1,041 research outputs found
Pressure-driven flow of solid helium
The recent torsional oscillator results of Kim and Chan suggest a supersolid
phase transition in solid 4He. We have used a piezoelectrically driven
diaphragm to study the flow of solid helium through an array of capillaries.
Our measurements showed no indication of low temperature flow, placing
stringent restrictions on supersolid flow in response to a pressure difference.
The average flow speed at low temperatures was less than 1.2x10-14 m/s,
corresponding to a supersolid velocity at least 7 orders of magnitude smaller
than the critical velocities inferred from the torsional oscillator
measurements.Comment: 4 pages, 3 figure
Dislocation networks in helium-4 crystals
The mechanical behavior of crystals is dominated by dislocation networks,
their structure and their interactions with impurities or thermal phonons.
However, in classical crystals, networks are usually random with impurities
often forming non-equilibrium clusters when their motion freezes at low
temperature. Helium provides unique advantages for the study of dislocations:
crystals are free of all but isotopic impurities, the concentration of these
can be reduced to the ppb level, and the impurities are mobile at all
temperatures and therefore remain in equilibrium with the dislocations. We have
achieved a comprehensive study of the mechanical response of 4He crystals to a
driving strain as a function of temperature, frequency and strain amplitude.
The quality of our fits to the complete set of data strongly supports our
assumption of string-like vibrating dislocations. It leads to a precise
determination of the distribution of dislocation network lengths and to
detailed information about the interaction between dislocations and both
thermal phonons and 3He impurities. The width of the dissipation peak
associated with impurity binding is larger than predicted by a simple Debye
model, and much of this broadening is due to the distribution of network
lengths.Comment: accepted by Phys. Rev.
A temperature-controlled device for volumetric measurements of Helium adsorption in porous media
We describe a set-up for studying adsorption of helium in silica aerogels,
where the adsorbed amount is easily and precisely controlled by varying the
temperature of a gas reservoir between 80 K and 180 K. We present validation
experiments and a first application to aerogels. This device is well adapted to
study hysteresis, relaxation, and metastable states in the adsorption and
desorption of fluids in porous media
Helium condensation in aerogel: avalanches and disorder-induced phase transition
We present a detailed numerical study of the elementary condensation events
(avalanches) associated to the adsorption of He in silica aerogels. We use
a coarse-grained lattice-gas description and determine the nonequilibrium
behavior of the adsorbed gas within a local mean-field analysis, neglecting
thermal fluctuations and activated processes. We investigate the statistical
properties of the avalanches, such as their number, size and shape along the
adsorption isotherms as a function of gel porosity, temperature, and chemical
potential. Our calculations predict the existence of a line of critical points
in the temperature-porosity diagram where the avalanche size distribution
displays a power-law behavior and the adsorption isotherms have a universal
scaling form. The estimated critical exponents seem compatible with those of
the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure
A cross sectional study of surgical training among United Kingdom general practitioners with specialist interests in surgery
OBJECTIVES: Increasing numbers of minor surgical procedures are being performed in the community. In the UK, general practitioners (family medicine physicians) with a specialist interest (GPwSI) in surgery frequently undertake them. This shift has caused decreases in available cases for junior surgeons to gain and consolidate operative skills. This study evaluated GPwSI's case-load, procedural training and perceptions of offering formalised operative training experience to surgical trainees. DESIGN: Prospective, questionnaire-based cross-sectional study. SETTING/PARTICIPANTS: A novel, 13-item, self-administered questionnaire was distributed to members of the Association of Surgeons in Primary Care (ASPC). A total 113 of 120 ASPC members completed the questionnaire, representing a 94% response rate. Respondents were general practitioners practising or intending to practice surgery in the community. RESULTS: Respondents performed a mean of 38 (range 5–150) surgical procedures per month in primary care. 37% (42/113) of respondents had previously been awarded Membership or Fellowship of a Surgical Royal College; 22% (25/113) had completed a surgical certificate or diploma or undertaken a course of less than 1 year duration. 41% (46/113) had no formal British surgical qualifications. All respondents believed that surgical training in primary care could be valuable for surgical trainees, and the majority (71/113, 63%) felt that both general practice and surgical trainees could benefit equally from such training. CONCLUSIONS: There is a significant volume of surgical procedures being undertaken in the community by general practitioners, with the capacity and appetite for training of prospective surgeons in this setting, providing appropriate standards are achieved and maintained, commensurate with current standards in secondary care. Surgical experience and training of GPwSI's in surgery is highly varied, and does not yet benefit from the quality assurance secondary care surgical training in the UK undergoes. The Royal Colleges of Surgery and General Practice are well placed to invest in such infrastructure to provide long-term, high-quality service and training in the community
Fermion Quasi-Spherical Harmonics
Spherical Harmonics, , are derived and presented (in a
Table) for half-odd-integer values of and . These functions are
eigenfunctions of and written as differential operators in the
spherical-polar angles, and . The Fermion Spherical Harmonics
are a new, scalar and angular-coordinate-dependent representation of fermion
spin angular momentum. They have symmetry in the angle , and hence
are not single-valued functions on the Euclidean unit sphere; they are
double-valued functions on the sphere, or alternatively are interpreted as
having a double-sphere as their domain.Comment: 16 pages, 2 Tables. Submitted to J.Phys.
Local mean-field study of capillary condensation in silica aerogels
We apply local mean-field (i.e. density functional) theory to a lattice model
of a fluid in contact with a dilute, disordered gel network. The gel structure
is described by a diffusion-limited cluster aggregation model. We focus on the
influence of porosity on both the hysteretic and the equilibrium behavior of
the fluid as one varies the chemical potential at low temperature. We show that
the shape of the hysteresis loop changes from smooth to rectangular as the
porosity increases and that this change is associated to disorder-induced
out-of-equilibrium phase transitions that differ on adsorption and on
desorption. Our results provide insight in the behavior of He in silica
aerogels.Comment: 19 figure
Representations of sport in the revolutionary socialist press in Britain, 1988–2012
This paper considers how sport presents a dualism to those on the far left of the political spectrum. A long-standing, passionate debate has existed on the contradictory role played by sport, polarised between those who reject it as a bourgeois capitalist plague and those who argue for its reclamation and reformation. A case study is offered of a political party that has consistently used revolutionary Marxism as the basis for its activity and how this party, the largest in Britain, addresses sport in its publications. The study draws on empirical data to illustrate this debate by reporting findings from three socialist publications. When sport did feature it was often in relation to high profile sporting events with a critical tone adopted and typically focused on issues of commodification, exploitation and alienation of athletes and supporters. However, readers’ letters, printed in the same publications, revealed how this interpretation was not universally accepted, thus illustrating the contradictory nature of sport for those on the far left
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