436 research outputs found
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 incidence of Gorlin syndrome in 173 consecutive cases of medulloblastoma.
We have investigated the incidence of Gorlin syndrome (GS) in patients with the childhood brain tumour, medulloblastoma. One hundred and seventy-three consecutive cases of medulloblastoma in the North-West Regional Health Authority between 1954 and 1989 (Manchester Regional Health Board before 1974) were studied. After review of case notes, X-rays and health surveys only 2/173 cases had evidence supporting a diagnosis of GS. A further case at 50% risk of GS died of a brain tumour aged 4 years. The incidence of GS in medulloblastoma is, therefore, probably between 1-2%. A population based study of GS in the region started in 1983 was used to assess the incidence of medulloblastoma in GS, which was found to be between 3-5%. This figure is lower than previous estimates, but this is the first population based study undertaken. In view of the early age of onset in GS (mean 2 years) children presenting with medulloblastoma, especially under 5 years, should be examined for signs of the syndrome. Those at high risk of developing multiple invasive basal cell carcinomata will then be identified
Phase diagram of bismuth in the extreme quantum limit
Elemental bismuth provides a rare opportunity to explore the fate of a
three-dimensional gas of highly mobile electrons confined to their lowest
Landau level. Coulomb interaction, neglected in the band picture, is expected
to become significant in this extreme quantum limit with poorly understood
consequences. Here, we present a study of the angular-dependent Nernst effect
in bismuth, which establishes the existence of ultraquantum field scales on top
of its complex single-particle spectrum. Each time a Landau level crosses the
Fermi level, the Nernst response sharply peaks. All such peaks are resolved by
the experiment and their complex angular-dependence is in very good agreement
with the theory. Beyond the quantum limit, we resolve additional Nernst peaks
signaling a cascade of additional Landau sub-levels caused by electron
interaction
Properties of a Gamma Ray Burst Host Galaxy at z ~ 5
We describe the properties of the host galaxy of the gamma-ray burst
GRB060510B based on a spectrum of the burst afterglow obtained with the Gemini
North 8m telescope. The galaxy lies at a redshift of z = 4.941 making it the
fourth highest spectroscopically identified burst host. However, it is the
second highest redshift galaxy for which the quality of the spectrum permits a
detailed metallicity analysis. The neutral hydrogen column density has a
logarithmic value of 21.0--21.2 cm^-2 and the weak metal lines of Ni, S and Fe
show that the metallicity is in excess of a tenth of solar which is far above
the metallicities in damped Lyman alpha absorbers at high redshift. The
tightest constraint is from the Fe lines which place [Fe/H] in excess of -0.8.
We argue that the results suggest that metallicity bias could be a serious
problem with inferring star formation from the GRB population and consider how
future higher quality measurements could be used to resolve this question.Comment: Accepted for publication in ApJ Letter
Nanospintronics with carbon nanotubes
One of the actual challenges of spintronics is the realization of a
spin-transistor allowing to control spin transport through an electrostatic
gate. In this review, we report on different experiments which demonstrate a
gate control of spin transport in a carbon nanotube connected to ferromagnetic
leads. We also discuss some theoretical approaches which can be used to analyze
spin transport in these systems. We emphasize the roles of the gate-tunable
quasi-bound states inside the nanotube and the coherent spin-dependent
scattering at the interfaces between the nanotube and its ferromagnetic
contacts.Comment: 35 pages, 15 figures, some figures in gi
Electrical transport between epitaxial manganites and carbon nanotubes
The possibility of performing spintronics at the molecular level may be
realized in devices that combine fully spin polarized oxides such as manganites
with carbon nanotubes. However, it is not clear whether electrical transport
between such different material systems is viable. Here we show that the room
temperature conductance of manganite-nanotube-manganite devices is only half
the value recorded in similar palladium-nanotube-palladium devices.
Interestingly, the former shows a pseudogap in the conductivity below the
relatively high temperature of 200 K. Our results suggest the possibility of
new spintronics heterostructures that exploit fully spin polarized sources and
drains
Transformation of spin information into large electrical signals via carbon nanotubes
Spin electronics (spintronics) exploits the magnetic nature of the electron,
and is commercially exploited in the spin valves of disc-drive read heads.
There is currently widespread interest in using industrially relevant
semiconductors in new types of spintronic devices based on the manipulation of
spins injected into a semiconducting channel between a spin-polarized source
and drain. However, the transformation of spin information into large
electrical signals is limited by spin relaxation such that the magnetoresistive
signals are below 1%. We overcome this long standing problem in spintronics by
demonstrating large magnetoresistance effects of 61% at 5 K in devices where
the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron
gap between epitaxial electrodes of the highly spin polarized manganite
La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes
is long due the small spin-orbit coupling of carbon, because the high nanotube
Fermi velocity permits the carrier dwell time to not significantly exceed this
spin lifetime, because the manganite remains highly spin polarized up to the
manganite-nanotube interface, and because the interfacial barrier is of an
appropriate height. We support these latter statements regarding the interface
using density functional theory calculations. The success of our experiments
with such chemically and geometrically different materials should inspire
adventure in materials selection for some future spintronicsComment: Content highly modified. New title, text, conclusions, figures and
references. New author include
The physics of dipolar bosonic quantum gases
This article reviews the recent theoretical and experimental advances in the
study of ultracold gases made of bosonic particles interacting via the
long-range, anisotropic dipole-dipole interaction, in addition to the
short-range and isotropic contact interaction usually at work in ultracold
gases. The specific properties emerging from the dipolar interaction are
emphasized, from the mean-field regime valid for dilute Bose-Einstein
condensates, to the strongly correlated regimes reached for dipolar bosons in
optical lattices.Comment: Review article, 71 pages, 35 figures, 350 references. Submitted to
Reports on Progress in Physic
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