4,676 research outputs found
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
Robot-Bone Attachment Device for Robot-Assisted Percutaneous Bone Fragment Manipulation
The treatment of joint-fractures is a common task in orthopaedic surgery causing considerable health costs and patient disabilities. Percutaneous techniques have been developed to mitigate the problems related to open surgery (e.g. soft tissue damage), although their application to joint-fractures is limited by the sub-optimal intra-operative imaging (2D-fluoroscopy) and by the high forces involved. Our earlier research toward improving percutaneous reduction of intra-articular fractures has resulted in the creation of a robotic system prototype, i.e. RAFS (Robot-Assisted Fracture Surgery) system.
We propose a robot-bone attachment device for percutaneous bone manipulation, which can be anchored to the bone fragment through one small incision, ensuring the required stability and reducing the “biological cost” of the procedure. The device has been evaluated through the reduction of 9 distal femur fractures on human cadavers using the RAFS system
Binaries in star clusters and the origin of the field stellar population
Many, possibly most, stars form in binary and higher-order multiple systems.
Therefore, the properties and frequency of binary systems provide strong clues
to the star-formation process, and constraints on star-formation models.
However, the majority of stars also form in star clusters in which the birth
binary properties and frequency can be altered rapidly by dynamical processing.
Thus, we almost never see the birth population, which makes it very difficult
to know if star formation (as traced by binaries, at least) is universal, or if
it depends on environment. In addition, the field population consists of a
mixture of systems from different clusters which have all been processed in
different ways.Comment: 16 pages, no figures. To appear as invited review article in a
special issue of the Phil. Trans. Royal Soc. A: Ch. 8 "Star clusters as
tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer
reviewed. LaTeX, requires rspublic.cls style fil
Does elevated atmospheric CO2 allow for sufficient wheat grain quality in the future?
To identify future impacts on biomass production and yield quality of important C3 crops, spring wheat was grown in association with 13 weed species in a Mini-FACE (free-air carbon dioxide (CO2) enrichment) system under ambient (375 μl l-1) and elevated (526 μl l-1) CO2 concentrations. Wheat productivity was assessed at maturity and grain yield was subjected to various chemical analyses and baking quality tests.CO2 enrichment acted as carbon ‘fertiliser’ and increased the aboveground biomass production of wheat by 18.8% as there was a trend towards higher stem biomass. Although not statistically significant, wheat grain yield was increased by 13.4% due to a significant establishment of more grains per unit ground area. At the same time, thousand grain weight was non-significantly shifted towards smaller grain size classes, which may result in negative consequences for the crop market value. As a result of the CO2- induced physiological and biochemical modifications, concentration of total grain protein was significantly decreased by 3.5%, reducing the wheat grain quality with potentially far-reaching impacts on the nutritional value and use for processing industry. Although often not significant, the concentrations of amino acids per unit of flour were decreased by 0.2 to 8.3% due to elevated CO2 thereby affecting the composition of proteinogenic amino acids.Furthermore, gluten proteins tended to decline. Within the significant decreased gliadins, α- and ω5-gliadins were significantly reduced under CO2 enrichment; there was also a negative trend for ω1,2- and γ-gliadins. Changes in certain essential minerals were found as well, although not statistically significant. Concentrations of sodium, calcium, phosphorus and sulphur were slightly lowered and those of potassium and magnesium were slightly increased due to CO2 enrichment. The micro-element molybdenum was increased, while concentrations of iron, zinc, copper, manganese and aluminium were decreased. With regard to rheological and baking parameters defining the cereal quality for industrial processing, the resistance of the dough was significantly reduced by about 30%, while the extensibility was non-significantly increased by 17.1% under CO2 enrichment. Moreover, the bread volume was decreased non-significantly by about 9%. Elevated CO2 is obviously affecting grain characteristics important for consumer nutrition and health, industrial processing and marketing. Experimental evidence for these changes is still poor but deserves further attention
Matrix-product-groundstates for one-dimensional spin-1 quantum antiferromagnets
We have found the exact groundstate for a large class of antiferromagnetic
spin-1 models with nearest-neighbour interactions on a linear chain. All
groundstate properties can be calculated. The groundstate is determined as a
matrix product of individual site states and has the properties of the Haldane
scenario.Comment: 8 pages (plain tex), preprint cologne-93-471
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
The evolution of rotating very massive stars with LMC composition
We present a dense model grid with tailored input chemical composition
appropriate for the Large Magellanic Cloud. We use a one-dimensional
hydrodynamic stellar evolution code, which accounts for rotation, transport of
angular momentum by magnetic fields, and stellar wind mass loss to compute our
detailed models. We calculate stellar evolution models with initial masses of
70-500 Msun and with initial surface rotational velocities of 0-550 km/s,
covering the core-hydrogen burning phase of evolution. We find our rapid
rotators to be strongly influenced by rotationally induced mixing of helium,
with quasi-chemically homogeneous evolution occurring for the fastest rotating
models. Above 160 Msun, homogeneous evolution is also established through mass
loss, producing pure helium stars at core hydrogen exhaustion independent of
the initial rotation rate. Surface nitrogen enrichment is also found for slower
rotators, even for stars that lose only a small fraction of their initial mass.
For models above 150 MZAMS, and for models in the whole considered mass range
later on, we find a considerable envelope inflation due to the proximity of
these models to their Eddington limit. This leads to a maximum zero-age main
sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of
stars in the mass range 50-100 Msun to the regime of luminous blue variables in
the HR diagram with high internal Eddington factors. Inflation also leads to
decreasing surface temperatures during the chemically homogeneous evolution of
stars above 180 Msun. The cool surface temperatures due to the envelope
inflation in our models lead to an enhanced mass loss, which prevents stars at
LMC metallicity from evolving into pair-instability supernovae. The
corresponding spin-down will also prevent very massive LMC stars to produce
long-duration gamma-ray bursts, which might, however, originate from lower
masses.Comment: 21 pages, 25 figure
Generalized Scaling for Models with Multiple Absorbing States
At a continuous transition into a nonunique absorbing state, particle systems
may exhibit nonuniversal critical behavior, in apparent violation of
hyperscaling. We propose a generalized scaling theory for dynamic critical
behavior at a transition into an absorbing state, which is capable of
describing exponents which vary according to the initial configuration. The
resulting hyperscaling relation is supported by simulations of two lattice
models.Comment: Latex 9 page
Non-instant collisions and two concepts of quasiparticle
The kinetic theory recently implemented in heavy ion reactions combines a
non-local and non-instant picture of binary collisions with quasiparticle
features. We show that the non-instant description is compatible with the
spectral concept of quasiparticles while the commonly used variational concept
is consistent only with instant collisions. The rearrangement energy, by which
the variational concept surpasses the spectral one, is shown to be covered by a
medium effect on non-instant collisions.Comment: PRE 59,1 rap. com
Break up of heavy fermions at an antiferromagnetic instability
We present results of high-resolution, low-temperature measurements of the
Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2.
They support earlier conclusions of an electronic (Kondo-breakdown) quantum
critical point concurring with a field induced antiferromagnetic one. We also
discuss the detachment of the two instabilities under chemical pressure. Volume
compression/expansion (via substituting Rh by Co/Ir) results in a
stabilization/weakening of magnetic order. Moderate Ir substitution leads to a
non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor
screened by the Kondo effect. The so-derived zero-temperature global phase
diagram promises future studies to explore the nature of the Kondo breakdown
quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS
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