583 research outputs found
Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder
A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by
inelastic neutron scattering. This frustrated magnet with quenched exchange
disorder remains in a strongly correlated paramagnetic state down to one 60th
of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering
develops and comprises 30 +/- 10% of the total magnetic scattering, as expected
for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/\delta
E=3.8 ps. The diffuse scattering is consistent with short range XY
antiferromagnetism with a correlation length of 16 \AA. The momentum (Q)
dependence of the inelastic intensity indicates relaxing XY-like
antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear
antiferromagnetic fluctuations above this energy. The relevant XY
configurations form a continuous manifold of symmetry-related states. Contrary
to well-known models that produce this continuous manifold, order-by-disorder
does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %)
exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters
that span this manifold.Comment: 9 pages, 9 figures. This updated version features modified figures
and some new discussio
Definitive Evidence for Order-by-Quantum-Disorder in Er2Ti2O7
Here we establish the systematic existence of a U(1) degeneracy of all
symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice,
at the mean-field level. By extracting the Hamiltonian of Er2Ti2O7 from
inelastic neutron scattering measurements, we then show that the
U(1)-degenerate states of Er2Ti2O7 are its classical ground states, and
unambiguously show that quantum fluctuations break the degeneracy in a way
which is confirmed by experiment. This is the first definitive observation of
order-by-disorder in any material. We provide further verifiable consequences
of this phenomenon, and several additional comparisons between theory and
experiment.Comment: 4.5 pages, 3 figures, 7.5 pages of Supplemental Material, 8
supplemental figure
Initiation to heroin injecting among heroin users in Sydney, Australia: cross sectional survey
BACKGROUND: Heroin injection is associated with health and social problems including hepatitis C virus (HCV) transmission. Few studies have examined the circumstances surrounding initiation to heroin injecting, especially current users initiating others. The current study aimed to examine the age of first heroin use and injection; administration route of first heroin use; relationship to initiator; the initiation of others among a group of heroin users; and to examine these factors in relation to HCV status and risk. METHOD: Heroin users in Sydney were recruited through needle and syringe programs, a methadone clinic and snowballing. Participants were interviewed about their own initiation to heroin use, blood-borne virus risk and knowledge, and whether they had initiated others to heroin injecting. Information on HCV status was collected via self-report. Data was analysed using univariate and multivariate statistical techniques for Normally distributed continuous and categorical data. RESULTS: The study recruited 399 heroin users, with a mean age of 31 years, 63% were male, 77% reported heroin as their primary drug and 59% were HCV positive (self-report). Mean age at first heroin use and injection was 19 and 21 years, respectively. The majority of heroin users commenced heroin use via injecting (65%), younger users (<25 years, 25–30 years) were less likely than older users (>30 years) to commence heroin use parenterally. Participants were initiated to injection mainly by friends (63%). Thirty-seven percent reported initiating others to heroin injection, but few factors were related to this behaviour. Those with longer heroin using careers were more likely to report initiating others to heroin injection, but were no more likely to have done so in the preceding 12 months. Participants who had initiated others were more likely to have shared injecting equipment (12 vs 23%), but were no more likely to be HCV positive (self-report) than those who did not. CONCLUSION: Interventions to prevent heroin users initiating others to injecting are necessary. Peer groups may be well positioned to implement such interventions
Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCoF and NaSrCoF with magnetic pair distribution function analysis
We present time-of-flight neutron total scattering and polarized neutron
scattering measurements of the magnetically frustrated compounds
NaCaCoF and NaSrCoF, which belong to a class of recently
discovered pyrochlore compounds based on transition metals and fluorine. The
magnetic pair distribution function (mPDF) technique is used to analyze and
model the total scattering data in real space. We find that a
previously-proposed model of short-range XY-like correlations with a length
scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic
correlations, accurately describes the mPDF data at low temperature, confirming
the magnetic ground state in these materials. This model is further verified by
the polarized neutron scattering data. From an analysis of the temperature
dependence of the mPDF and polarized neutron scattering data, we find that
short-range correlations persist on the nearest-neighbor length scale up to 200
K, approximately two orders of magnitude higher than the spin freezing
temperatures of these compounds. These results highlight the opportunity
presented by these new pyrochlore compounds to study the effects of geometric
frustration at relatively high temperatures, while also advancing the mPDF
technique and providing a novel opportunity to investigate a genuinely
short-range-ordered magnetic ground state directly in real space
Salt-assisted vapor-liquid-solid growth of one-dimensional van der Waals materials
We have combined the benefits of two catalytic growth phenomena to form
nanostructures of transition metal trichalcogenides (TMTs), materials that are
challenging to grow in a nanostructured form by conventional techniques, as
required to exploit their exotic physics. Our growth strategy combines the
benefits of vapor-liquid-solid (VLS) growth in controlling dimension and growth
location, and salt-assisted growth for fast growth at moderate temperatures.
This salt-assisted VLS growth is enabled through use of a catalyst that
includes Au and an alkali metal halide. We demonstrate high yields of NbS3 1D
nanostructures with sub-ten nanometer diameter, tens of micrometers length, and
distinct 1D morphologies consisting of nanowires and nanoribbons with [010] and
[100] growth orientations, respectively. We present strategies to control the
growth location, size, and morphology. We extend the growth method to
synthesize other TMTs, NbSe3 and TiS3, as nanowires. Finally, we discuss the
growth mechanism based on the relationships we measure between the materials
characteristics (growth orientation, morphology and dimensions) and the growth
conditions (catalyst volume and growth time). Our study introduces
opportunities to expand the library of emerging 1D vdW materials and their
heterostructures with controllable nanoscale dimensions.Comment: 16 pages, 5 figure
Englacial Architecture of Lambert Glacier, East Antarctica
The analysis of englacial layers using radio-echo sounding data enables the characterisation and reconstruction of current and past ice-sheet flow. Despite the Lambert Glacier catchment being one of the largest in Antarctica, discharging ~16 % of East Antarctica’s ice, its englacial architecture has been little analysed. Here, we present a comprehensive analysis of Lambert Glacier’s englacial architecture using radio-echo sounding data collected by the Antarctica's Gamburtsev Province Project (AGAP) North survey. We used an “internal-layering continuity index” (ILCI) to characterise the internal architecture of the ice and identify four macro-scale ILCI zones with distinct glaciological contexts. Whilst the catchment is dominated by continuous englacial layering, disrupted or discontinuous layering is highlighted by the ILCI at both the onset of enhanced ice flow (defined here as >15 ma−1) and along the shear margin, revealing the transition from internal-deformation-controlled to basal-sliding-dominated ice flow. These zones are characterised by buckled and folded englacial layers which align with the current ice-flow regime, and which we interpret as evidence that the flow direction of the Lambert Glacier trunk has changed little, if at all, during the Holocene. However, disturbed englacial layers along a deep subglacial channel that does not correspond to modern ice-flow routing suggest that ice-flow change has occurred in a former tributary which fed Lambert Glacier from grid north. As large outlet systems such as Lambert Glacier are likely to play a vital role in the future drainage of the East Antarctic Ice Sheet, constraining their englacial architecture to reconstruct their past ice flow and assess basal conditions is important.</p
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