7,642 research outputs found
A cluster model with random anisotropy for hysteresis jumps in CeNiCu alloys
Some Cerium compounds exhibit hysteresis cycles with sharp macroscopic jumps
in the magnetization at very low temperatures. This effect is attributed to the
formation of clusters in which the anisotropy competes with the applied
magnetic field. Here, we present a simple model where a lattice of
ferromagnetically coupled spins is separated in clusters of random sizes and
with random anisotropy. Within this model, we obtain hysteresis cycles
presenting jumps that behave in a similar way that the experimental ones, and
that disappear when increasing the temperature. The results are in good
agreement with the hysteresis cycles measured at very low temperatures in
CeNiCu and the comparison with these experimental results allows
to discriminate the relative importance of the mechanisms driving the thermal
evolution of the cycles.Comment: Accepted in PR
Entropy and equilibrium state of free market models
Many recent models of trade dynamics use the simple idea of wealth exchanges
among economic agents in order to obtain a stable or equilibrium distribution
of wealth among the agents. In particular, a plain analogy compares the wealth
in a society with the energy in a physical system, and the trade between agents
to the energy exchange between molecules during collisions. In physical
systems, the energy exchange among molecules leads to a state of equipartition
of the energy and to an equilibrium situation where the entropy is a maximum.
On the other hand, in the majority of exchange models, the system converges to
a very unequal condensed state, where one or a few agents concentrate all the
wealth of the society while the wide majority of agents shares zero or almost
zero fraction of the wealth. So, in those economic systems a minimum entropy
state is attained. We propose here an analytical model where we investigate the
effects of a particular class of economic exchanges that minimize the entropy.
By solving the model we discuss the conditions that can drive the system to a
state of minimum entropy, as well as the mechanisms to recover a kind of
equipartition of wealth
Electromagnetic emission of white dwarf binary mergers
It has been recently proposed that the ejected matter from white dwarf (WD)
binary mergers can produce transient, optical and infrared emission similar to
the "kilonovae" of neutron star (NS) binary mergers. To confirm this we
calculate the electromagnetic emission from WD-WD mergers and compare with
kilonova observations. We simulate WD-WD mergers leading to a massive, fast
rotating, highly magnetized WD with an adapted version of the
smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial
conditions for the ejecta such as escape velocity, mass and initial position
and distribution. The subsequent thermal and dynamical evolution of the ejecta
is obtained by integrating the energy-conservation equation accounting for
expansion cooling and a heating source given by the fallback accretion onto the
newly-formed WD and its magneto-dipole radiation. We show that magnetospheric
processes in the merger can lead to a prompt, short gamma-ray emission of up to
erg in a timescale of - s. The bulk of the ejecta
initially expands non-relativistically with velocity and then it
accelerates to due to the injection of fallback accretion energy. The
ejecta become transparent at optical wavelengths around days
post-merger with a luminosity - erg s. The X-ray
emission from the fallback accretion becomes visible around -
day post-merger with a luminosity of erg s. We also predict
the post-merger time at which the central WD should appear as a pulsar
depending on the value of the magnetic field and rotation period.Comment: 12 pages, Accepted for publication in JCA
Randomised, controlled trial of alternating pressure mattresses compared with alternating pressure overlays for the prevention of pressure ulcers : PRESSURE (pressure relieving support surfaces) trial
Objective To compare whether differences exist between alternating pressure overlays and alternating pressure mattresses in the development of new pressure ulcers, healing of existing pressure ulcers, and patient acceptability. Design Pragmatic, open, multicentre, randomised controlled trial. Setting 11 hospitals in six NHS trusts. Participants 1972 people admitted to hospital as acute or elective patients. Interventions Participants were randomised to an alternating pressure mattress (n = 982) of- an alternating pressure overlay (n = 990). Main outcome measures The proportion of participants developing a new pressure ulcer of grade 2 or worse; time to development of new pressure ulcers; proportions of participants developing a new ulcer within 30 days; healing of existing pressure ulcers; and patient acceptability Results Intention to treat analysis found no difference in the proportions of participants developing a new pressure ulcer of grade 2 or worse (10.7% overlay patients, 10.3% mattress patients; difference 0.4%, 95% confidence interval - 23% to 3.1%, P = 0.75). More overlay patients requested change owing to dissatisfaction (23.3%) than mattress patients (18.9%, P = 0.02). Conclusion No difference was found between alternating pressure mattresses and alternating pressure overlays in the proportion of people who develop a pressure ulcer
Application of the S=1 underscreened Anderson lattice model to Kondo uranium and neptunium compounds
Magnetic properties of uranium and neptunium compounds showing the
coexistence of Kondo screening effect and ferromagnetic order are investigated
within the Anderson lattice Hamiltonian with a two-fold degenerate -level in
each site, corresponding to electronic configuration with spins. A
derivation of the Schrieffer-Wolff transformation is presented and the
resulting Hamiltonian has an effective -band term, in addition to the
regular exchange Kondo interaction between the -spins and the
spins of the conduction electrons. The obtained effective Kondo lattice model
can describe both the Kondo regime and a weak delocalization of -electron.
Within this model we compute the Kondo and Curie temperatures as a function of
model parameters, namely the Kondo exchange interaction constant , the
magnetic intersite exchange interaction and the effective -bandwidth.
We deduce, therefore, a phase diagram of the model which yields the coexistence
of Kondo effect and ferromagnetic ordering and also accounts for the pressure
dependence of the Curie temperature of uranium compounds such as UTe.Comment: 9 pages, 4 figure
Recent Developments in Engineering Measurements Lab
Over the past two years, the Engineering Measurements Lab has attempted to increase the breadth and depth of course material introduced to students to allow them to design and perform successful experimental tests. Over that time, the following structural changes have been made to this course: (i) a single lecture contact hour per week was added, (ii) lab contact hours focus more on practical aspects of each lab, and (iii) the number of experiments run in the course has increased from four to seven. To reflect these changes, the course has grown from one credit to two credits. Material for each lab was delivered in a two-week cycle with a one-hour lecture and two-hour lab period every week. Each lab had one dedicated lecture and additional lectures were added to further emphasize broader topics including data acquisition, measurement uncertainty, and statistical analysis. In addition to the updated course content, the Toyota A3 report format has been adopted for all labs to expose students to a wider variety of tools for technical communication and to foster a spirit of creative and innovative problem solving. In keeping with the iterative nature of these reports, the general process for each lab involves multiple events with feedback from peers and instructors. During the week “A†lab period, students are introduced to the lab facility and perform an ungraded activity where they manually perform relevant calculations using a small subset of previously recorded data. They are then presented with a full set of previous data so they can perform relevant calculations and plot pertinent information. This prelab data exercise is submitted before the week “B†lab period. During the week “B†lab period, students run the laboratory to generate their own data set. A draft A3 report is then submitted prior to the following week “A†lab period. Students peer-review the draft A3 reports in lab before they perform the manual activity for the next laboratory. Final A3 drafts are due at 11:59 pm the following day. Lab topics for this course include characterization of (i) vortex tubes, (ii) vapor compression refrigeration, (iii) centrifugal pumps, and (iv) frictional pipe losses. New labs have been developed for this course examining (v) error propagation in measurement of complex geometries, (vi) measuring Poiseuille flow velocity profiles, and (vii) thermocouple calibration. This work will describe the changes made to this course over the past two years and discuss their suitability based on effectiveness and student satisfaction. Plans for future development of the course will also be discussed
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