7,537 research outputs found
Implications of non-feasible transformations among icosahedral orbitals
The symmetric group that permutes the six five-fold axes of an
icosahedron is introduced to go beyond the simple rotations that constitute the
icosahedral group . Owing to the correspondence , the
calculation of the Coulomb energies for the icosahedral configurations
based on the sequence can be brought
to bear on Racah's classic theory for the atomic d shell based on . Among the elements of is the kaleidoscope
operator that rotates the weight space of SO(5) by . Its use
explains some puzzling degeneracies in d^3 involving the spectroscopic terms
^2P, ^2F, ^2G and ^2H.Comment: Tentatively scheduled to appear in Physical Preview Letters Apr 5,
99. Revtex, 1 ps figur
Exploiting soliton decay and phase fluctuations in atom chip interferometry of Bose-Einstein condensates
We show that the decay of a soliton into vortices provides a mechanism for
measuring the initial phase difference between two merging Bose-Einstein
condensates. At very low temperatures, the mechanism is resonant, operating
only when the clouds start in anti-phase. But at higher temperatures, phase
fluctuations trigger vortex production over a wide range of initial relative
phase, as observed in recent experiments at MIT. Choosing the merge time to
maximize the number of vortices created makes the interferometer highly
sensitive to spatially varying phase patterns and hence atomic movement.Comment: 5 pages, 5 figure
Quantifying Finite Temperature Effects in Atom Chip Interferometry of Bose-Einstein Condensates
We quantify the effect of phase fluctuations on atom chip interferometry of
Bose-Einstein condensates. At very low temperatures, we observe small phase
fluctuations, created by mean-field depletion, and a resonant production of
vortices when the two clouds are initially in anti-phase. At higher
temperatures, we show that the thermal occupation of Bogoliubov modes makes
vortex production vary smoothly with the initial relative phase difference
between the two atom clouds. We also propose a technique to observe vortex
formation directly by creating a weak link between the two clouds. The position
and direction of circulation of the vortices is subsequently revealed by kinks
in the interference fringes produced when the two clouds expand into one
another. This procedure may be exploited for precise force measurement or
motion detection.Comment: 7 pages, 5 figure
Improving the theoretical prediction for the width difference: matrix elements of next-to-leading order operators
We present lattice QCD results for the matrix elements of and other
dimension-7, operators relevant for calculations of , the width difference. We have computed correlation
functions using 5 ensembles of the MILC Collaboration's 2+1+1-flavour gauge
field configurations, spanning 3 lattice spacings and light sea quarks masses
down to the physical point. The HISQ action is used for the valence strange
quarks, and the NRQCD action is used for the bottom quarks. Once our analysis
is complete, the theoretical uncertainty in the Standard Model prediction for
will be substantially reduced.Comment: 8 pages. To appear in the Proceedings of the 35th International
Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai
More on coupling coefficients for the most degenerate representations of SO(n)
We present explicit closed-form expressions for the general group-theoretical
factor appearing in the alpha-topology of a high-temperature expansion of
SO(n)-symmetric lattice models. This object, which is closely related to
6j-symbols for the most degenerate representation of SO(n), is discussed in
detail.Comment: 9 pages including 1 table, uses IOP macros Update of Introduction and
Discussion, References adde
Finding the cost of control
A large and growing literature has demonstrated that explicit incentives, such as enforceable contracts, can lead agents to withhold effort. We investigate when this behavioral result arises. In an extensive laboratory experiment, we find that imposing control through an enforceable contract is only detrimental to principals in a special case when: (1) there is a preexisting norm that agents provide high effort; (2) control is imposed unilaterally and has an asymmetric effect on the agent; (3) control is weak (i.e. it cannot induce significant effort); and (4) the agent does not use control when acting as a principal
Quantum reflection of ultracold atoms from thin films, graphene, and semiconductor heterostructures
We show that thin dielectric films can be used to enhance the performance of
passive atomic mirrors by enabling quantum reflection probabilities of over 90%
for atoms incident at velocities ~1 mm/s, achieved in recent experiments. This
enhancement is brought about by weakening the Casimir-Polder attraction between
the atom and the surface, which induces the quantum reflection. We show that
suspended graphene membranes also produce higher quantum reflection
probabilities than bulk matter. Temporal changes in the electrical resistance
of such membranes, produced as atoms stick to the surface, can be used to
monitor the reflection process, non-invasively and in real time. The resistance
change allows the reflection probability to be determined purely from
electrical measurements without needing to image the reflected atom cloud
optically. Finally, we show how perfect atom mirrors may be manufactured from
semiconductor heterostructures, which employ an embedded two-dimensional
electron gas to tailor the atom-surface interaction and so enhance the
reflection by classical means.Comment: 8 pages, 4 figure
Design of a microwave radiometer for monitoring high voltage insulator contamination level
Microwave radiometry is a novel method for monitoring contamination levels on high voltage insulators. The microwave radiometer described measures energy emitted from the contamination layer and could provide a safe, reliable, contactless monitoring method that is effective under dry conditions. The design of the system has focused on optimizing accuracy, stability and sensitivity using a relatively low cost architecture. Experimental results demonstrate that the output from the radiometer is able to clearly distinguish between samples with different contamination levels under dry conditions. This contamination monitoring method could potentially provide advance warning of the future failure of wet insulators in climates where insulators can experience dry conditions for extended periods
Detecting periodicity in experimental data using linear modeling techniques
Fourier spectral estimates and, to a lesser extent, the autocorrelation
function are the primary tools to detect periodicities in experimental data in
the physical and biological sciences. We propose a new method which is more
reliable than traditional techniques, and is able to make clear identification
of periodic behavior when traditional techniques do not. This technique is
based on an information theoretic reduction of linear (autoregressive) models
so that only the essential features of an autoregressive model are retained.
These models we call reduced autoregressive models (RARM). The essential
features of reduced autoregressive models include any periodicity present in
the data. We provide theoretical and numerical evidence from both experimental
and artificial data, to demonstrate that this technique will reliably detect
periodicities if and only if they are present in the data. There are strong
information theoretic arguments to support the statement that RARM detects
periodicities if they are present. Surrogate data techniques are used to ensure
the converse. Furthermore, our calculations demonstrate that RARM is more
robust, more accurate, and more sensitive, than traditional spectral
techniques.Comment: 10 pages (revtex) and 6 figures. To appear in Phys Rev E. Modified
styl
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