680 research outputs found
Multi-focal laser surgery: cutting enhancement by hydrodynamic interactions between cavitation bubbles
Transparent biological tissues can be precisely dissected with ultrafast
lasers using optical breakdown in the tight focal zone. Typically, tissues are
cut by sequential application of pulses, each of which produces a single
cavitation bubble. We investigate the hydrodynamic interactions between
simultaneous cavitation bubbles originating from multiple laser foci.
Simultaneous expansion and collapse of cavitation bubbles can enhance the
cutting efficiency by increasing the resulting deformations in tissue, and the
associated rupture zone. An analytical model of the flow induced by the bubbles
is presented and experimentally verified. The threshold strain of the material
rupture is measured in a model tissue. Using the computational model and the
experimental value of the threshold strain one can compute the shape of the
rupture zone in tissue resulting from application of multiple bubbles. With the
threshold strain of 0.7 two simultaneous bubbles produce a continuous cut when
applied at the distance 1.35 times greater than that required in sequential
approach. Simultaneous focusing of the laser in multiple spots along the line
of intended cut can extend this ratio to 1.7. Counter-propagating jets forming
during collapse of two bubbles in materials with low viscosity can further
extend the cutting zone - up to a factor of 1.54.Comment: 16 pages, 8 figures. Paper is accepted for publication in Physical
Review
Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548
The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor
the radio continuum counterpart to the optical broad line region (BLR) in the
Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained
at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited
to observations spanning 10-60 days and 3-4 yr, and acquired at matched
resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA
observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is
photometrically variable at 8.4 GHz by % and % between VLA
observations separated by 41 days and 4.1 yr, respectively. The 41-day changes
are milder (%) at 4.9 GHz and exhibit an inverted spectrum (, ) from 4.9 to 8.4 GHz. The nucleus
is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between
VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc
between VLBA observations separated by 3.1 yr. Such photometric variability and
astrometric stability is consistent with a black hole being the ultimate energy
source for the BLR, but is problematic for star cluster models that treat the
BLR as a compact supernova remnant and, for NGC 5548, require a new supernova
event every 1.7 yr within an effective radius 42 mas = 22 pc. A deep
image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data
from quiescent VLA observations. This image shows faint bipolar lobes
straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These
synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from
the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March
10, volume 53
The Effect of Electronic Structure on the Phases Present in High Entropy Alloys
Multicomponent systems, termed High Entropy Alloys (HEAs), with predominantly single solid solution phases are a current area of focus in alloy development. Although different empirical rules have been introduced to understand phase formation and determine what the dominant phases may be in these systems, experimental investigation has revealed that in many cases their structure is not a single solid solution phase, and that the rules may not accurately distinguish the stability of the phase boundaries. Here, a combined modelling and experimental approach that looks into the electronic structure is proposed to improve accuracy of the predictions of the majority phase. To do this, the Rigid Band model is generalised for magnetic systems in prediction of the majority phase most likely to be found. Good agreement is found when the predictions are confronted with data from experiments, including a new magnetic HEA system (CoFeNiV). This also includes predicting the structural transition with varying levels of constituent elements, as a function of the valence electron concentration, n, obtained from the integrated spin-polarised density of states. This method is suitable as a new predictive technique to identify compositions for further screening, in particular for magnetic HEAs
Sub-Relativistic Radio Jets and Parsec-Scale Absorption in Two Seyfert Galaxies
The Very Long Baseline Array has been used at 15 GHz to image the
milliarcsecond structure of the Seyfert galaxies Mrk 231 and Mrk 348 at two
epochs separated by about 1.7 yr. Both galaxies contain parsec-scale double
radio sources whose components have brightness temperatures of 10^9-10^{11} K,
implying that they are generated by synchrotron emission. The nuclear
components are identified by their strong variability between epochs,
indicating that the double sources represent apparently one-sided jets.
Relative component speeds are measured to be ~0.1c at separations of 1.1 pc or
less (for H_0 = 65 km/s/Mpc), implying that parsec-scale Seyfert jets are
intrinsically different from those in most powerful radio galaxies and quasars.
The lack of observed counterjets is most likely due to free-free absorption by
torus gas, with an ionized density n_e > 2 X 10^5 cm^{-3} at T~8000 K, or n_e >
10^7 cm^{-3} at T~10^{6.6} K, in the inner parsec of each galaxy. The lower
density is consistent with values found from X-ray absorption measurements,
while the higher temperature and density are consistent with direct radio
imaging of the NGC 1068 torus by Gallimore et al.Comment: 12 pages, 2 postscript figures, LaTeX file in AASTeX format, accepted
by ApJ Letter
Bi2V1-x (Mg0.25Cu0.25Ni0.25Zn0.25) x O5.5-3x/2: A high entropy dopant BIMEVOX
A high entropy dopant approach has been used to prepare a new BIMEVOX ceramic system, Bi2V1-x(Mg0.25Cu0.25Ni0.25Zn0.25)xO5.5-3x/2. Structures were investigated using a combination of X-ray and neutron powder diffraction, with electrical characterisation by A.C. impedance spectroscopy. A γ-type phase is observed at room temperature over the compositional range 0.10 ≤ x ≤ 0.30, the upper limit of which is beyond that seen for all the single substituted systems based on these substituents, apart from BIMGVOX. No stabilisation of the fully disordered γ-phase is seen at room temperature over this compositional range, with only the incommensurately ordered γ'-phase evident below around 450 °C. Changes in defect structure are used to explain an apparent transition in the compositional variation of lattice parameters. The HE dopant approach has no detrimental effect on ionic conductivity, with values comparable to those of the single substituted systems based on the component oxides
Local structure and conductivity behaviour in Bi7WO13.5
Total neutron scattering analysis reveals details of cation coordination and vacancy distribution in Bi7WO13.5.</p
Spin transport in inhomogeneous magnetic fields: a proposal for Stern-Gerlach-like experiments with conduction electrons
Spin dynamics in spatially inhomogeneous magnetic fields is studied within
the framework of Boltzmann theory. Stern-Gerlach-like separation of spin up and
spin down electrons occurs in ballistic and diffusive regimes, before spin
relaxation sets in. Transient dynamics and spectral response to time-dependent
inhomogeneous magnetic fields are investigated, and possible experimental
observations of our findings are discussed.Comment: 7 pages, 4 figures; revised and extended version, to appear in PR
Dopant clustering and vacancy ordering in neodymium doped ceria
Lanthanide doped cerias, show fast oxide ion conduction and have applications as electrolytes in intermediate temperature solid oxide fuel cells. Here, we examine the long- and short-range structures of Ce1−xNdxO2−x/2 (0.05 ≤ x ≤ 0.30, NDC) using reverse Monte Carlo modelling of total neutron scattering data, supported by measurements of electrical behaviour using a.c. impedance spectroscopy. Three distinct features are evident in the local structure of NDC, viz.: clustering of Nd3+ cations, preferred Nd3+-oxide ion vacancy association and oxide ion vacancy clustering with preferential alignment in the 〈100〉 direction. Interestingly, the presence of preferential dopant cation-oxide ion vacancy association is also observed at 600 °C, although diminished compared to the level at room temperature. This suggests a continued contribution of defect association enthalpy to activation energy at elevated temperatures and is reflected in similar compositional variation of high- and low-temperature activation energies
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