621 research outputs found
Gauged Six-dimensional Supergravity from Massive Type IIA
We obtain the complete non-linear Kaluza-Klein ansatz for the reduction of
the bosonic sector of massive type IIA supergravity to the Romans F(4) gauged
supergravity in six dimensions. The latter arises as a consistent warped S^4
reduction.Comment: Latex, 7 page
Investigating the intrinsic noise limit of Dayem bridge NanoSQUIDs
NanoSQUIDs made from Nb thin films have been produced with nanometre loop sizes down to 200 nm, using weak-link junctions with dimensions less than 60 nm. These composite (W/Nb) single layer thin film devices, patterned by FIB milling, show extremely good low-noise performance âŒ170 nΊ0 at temperatures between 5 and 8.5 K and can operate in rather high magnetic fields (at least up to 1 T). The devices produced so far have a limited operating temperature range, typically only 1â2 K. We have the goal of achieving operation at 4.2 K, to be compatible with the best SQUID series array (SSA) preamplifier available. Using the SSA to readout the nanoSQUIDs provides us with a means of investigating the intrinsic noise of the former. In this paper we report improved white noise levels of these nanoSQUIDs, enabling potential detection of a single electronic spin flip in a 1-Hz bandwidth. At low frequencies the noise performance is already limited by SSA preamplifier noise
Intersecting branes and Supersymmetry
We consider intersecting M-brane solutions of supergravity in eleven
dimensions. Supersymmetry turns out to be a powerful tool in obtaining such
solutions and their generalizations.Comment: 6 pages, Latex, Presented at Supersymmetry and Quantum Field Theory,
International Seminar dedicated to the memory of D. V. Volkov, Kharkov, 199
Heat propagation models for superconducting nanobridges at millikelvin temperatures
Nanoscale superconducting quantum interference devices (nanoSQUIDs) most commonly use Dayem bridges as Josephson elements to reduce the loop size and achieve high spin sensitivity. Except at temperatures close to the critical temperature T c, the electrical characteristics of these bridges exhibit undesirable thermal hysteresis which complicates device operation. This makes proper thermal analysis an essential design consideration for optimising nanoSQUID performance at ultralow temperatures. However the existing theoretical models for this hysteresis were developed for micron-scale devices operating close to liquid helium temperatures, and are not fully applicable to a new generation of much smaller devices operating at significantly lower temperatures. We have therefore developed a new analytic heat model which enables a more accurate prediction of the thermal behaviour in such circumstances. We demonstrate that this model is in good agreement with experimental results measured down to 100 mK and discuss its validity for different nanoSQUID geometries
Hearing capacities and otolith size in two ophidiiform species (<i>Ophidion rochei</i> and <i>Carapus acus</i>)
Numerous studies have highlighted the diversity of fish inner ear morphology. However, the function of the shape, size and orientation of the different structures remains poorly understood. The saccule (otolithic endorgan) is considered to be the principal hearing organ in fishes and it has been hypothesized that sagitta (saccular otolith) shape and size affect hearing capacities: large sagittae are thought to increase sensitivity. The sagittae of many ophidiids and carapids occupy a large volume inside the neurocranium. Hence they are a good structure with which to test the size hypothesis. The main aim of this study was to investigate hearing capacities and inner ear morphology in two ophidiiform species: Ophidion rochei and Carapus acus. We used a multidisciplinary approach that combines dissections, ”CT-scan examinations and auditory evoked potential techniques. Carapus acus and O. rochei sagittae have similar maximal diameters; both species have larger otoliths than many non-ophidiiform species, especially compared with the intra-neurocranium volume. Both species are sensitive to sounds up to 2100 Hz. Relative to the skull, O. rochei has smaller sagittae than the carapid, but better hearing capacities from 300 to 900 Hz and similar sensitivities at 150 Hz and from 1200 to 2100 Hz. Results show that hearing capacities of a fish species cannot be predicted only based on sagitta size. Larger otoliths (in size relative to the skull) may have evolved mainly for performing vestibular functions in fishes, especially those species that need to execute precise and complex movements
Anti-de Sitter space, branes, singletons, superconformal field theories and all that
There has recently been a revival of interest in anti de-Sitter space (AdS)
brought about by the conjectured duality beteeen physics in the bulk of AdS and
a conformal field theory on the boundary. Since the whole subject of branes,
singletons and superconformal field theories on the AdS boundary was an active
area of research about ten years ago, I begin with a historical review,
including the ``Membrane at the end of the universe'' idea. Next I discuss two
recent papers with Lu and Pope on on and on , respectively. In each case we note that odd-dimensional spheres
may be regarded as U(1) bundles over and that this
permits an unconventional ``Hopf''duality along the U(1) fibre. This leads in
particular to the phenomenon of BPS without BPS whereby states which appear to
be non-BPS in one picture are seen to be BPS in the dual picture.Comment: Minor improvements. 37 pages Latex. Based on talks delivered at the
the PASCOS 98 conference, Northeastern University, March 1998; the
Superfivebranes and Physics in 5+1 Dimensions conference, ICTP, Trieste,
Italy, April 1998; the Arnowitt Fest, Texas A&M University, April 1998; the
Strings 98 conference, ITP, Santa Barbara, June 199
Higher-loop anomalies in chiral gravities
The one-loop anomalies for chiral gravity are derived using the
Fujikawa regularisation method. The expected two-loop anomalies are then
obtained by imposing the Wess-Zumino consistency conditions on the one-loop
results. The anomalies found in this way agree with those already known from
explicit Feynman diagram calculations. We then directly verify that the order
non-local BRST Ward identity anomalies, arising from the ``dressing''
of the one-loop results, satisfy Lam's theorem. It is also shown that in a
rigorous calculation of anomaly for the BRST charge, one recovers both
the non-local as well as the local anomalies. We further verify that, in chiral
gravities, the non-local anomalies in the BRST Ward identity can be obtained by
the application of the anomalous operator , calculated using operator
products, to an appropriately defined gauge fermion. Finally, we give arguments
to show why this relation should hold generally in reparametrisation-invariant
theories.Comment: 21 pages, latex, 12 figures as uuencoded postscript. To appear in
Nucl. Phys.
Symmetric Potentials of Gauged Supergravities in Diverse Dimensions and Coulomb Branch of Gauge Theories
A class of conformally flat and asymptotically anti-de Sitter geometries
involving profiles of scalar fields is studied from the point of view of gauged
supergravity. The scalars involved in the solutions parameterise the
SL(N,R)/SO(N) submanifold of the full scalar coset of the gauged supergravity,
and are described by a symmetric potential with a universal form. These
geometries descend via consistent truncation from distributions of D3-branes,
M2-branes, or M5-branes in ten or eleven dimensions. We exhibit analogous
solutions asymptotic to AdS_6 which descend from the D4-D8-brane system. We
obtain the related six-dimensional theory by consistent reduction from massive
type IIA supergravity. All our geometries correspond to states in the Coulomb
branch of the dual conformal field theories. We analyze linear fluctuations of
minimally coupled scalars and find both discrete and continuous spectra, but
always bounded below.Comment: Latex, 38 pages, minor correction
Kaehler forms and cosmological solutions in type II supergravities
We consider cosmological solutions to type II supergravity theories where the
spacetime is split into a FRW universe and a K\"ahler space, which may be taken
to be Calabi-Yau. The various 2-forms present in the theories are taken to be
proportional to the K\"ahler form associated to the K\"ahler space.Comment: 6 pages, LaTeX2
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