220 research outputs found
Valence bond glass on an fcc lattice in the double perovskite Ba2YMoO6
Peer reviewedPublisher PD
Spin Susceptibility of the Topological Superconductor UPt3 from Polarized Neutron Diffraction
Experiment and theory indicate that UPt3 is a topological superconductor in
an odd-parity state, based in part from temperature independence of the NMR
Knight shift. However, quasiparticle spin-flip scattering near a surface, where
the Knight shift is measured, might be responsible. We use polarized neutron
scattering to measure the bulk susceptibility with H||c, finding consistency
with the Knight shift but inconsistent with theory for this field orientation.
We infer that neither spin susceptibility nor Knight shift are a reliable
indication of odd-parity
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
Spins in the Vortices of a High Temperature Superconductor
Neutron scattering is used to characterise the magnetism of the vortices for
the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO(4)
(x=0.163) in an applied magnetic field. As temperature is reduced, low
frequency spin fluctuations first disappear with the loss of vortex mobility,
but then reappear. We find that the vortex state can be regarded as an
inhomogeneous mixture of a superconducting spin fluid and a material containing
a nearly ordered antiferromagnet. These experiments show that as for many other
properties of cuprate superconductors, the important underlying microscopic
forces are magnetic
Competing superconducting and magnetic order parameters and field-induced magnetism in electron doped Ba(FeCo)As
We have studied the magnetic and superconducting properties of
Ba(FeCo)As as a function of temperature and
external magnetic field using neutron scattering and muon spin rotation. Below
the superconducting transition temperature the magnetic and superconducting
order parameters coexist and compete. A magnetic field can significantly
enhance the magnetic scattering in the superconducting state, roughly doubling
the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data
by use of a five-band Hamiltonian relevant to iron pnictides. In the
superconducting state, vortices can slow down and freeze spin fluctuations
locally. When such regions couple they result in a long-range ordered
antiferromagnetic phase producing the enhanced magnetic elastic scattering in
agreement with experiments.Comment: 9 pages, 6 figure
Halving Balls in Deterministic Linear Time
Let \D be a set of pairwise disjoint unit balls in and the
set of their center points. A hyperplane \Hy is an \emph{-separator} for
\D if each closed halfspace bounded by \Hy contains at least points
from . This generalizes the notion of halving hyperplanes, which correspond
to -separators. The analogous notion for point sets has been well studied.
Separators have various applications, for instance, in divide-and-conquer
schemes. In such a scheme any ball that is intersected by the separating
hyperplane may still interact with both sides of the partition. Therefore it is
desirable that the separating hyperplane intersects a small number of balls
only. We present three deterministic algorithms to bisect or approximately
bisect a given set of disjoint unit balls by a hyperplane: Firstly, we present
a simple linear-time algorithm to construct an -separator for balls
in , for any , that intersects at most
balls, for some constant that depends on and . The number of
intersected balls is best possible up to the constant . Secondly, we present
a near-linear time algorithm to construct an -separator in
that intersects balls. Finally, we give a linear-time algorithm to
construct a halving line in that intersects
disks.
Our results improve the runtime of a disk sliding algorithm by Bereg,
Dumitrescu and Pach. In addition, our results improve and derandomize an
algorithm to construct a space decomposition used by L{\"o}ffler and Mulzer to
construct an onion (convex layer) decomposition for imprecise points (any point
resides at an unknown location within a given disk)
Staging superstructures in high- Sr/O co-doped LaSrCuO
We present high energy X-ray diffraction studies on the structural phases of
an optimal high- superconductor LaSrCuO tailored by
co-hole-doping. This is specifically done by varying the content of two very
different chemical species, Sr and O, respectively, in order to study the
influence of each. A superstructure known as staging is observed in all
samples, with the staging number increasing for higher Sr dopings . We
find that the staging phases emerge abruptly with temperature, and can be
described as a second order phase transition with transition temperatures
slightly depending on the Sr doping. The Sr appears to correlate the
interstitial oxygen in a way that stabilises the reproducibility of the staging
phase both in terms of staging period and volume fraction in a specific sample.
The structural details as investigated in this letter appear to have no direct
bearing on the electronic phase separation previously observed in the same
samples. This provides new evidence that the electronic phase separation is
determined by the overall hole concentration rather than specific Sr/O content
and concommittant structural details.Comment: 8 pages, incl. 4 figure
Distinct nature of static and dynamic magnetic stripes in cuprate superconductors
We present detailed neutron scattering studies of the static and dynamic
stripes in an optimally doped high-temperature superconductor,
LaCuO. We find that the dynamic stripes do not disperse towards the
static stripes in the limit of vanishing energy transfer. We conclude that the
dynamic stripes observed in neutron scattering experiments are not the
Goldstone modes associated with the broken symmetry of the simultaneously
observed static stripes, but rather that the signals originate from different
domains in the sample. These domains may be related by structural twinning, or
may be entirely different phases, where the static stripes in one phase are
pinned versions of the dynamic stripes in the other. Our results explain
earlier observations of unusual dispersions in underdoped
LaSrCuO () and LaBaCuO ().
Our findings are relevant for all compounds exhibiting magnetic stripes, and
may thus be a vital part in unveiling the nature of high temperature
superconductivity
Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain
Measurements are reported of the magnetic field dependence of excitations in
the quantum critical state of the spin S=1/2 linear chain Heisenberg
antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was
measured at k_B T/J <= 0.025 for H=0 and H=8.7 Tesla where the system is ~30%
magnetized. At H=0, the results are in quantitative agreement with exact
calculations of the dynamic spin correlation function for a two-spinon
continuum. At high magnetic field, there are multiple overlapping continua with
incommensurate soft modes. The boundaries of these continua confirm
long-standing predictions, and the intensities are consistent with exact
diagonalization and Bethe Ansatz calculations.Comment: 4 pages, 4 figure
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