions, low temperatures
have been successfully obtained by adiabatic demagnetization, in which a strong
effective magnetic field is applied to a spin-polarized system, and the
magnetic field is adiabatically reduced to zero. Unfortunately, applying this
approach to the Fermi-Hubbard model encounters a fundamental obstacle: the
symmetry introduces many level crossings that prevent the system from
reaching the ground state, even in principle. However, by breaking the
symmetry with a spin-dependent tunneling, we show that adiabatic
demagnetization can achieve low temperature states. Using density matrix
renormalization group (DMRG) calculations in one dimension, we numerically find
that demagnetization protocols successfully reach low temperature states of a
spin-anisotropic Hubbard model, and we discuss how to optimize this protocol
for experimental viability. By subsequently ramping spin-dependent tunnelings
to spin-independent tunnelings, we expect that our protocol can be employed to
produce low-temperature states of the Fermi-Hubbard Model.Comment: References adde
Bosonic molecules in a lattice: unusual fluid phase from multichannel interactions
We show that multichannel interactions significantly alter the phase diagram
of ultracold bosonic molecules in an optical lattice. Most prominently, an
unusual fluid region intervenes between the conventional superfluid and the
Mott insulator. In it, number fluctuations remain but phase coherence is
suppressed by a significant factor. This factor can be made arbitrarily large,
at least in a two-site configuration. We calculate the phase diagram using
complementary methods, including Gutzwiller mean-field and density matrix
renormalization group (DMRG) calculations. Although we focus on bosonic
molecules without dipolar interactions, we expect multichannel interactions to
remain important for dipolar interacting and fermionic molecules.Comment: 6 pages incl. refs, 4 figure
Ultracold nonreactive molecules in an optical lattice: connecting chemistry to many-body physics
We derive effective lattice models for ultracold bosonic or fermionic
nonreactive molecules (NRMs) in an optical lattice, analogous to the Hubbard
model that describes ultracold atoms in a lattice. In stark contrast to the
Hubbard model, which is commonly assumed to accurately describe NRMs, we find
that the single on-site interaction parameter is replaced by a
multi-channel interaction, whose properties we elucidate. The complex,
multi-channel collisional physics is unrelated to dipolar interactions, and so
occurs even in the absence of an electric field or for homonuclear molecules.
We find a crossover between coherent few-channel models and fully incoherent
single-channel models as the lattice depth is increased. We show that the
effective model parameters can be determined in lattice modulation experiments,
which consequently measure molecular collision dynamics with a vastly sharper
energy resolution than experiments in an ultracold gas.Comment: 4 pages+refs, 3 figures; 2.5 pages+1 figure Supplemental Materia
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