743 research outputs found
The Abundance of Kaluza-Klein Dark Matter with Coannihilation
In Universal Extra Dimension models, the lightest Kaluza-Klein (KK) particle
is generically the first KK excitation of the photon and can be stable, serving
as particle dark matter. We calculate the thermal relic abundance of the KK
photon for a general mass spectrum of KK excitations including full
coannihilation effects with all (level one) KK excitations. We find that
including coannihilation can significantly change the relic abundance when the
coannihilating particles are within about 20% of the mass of the KK photon.
Matching the relic abundance with cosmological data, we find the mass range of
the KK photon is much wider than previously found, up to about 2 TeV if the
masses of the strongly interacting level one KK particles are within five
percent of the mass of the KK photon. We also find cases where several
coannihilation channels compete (constructively and destructively) with one
another. The lower bound on the KK photon mass, about 540 GeV when just
right-handed KK leptons coannihilate with the KK photon, relaxes upward by
several hundred GeV when coannihilation with electroweak KK gauge bosons of the
same mass is included.Comment: 38 pages, 4 figure
Classical spin liquids in stacked triangular lattice Ising antiferromagnets
We study Ising antiferromagnets that have nearest-neighbour interactions on
multilayer triangular lattices with frustrated ( and ) stacking, and
make comparisons with the unfrustrated () stacking. If interlayer
couplings are much weaker than in-plane ones, the paramagnetic phase of models
with frustrated stackings has a classical spin-liquid regime at low
temperature, in which correlations are strong both within and between planes,
but there is no long-range order. We investigate this regime using Monte Carlo
simulations and by mapping the spin models to coupled height models, which are
treated using renormalisation group methods and an analysis of the effects of
vortex excitations. The classical spin-liquid regime is parametrically wide at
small interlayer coupling in models with frustrated stackings. By contrast, for
the unfrustrated stacking there is no extended regime in which interlayer
correlations are strong without three-dimensional order.Comment: 25 pages, 21 figures; version to appear in Physical Review B,
includes minor correction
The Place of Widening Participation
This chapter reports on a small-scale study to explore the learning careers of mature, non-traditionally qualified students. It reveals how they (re)engage with learning, and their transition from studying on a bespoke access course delivered in a university to study at undergraduate level. The study highlights the barriers and challenges students overcome and identifies their motivations and dispositions to learning. Whilst the majority of access courses are delivered in further education colleges, a small number of universities deliver bespoke access courses. These courses, which are usually designed to facilitate progression within the institution, enable students to experience teaching and learning in a university setting, facilities and support services.
The research adopted a qualitative approach, using interviews as the main method of data collection. Emerging findings reveal that (re)engaging with education and the transition from ‘access student’ to undergraduate student is not seamless and without challenges. Such students ‘often undergo a unique and profound experience’ (Burnell, 2015:6) as they enter this new space which provides an opportunity to create new social and class-related identities. The transition to undergraduate study is easier when learners are familiar with the routines and rhythms of higher education in general and the facilities and services of a particular institution
Scenario for Fractional Quantum Hall Effect in Bulk Isotropic Materials
We investigate the possibility of a strongly correlated Fractional Quantum
Hall (FQH) state in bulk three dimensional isotropic (not layered) materials.
We find that a FQH state can exist at low densities only if it is accompanied
by a staging transition in which the electrons re-organize themselves in
layers, perpendicular to the magnetic field, at distances of order the magnetic
length apart. The Hartree energy associated to the staging transition is
off-set by the correlation Fock energy of the 3D FQH state. We obtain the phase
diagram of bulk electrons in a magnetic field subject to Coulomb interactions
as a function of carrier density and lattice constant. At very low densities,
the 3D FQH state exhibits a transition to a 3D Wigner crystal state stabilized
by phonon correlations
Spin-transfer switching and low-field precession in exchange-biased spin valve nano-pillars
Using a three-dimensional focused-ion beam lithography process we have
fabricated nanopillar devices which show spin transfer torque switching at zero
external magnetic fields. Under a small in-plane external bias field, a
field-dependent peak in the differential resistance versus current is observed
similar to that reported in asymmetrical nanopillar devices. This is
interpreted as evidence for the low-field excitation of spin waves which in our
case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange
bias layer coupled to a relatively thin CoFe fixed layer.Comment: 11 pages, 4 figures. To appear in APL, April 200
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions
We report magnetic and electrical measurements of Nb Josephson junctions with
strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these
materials show multiple oscillations of critical current with barrier thickness
implying repeated 0-pi phase-transitions in the superconducting order
parameter. We show in particular that the Co barrier devices can be accurately
modelled using existing clean limit theories and so that, despite the high
exchange energy (309 meV), the large IcRN value in the pi-state means Co
barriers are ideally suited to the practical development of superconducting
pi-shift devices.Comment: 4 pages 3 figures 1 table. Revised version as accepted for
publication. To appear in Physical Review Letter
Legislative strengthening meets party support in international assistance: a closer relationship?
Recent reports recommend that international efforts to help strengthen legislatures in emerging democracies should work more closely with support for building stronger political parties and competitive party systems. This article locates the recommendations within international assistance more generally and reviews the arguments. It explores problems that must be addressed if the recommendations are to be implemented effectively. The article argues that an alternative, issue-based approach to strengthening legislatures and closer links with civil society could gain more traction. However, that is directed more centrally at promoting good governance for the purpose of furthering development than at democratisation goals sought by party aid and legislative strengtheners in the democracy assistance industry
Cygnus X-3 in outburst : quenched radio emission, radiation losses and variable local opacity
We present multiwavelength observations of Cygnus X-3 during an extended
outburst in 1994 February - March. Intensive radio monitoring at 13.3, 3.6 &
2.0 cm is complemented by observations at (sub)millimetre and infrared
wavelengths, which find Cyg X-3 to be unusually bright and variable, and
include the first reported detection of the source at 0.45 mm. We report the
first confirmation of quenched radio emission prior to radio flaring
independent of observations at Green Bank. The observations reveal evidence for
wavelength-dependent radiation losses and gradually decreasing opacity in the
environment of the radio jet. We find that the radiation losses are likely to
be predominantly inverse Compton losses experienced by the radio-emitting
electrons in the strong radiation field of a luminous companion to the compact
object. We interpret the decreasing opacity during the flare sequence as
resulting from a decreasing proportion of thermal electrons entrained in the
jet, reflecting a decreasing density in the region of jet formation. We
present, drawing in part on the work of other authors, a model based upon
mass-transfer rate instability predicting gamma-ray, X-ray, infrared and radio
trends during a radio flaring sequence.Comment: LaTeX, 11 pages, 6 figures. Submitted to MNRA
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