5,010 research outputs found
A Simple Bayesian Method for Improved Analysis of Quasi-2D Scattering Data
A new method is presented for the analysis of small angle neutron scattering
data from quasi-2D systems such as flux lattices, Skyrmion lattices, and
aligned liquid crystals. A significant increase in signal to noise ratio, and a
natural application of the Lorentz factor can be achieved by taking advantage
of the knowledge that all relevant scattering is centered on a plane in
reciprocal space. The Bayesian form ensures that missing information is treated
in a controlled way and can be subsequently included in the analysis. A simple
algorithm based on Gaussian probability assumptions is provided which provides
very satisfactory results. Finally, it is argued that a generalised
model-independent Bayesian data analysis method would be highly advantageous
for the processing of neutron and x-ray scattering data
Valence Instability and Superconductivity in Heavy Fermion Systems
Many cerium-based heavy fermion (HF) compounds have pressure-temperature
phase diagrams in which a superconducting region extends far from a magnetic
quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an
enhancement of the superconducting transition temperature was found to coincide
with an abrupt valence change, with strong circumstantial evidence for pairing
mediated by critical valence, or charge transfer, fluctuations. This pairing
mechanism, and the valence instability, is a consequence of a f-c Coulomb
repulsion term U_fc in the hamiltonian. While some non-superconducting Ce
compounds show a clear first order valence instability, analogous to the Ce
alpha-gamma transition, we argue that a weakly first order valence transition
may be a general feature of Ce-based HF systems, and both magnetic and critical
valence fluctuations may be responsible for the superconductivity in these
systems.Comment: 11 pages, 16 figure
Valence fluctuation mediated superconductivity in CeCu2Si2
It has been proposed that there are two types of superconductivity in
CeCu2Si2, mediated by spin fluctuations at ambient pressure, and by critical
valence fluctuations around a charge instability at a pressure P_v \simeq 4.5
GPa. We present in detail some of the unusual features of this novel type of
superconducting state, including the coexistence of superconductivity and huge
residual resistivity of the order of the Ioffe-Regel limit, large and pressure
dependent resistive transition widths in a single crystal measured under
hydrostatic conditions, asymmetric pressure dependence of the specific heat
jump shape, unrelated to the resistivity width, and negative temperature
dependence of the normal state resistivity below 10 K at very high pressure.Comment: 4 pages, 4 figures; Proceedings SCES '0
Reference Standards for Body Fat Measure Using GE Dual Energy X-Ray Absorptiometry in Caucasian Adults
Background Dual energy x-ray absorptiometry (DXA) is an established technique for the measurement of body composition. Reference values for these variables, particularly those related to fat mass, are necessary for interpretation and accurate classification of those at risk for obesityrelated health complications and in need of lifestyle modifications (diet, physical activity, etc.). Currently, there are no reference values available for GE-Healthcare DXA systems and it is known that whole-body and regional fat mass measures differ by DXA manufacturer.
Objective To develop reference values by age and sex for DXA-derived fat mass measurements with GE-Healthcare systems.
Methods A de-identified sample of 3,327 participants (2,076 women, 1,251 men) was obtained from Ball State University\u27s Clinical Exercise Physiology Laboratory and University of Wisconsin- Milwaukee\u27s Physical Activity & Health Research Laboratory. All scans were completed using a GE Lunar Prodigy or iDXA and data reported included percent body fat (%BF), fat mass index (FMI), and ratios of android-to-gynoid (A/G), trunk/limb, and trunk/leg fat measurements. Percentiles were calculated and a factorial ANOVA was used to determine differences in the mean values for each variable between age and sex.
Results Normative reference values for fat mass variables from DXA measurements obtained from GE-Healthcare DXA systems are presented as percentiles for both women and men in 10- year age groups. Women had higher (p\u3c0.01) mean %BF and FMI than men, whereas men had higher (p\u3c0.01) mean ratios of A/G, trunk/limb, and trunk/leg fat measurements than women
Influence of the Fermi Surface Morphology on the Magnetic Field-Driven Vortex Lattice Structure Transitions in YBaCuO0, 0.15
We report small-angle neutron scattering measurements of the vortex lattice
(VL) structure in single crystals of the lightly underdoped cuprate
superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied
parallel to the crystal c-axis, we observe a sequence of field-driven and
first-order transitions between different VL structures. By rotating the field
away from the c-axis, we observe each structure transition to shift to either
higher or lower field dependent on whether the field is rotated towards the
[100] or [010] direction. We use this latter observation to argue that the
Fermi surface morphology must play a key role in the mechanisms that drive the
VL structure transitions. Furthermore, we show this interpretation is
compatible with analogous results obtained previously on lightly overdoped
YBa2Cu3O7. In that material, it has long-been suggested that the high field VL
structure transition is driven by the nodal gap anisotropy. In contrast, the
results and discussion presented here bring into question the role, if any, of
a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and
YBa2Cu3O7
Signatures of valence fluctuations in CeCu2Si2 under high pressure
Simultaneous resistivity and a.c.-specific heat measurements have been
performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a
hydrostatic helium pressure medium. A series of anomalies were observed around
the pressure coinciding with a maximum in the superconducting critical
temperature, . These anomalies can be linked with an abrupt change
of the Ce valence, and suggest a second quantum critical point at a pressure
GPa, where critical valence fluctuations provide the
superconducting pairing mechanism, as opposed to spin fluctuations at ambient
pressure. Such a valence instability, and associated superconductivity, is
predicted by an extended Anderson lattice model with Coulomb repulsion between
the conduction and f-electrons. We explain the T-linear resistivity found at
in this picture, while other anomalies found around can be
qualitatively understood using the same model.Comment: Submitted to Phys. Rev.
Long-range electronic reconstruction to a dxz,yz-dominated Fermi surface below the LaAlO3/SrTiO3 interface
Low dimensionality, broken symmetry and easily-modulated carrier concentrations provoke novel electronic phase emergence at oxide interfaces. However, the spatial extent of such reconstructions - i.e. the interfacial ‘‘depth’’ - remains unclear. Examining LaAlO3/SrTiO3 heterostructures at previously unexplored carrier densities n2D 14 cm-2 , we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic of an anisotropic 3D Fermi surface with preferential dxz,yz orbital occupancy extending over at least 100 nm perpendicular to the interface. Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO3 emerge simultaneously at higher n2D. We distinguish three areas in doped perovskite heterostructures: narrow (nm) 2D interfaces housing superconductivity and/or other emergent phases, electronically isotropic regions far (\u3e120 nm) from the interface and new intermediate zones where interfacial proximity renormalises the electronic structure relative to the bulk
Magnetic Field Mapping and Correction for Moving OP-MEG
Background: Optically pumped magnetometers (OPMs) have made moving, wearable magnetoencephalography (MEG) possible. The OPMs typically used for MEG require a low background magnetic field to operate, which is achieved using both passive and active magnetic shielding. However, the background magnetic field is never truly zero Tesla, and so the field at each of the OPMs changes as the participant moves. This leads to position and orientation dependent changes in the measurements, which manifest as low frequency artefacts in MEG data. Objective: We modelled the spatial variation in the magnetic field and used the model to predict the movement artefact found in a dataset. Methods: We demonstrate a method for modelling this field with a triaxial magnetometer, then showed that we can use the same technique to predict the movement artefact in a real OPM-based MEG (OP-MEG) dataset. Results: Using an 86-channel OP-MEG system, we found that this modelling method maximally reduced the power spectral density of the data by 27.8 0.6 dB at 0 Hz, when applied over 5 s non-overlapping windows. Conclusion: The magnetic field inside our state-of-the art magnetically shielded room can be well described by low-order spherical harmonic functions. We achieved a large reduction in movement noise when we applied this model to OP-MEG data. Significance: Real-time implementation of this method could reduce passive shielding requirements for OP-MEG recording and allow the measurement of low-frequency brain activity during natural participant movement
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