1,588 research outputs found
Elastic properties of cubic crystals: Every's versus Blackman's diagram
Blackman's diagram of two dimensionless ratios of elastic constants is
frequently used to correlate elastic properties of cubic crystals with
interatomic bondings. Every's diagram of a different set of two dimensionless
variables was used by us for classification of various properties of such
crystals. We compare these two ways of characterization of elastic properties
of cubic materials and consider the description of various groups of materials,
e.g. simple metals, oxides, and alkali halides. With exception of intermediate
valent compounds, the correlation coefficients for Every's diagrams of various
groups of materials are greater than for Blackaman's diagrams, revealing the
existence of a linear relationship between two dimensionless Every's variables.
Alignment of elements and compounds along lines of constant Poisson's ratio
, ( arbitrary perpendicular to ) is
observed. Division of the stability region in Blackman's diagram into region of
complete auxetics, auxetics and non-auxetics is introduced. Correlations of a
scaling and an acoustic anisotropy parameter are considered.Comment: 8 pages, 9 figures, presented on The Ninth International School on
Theoretical Physics "Symmetry and Structural Properties of Condensed Matter",
5 - 12 September 2007, Myczkowce, Polan
Understanding the performance of water supply systems during mild to extreme droughts
This project assessed the performance of different types of public water supply systems in England and Wales in a range of droughts, including those that are more severe than the worst droughts in the historical record
Gyroscopes based on nitrogen-vacancy centers in diamond
We propose solid-state gyroscopes based on ensembles of negatively charged
nitrogen-vacancy () centers in diamond. In one scheme, rotation of
the nitrogen-vacancy symmetry axis will induce Berry phase shifts in the electronic ground-state coherences proportional to the solid angle
subtended by the symmetry axis. We estimate sensitivity in the range of
in a 1 sensor volume using
a simple Ramsey sequence. Incorporating dynamical decoupling to suppress
dipolar relaxation may yield sensitivity at the level of . With a modified Ramsey scheme, Berry phase shifts in the
hyperfine sublevels would be employed. The projected sensitivity
is in the range of , however the smaller
gyromagnetic ratio reduces sensitivity to magnetic-field noise by several
orders of magnitude. Reaching would represent
an order of magnitude improvement over other compact, solid-state gyroscope
technologies.Comment: 3 figures, 5 page
Cancellation of nonlinear Zeeman shifts with light shifts
Nonlinear Zeeman (NLZ) shifts arising from magnetic-field mixing of the two
hyperfine ground-states in alkali atoms lead to splitting of magnetic-resonance
lines. This is a major source of sensitivity degradation and the so-called
"heading errors" of alkali-vapor atomic magnetometers operating in the
geophysical field range (B approx. 0.2-0.7 G). Here, it is shown theoretically
and experimentally that NLZ shifts can be effectively canceled by light shifts
caused by a laser field of appropriate intensity, polarization and frequency, a
technique that can be readily applied in practical situations.Comment: 5 pages, 5 figures, to be published in PR
Influence of Mechanical Layering and Natural Fractures on Undercutting and Rapid Headward Erosion (Recession) at Canyon Lake Spillway, Texas, U.S.A
This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting and resulting rapid spillway recession. In the summer of 2002, 86 cm of rain fell in an 8-day period across the Guadalupe River drainage basin in central Texas, causing Canyon Lake reservoir to completely fill and overtop the emergency spillway for the first time. The resulting flood incised a gorge into the mechanically layered Glen Rose Formation and caused headward erosion (recession) at the downstream edge of the emergency spillway. Comparison of pre- and post-flood imagery and assessment of flood records indicates that maximum recession localized at the northern end of the emergency spillway where 28 m recession occurred. This recession occurred at an estimated rate of up to 10 m/day during the first ~3 days of the flood, which is among the highest rates of recorded bedrock recession. Analysis of historical photographs, field observations and measurement of erosional undercutting, along with measurements of fracture orientation, fracture spacing, and mechanical rebound are used to understand rock mass characteristics that influenced erosional undercutting and rapid recession of the spillway. Evidence of significant undercutting was observed where incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal that the greatest amount and rate of recession of the spillway was associated with undercutting and toppling collapse of fracture-bounded limestone blocks. Block size may be a factor in continuation of the process, in that large blocks may accumulate at the base of the scarp and inhibit continued erosional undercutting, whereas in other areas smaller eroded blocks can be carried away by the floodwaters and undercutting may continue, facilitating recession. The combination of mechanical contrast between layers and natural fractures in competent layers together contributed to exceptionally high rates of headward erosion. Observed rock mass erodibility behavior was in the range of medium to high erodibility in limestone with widely spaced fractures that would normally be expected to have very low erodibility. Bulk rock mass erodibility in this situation was similar to the most erodibile layer, specifically, the marl at base of spillway pour-off cliff
Temperature dependence of the nitrogen-vacancy magnetic resonance in diamond
The temperature dependence of the magnetic resonance spectra of
nitrogen-vacancy (NV-) ensembles in the range of 280-330 K was studied. Four
samples prepared under different conditions were studied with NV-
concentrations ranging from 10 ppb to 15 ppm. For all of these samples, the
axial zero-field splitting (ZFS) parameter, D, was found to vary significantly
with temperature, T, as dD/dT = -74.2(7) kHz/K. The transverse ZFS parameter,
E, was non-zero (between 4 and 11 MHz) in all samples, and exhibited a
temperature dependence of dE/(EdT) = -1.4(3) x 10^(-4) K^(-1). The results
might be accounted for by considering local thermal expansion. The observation
of the temperature dependence of the ZFS parameters presents a significant
challenge for room-temperature diamond magnetometers and may ultimately limit
their bandwidth and sensitivity.Comment: 5 pages, 2 figures, 1 tabl
How do you know if you ran through a wall?
Stable topological defects of light (pseudo)scalar fields can contribute to
the Universe's dark energy and dark matter. Currently the combination of
gravitational and cosmological constraints provides the best limits on such a
possibility. We take an example of domain walls generated by an axion-like
field with a coupling to the spins of standard-model particles, and show that
if the galactic environment contains a network of such walls, terrestrial
experiments aimed at detection of wall-crossing events are realistic. In
particular, a geographically separated but time-synchronized network of
sensitive atomic magnetometers can detect a wall crossing and probe a range of
model parameters currently unconstrained by astrophysical observations and
gravitational experiments.Comment: 5 pages, 2 figure; to appear in the PR
Toward lowering the cost of mission operations
The mission operations system is one of the more significant drivers of the cost of the mission operations and data analysis segment of missions. In large or long-lived projects, the MOS can also be a driver in total mission cost. Larger numbers of missions, together with an increasingly cost-conscious environment, dictate that future missions must more strictly control costs as they perform to their requirements. It is therefore prudent to examine the conduct of past missions for ways to conserve resources. In this paper we review inputs made to past projects' 'lessons-learned' activities, in which personnel from past projects (among other things) identified major cost drivers of MOS's and considered how economies were or might have been realized in both design and performance of their MOS. Common themes among four such reviews are summarized in an attempt to provide suggestions for cost reduction in future missions
Measurement of Untruncated Nuclear Spin Interactions via Zero- to Ultra-Low-Field Nuclear Magnetic Resonance
Zero- to ultra-low-field nuclear magnetic resonance (ZULF NMR) provides a new
regime for the measurement of nuclear spin-spin interactions free from effects
of large magnetic fields, such as truncation of terms that do not commute with
the Zeeman Hamiltonian. One such interaction, the magnetic dipole-dipole
coupling, is a valuable source of spatial information in NMR, though many terms
are unobservable in high-field NMR, and the coupling averages to zero under
isotropic molecular tumbling. Under partial alignment, this information is
retained in the form of so-called residual dipolar couplings. We report zero-
to ultra-low-field NMR measurements of residual dipolar couplings in
acetonitrile-2-C aligned in stretched polyvinyl acetate gels. This
represents the first investigation of dipolar couplings as a perturbation on
the indirect spin-spin -coupling in the absence of an applied magnetic
field. As a consequence of working at zero magnetic field, we observe terms of
the dipole-dipole coupling Hamiltonian that are invisible in conventional
high-field NMR. This technique expands the capabilities of zero- to
ultra-low-field NMR and has potential applications in precision measurement of
subtle physical interactions, chemical analysis, and characterization of local
mesoscale structure in materials.Comment: 6 pages, 3 figure
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