67,969 research outputs found
Non-equilibrium conductivity at quantum critical points
Quantum criticality provides an important route to revealing universal
non-equilibrium behaviour. A canonical example of a quantum critical point is
the Bose-Hubbard model, which we study under the application of an electric
field. A Boltzmann transport formalism and -expansion are used to
obtain the non-equilibrium conductivity and current noise. This approach allows
us to explicitly identify how a universal non-equilibrium steady state is
maintained, by identifying the rate-limiting step in balancing Joule heating
and dissipation to a heat bath. It also reveals that the non-equilibrium
distribution function is very far from a thermal distribution.Comment: 5 pages, 2 figure
Ward round documentation in a major trauma centre: can we improve patient safety?
Our objective was to improve documentation and patient safety in a major trauma centre. A retrospective audit was undertaken in March 2014. Ward round entries for each orthopaedic patients on three dates were assessed against standards and analysed. The audit was repeated in April 2014, and again in August 2014. Thorough documentation is paramount in a major trauma centre. It forms a useful record of the patients hospital stay, is a legal document and is highlighted in national guidelines. It provides a basis for good handover, ensuring continuation of care and maintaining patient safety. Resultant poor compliance with Royal College guidelines in the initial audit led to the production of a new electronic based note keeping system. A meeting was held with all staff prior to introduction. Our initial results gained 75 entries, and none showed full compliance. Mean compliance per entry was 59% (0-81%). The second attempt gained 90 entries, with 30 from the weekend. Mean compliance per entry 97%. Third attempt received 61 entries, with 27 from the weekend. Mean compliance was 96%, meaning that the improvement was being maintained. Recent distressing reports regarding patient highlighted the importance of patient. Our initial audit proved there were many areas lacking in our documentation and improvement was necessary. Prior to introducing electronic systems, the implemented change has produced improvement in documentation, and provides a useful handover tool for staff
Non-linear quantum critical transport and the Schwinger Mechanism
Scaling arguments imply that quantum critical points exhibit universal
non-linear responses to external probes. We investigate the origins of such
non-linearities in transport, which is especially problematic since the system
is necessarily driven far from equilibrium. We argue that for a wide class of
systems the new ingredient that enters is the Schwinger mechanism--the
production of carriers from the vacuum by the applied field-- which is then
balanced against a scattering rate which is itself set by the field. We show by
explicit computation how this works for the case of the symmetric
superfluid-Mott insulator transition of bosons
Many-body theory for positronium-atom interactions
A many-body-theory approach has been developed to study positronium-atom
interactions. As first applications, we calculate the elastic scattering and
momentum-transfer cross sections and the pickoff annihilation rate
for Ps collisions with He and Ne. The cross section for He is
in agreement with previous coupled-state calculations, and the
momentum-transfer cross section for Ne agrees with available experimental data.
is found to be 0.13 and 0.26 for He and Ne, respectively, in
excellent agreement with the measured values.Comment: Accepted by Phys. Rev. Lett. (V2 contains update to text and Figs. 3
and 5. V3 contains further discussion on the calculation of pickoff
annihilation rates.
Positron scattering and annihilation on noble gas atoms
Positron scattering and annihilation on noble gas atoms below the positronium
formation threshold is studied ab initio using many-body theory methods. The
many-body theory provides a near-complete understanding of the
positron-noble-gas-atom system at these energies and yields accurate numerical
results. It accounts for positron-atom and electron-positron correlations,
e.g., polarization of the atom by the incident positron and the
non-perturbative process of virtual positronium formation. These correlations
have a large effect on the scattering dynamics and result in a strong
enhancement of the annihilation rates compared to the independent-particle
mean-field description. Computed elastic scattering cross sections are found to
be in good agreement with recent experimental results and Kohn variational and
convergent close-coupling calculations. The calculated values of the
annihilation rate parameter (effective number of electrons
participating in annihilation) rise steeply along the sequence of noble gas
atoms due to the increasing strength of the correlation effects, and agree well
with experimental data.Comment: 24 pages, 17 figure
Electronic spin-triplet nematic with a twist
We analyze a model of itinerant electrons interacting through a quadrupole
density-density repulsion in three dimensions. At the mean field level, the
interaction drives a continuous Pomeranchuk instability towards -wave,
spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation
and spatial rotational symmetries. This order results in spin antisymmetric,
elliptical deformations of the Fermi surfaces of up and down spins. We show
that the effects of quantum fluctuations are similar to those in metallic
ferromagnets, rendering the nematic transition first-order at low temperatures.
Using the fermionic quantum order-by-disorder approach to self-consistently
calculate fluctuations around possible modulated states, we show that the
first-order transition is pre-empted by the formation of a nematic state that
is intertwined with a helical modulation in spin space. Such a state is closely
related to -wave bond density wave order in square-lattice systems.
Moreover, we show that it may coexist with a modulated, -wave
superconducting state.Comment: 15 pages, 9 figure
Experimental and numerical investigation on micro deep drawing process of stainless steel 304 foil using flexible tools
Flexible forming technology provides significant application potential in various areas of manufacturing, particularly at a miniaturized level. Simplicity, versatility of process and feasibility of prototyping makes forming techniques by using flexible tools suitable for micro sheet metal forming. This paper reports the results of FE simulation and experimental research on micro deep drawing processes of stainless steel 304 sheets utilising a flexible die. The study presents a novel technique in which an initial gap (positive or negative) is adopted between an adjustment ring and a blank holder employed in the developed forming system. The blank holder is moveable part and supported by a particular spring that provides the required holding force. The forming parameters (anisotropy of SS 304 material, initial gap, friction conditions at various contact interfaces and initial sheet thickness) related with the forming process are in details investigated. The FE models are built using the commercial code Abaqus/Standard. The numerical predictions reveal the capability of the proposed technique on producing micro metallic cups with high quality and large aspect ratio. To verify these results, number of micro deep drawing experiments is conducted using a special set up developed for this purpose. As providing a fundamental understanding is required for the commercial development of this novel forming technique, hence the optimization of the initial gap in accordance with each sheet thickness, thickness distribution and punch force/stroke relationship are detected
Four-quark flux distribution and binding in lattice SU(2)
The full spatial distribution of the color fields of two and four static
quarks is measured in lattice SU(2) field theory at separations up to 1 fm at
beta=2.4. The four-quark case is equivalent to a qbar q qbar q system in SU(2)
and is relevant to meson-meson interactions. By subtracting two-body flux tubes
from the four-quark distribution we isolate the flux contribution connected
with the four-body binding energy. This contribution is further studied using a
model for the binding energies. Lattice sum rules for two and four quarks are
used to verify the results.Comment: 46 pages including 71 eps figures. 3D color figures are available at
www.physics.helsinki.fi/~ppennane/pics
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