122 research outputs found
Gains from the upgrade of the cold neutron triple-axis spectrometer FLEXX at the BER-II reactor
The upgrade of the cold neutron triple-axis spectrometer FLEXX is described.
We discuss the characterisation of the gains from the new primary spectrometer,
including a larger guide and double focussing monochromator, and present
measurements of the energy and momentum resolution and of the neutron flux of
the instrument. We found an order of magnitude gain in intensity (at the cost
of coarser momentum resolution), and that the incoherent elastic energy widths
are measurably narrower than before the upgrade. The much improved count rate
should allow the use of smaller single crystals samples and thus enable the
upgraded FLEXX spectrometer to continue making leading edge measurements.Comment: 8 pages, 7 figures, 5 table
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
A simulational study of the indirect geometry neutron spectrometer, BIFROST at the European Spallation Source, from neutron source position to detector position
The European Spallation Source (ESS) is intended to become the most powerful
spallation neutron source in the world and the flagship of neutron science in
the upcoming decades. The exceptionally high neutron flux will provide unique
opportunities for scientific experiments, but also set high requirements for
the detectors. One of the most challenging aspects is the rate capability and
in particular the peak instantaneous rate capability, i.e. the number of
neutrons hitting the detector per channel or cm at the peak of the neutron
pulse. The primary purpose of this paper is to estimate the incident rates that
are anticipated for the BIFROST instrument planned for ESS, and also to
demonstrate the use of powerful simulation tools for the correct interpretation
of neutron transport in crystalline materials. A full simulation model of the
instrument from source to detector position, implemented with the use of
multiple simulation software packages is presented. For a single detector tube
instantaneous incident rates with a maximum of 1.7 GHz for a Bragg peak from a
single crystal, and 0.3 MHz for a vanadium sample are found. This paper also
includes the first application of a new pyrolytic graphite model, and a
comparison of different simulation tools to highlight their strengths and
weaknesses.Comment: 45 pages, 20 figure
Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point
Spontaneous symmetry-breaking quantum phase transitions play an essential
role in condensed matter physics. The collective excitations in the
broken-symmetry phase near the quantum critical point can be characterized by
fluctuations of phase and amplitude of the order parameter. The phase
oscillations correspond to the massless NambuGoldstone modes whereas the
massive amplitude mode, analogous to the Higgs boson in particle physics, is
prone to decay into a pair of low-energy NambuGoldstone modes in low
dimensions. Especially, observation of a Higgs amplitude mode in two dimensions
is an outstanding experimental challenge. Here, using the inelastic neutron
scattering and applying the bond-operator theory, we directly and unambiguously
identify the Higgs amplitude mode in a two-dimensional S=1/2 quantum
antiferromagnet CHNCuBr near a quantum critical point in two
dimensions. Owing to an anisotropic energy gap, it kinematically prevents such
decay and the Higgs amplitude mode acquires an infinite lifetime.Comment: 12 pages, 4 figures in the main text+3 figures in Supplementary
Informatio
Phonons in mesoporous silicon The influence of nanostructuring on the dispersion in the Debye regime
We present a comprehensive scattering study of nanostructured silicon. Neutron and x ray scattering experiments elucidate structural and dynamical properties of electrochemically etched, porous silicon membranes with pores roughly 8 nm across. In particular, inelastic cold neutron scattering techniques reveal the phonon dispersion of the nanostructured, single crystalline samples in the linear Debye regime for energy transfers up to 4 meV. A modified dispersion relation characterized by systematically reduced sound velocities manifests itself in altered elastic properties of porous silicon when compared to bulk silicon. Its relevance for nanostructured silicon as thermoelectric material of interest is discusse
Anomalous magnetic structure and spin dynamics in magnetoelectric LiFePO4
We report significant details of the magnetic structure and spin dynamics of LiFePO4 obtained by single-crystal neutron scattering. Our results confirm a previously reported collinear rotation of the spins away from the principal b axis, and they determine that the rotation is toward the a axis. In addition, we find a significant spin-canting component along c. The possible causes of these components are discussed, and their significance for the magnetoelectric effect is analyzed. Inelastic neutron scattering along the three principal directions reveals a highly anisotropic hard plane consistent with earlier susceptibility measurements. Using a spin Hamiltonian, we show that the spin dimensionality is intermediate between XY- and Ising-like, with an easy b axis and a hard c axis. It is shown that both next-nearest neighbor exchange couplings in the bc plane are in competition with the strongest nearest neighbor coupling.
©2015 American Physical Society8
Quantum spin liquid in frustrated one dimensional LiCuSbO4
A quantum magnet, LiCuSbO4, with chains of edge-sharing S = 1/2 CuO6
octahedra is reported. While the Curie-Weiss constant is ferromagnetic, \theta
= 30 K, no phase transition or spin freezing occurs down to 100 mK. Specific
heat indicates a distinct high field phase near the 12 T saturation field.
Neutron scattering shows incommensurate spin correlations with q =
0.47\pm0.01{\pi}/a and places an upper limit of 70 \mueV on a potential spin
gap. Exact diagonalization of easy plane S = 1/2 chains with competing ferro-
and antiferromagnetic interactions (J1 = - 75 K, J2 = 34 K) accounts for the T
> 2 K data.Comment: 14 pages, 4 figure
The upgraded cold neutron triple axis spectrometer FLEXX enhanced capabilities by new instrumental options
The upgrade of the cold neutron triple axis spectrometer FLEXX, a work-horse instrument for inelastic neutron scattering matching the sample environment capabilities at Helmholtz-Zentrum Berlin, has been successfully accomplished. Experiments confirmed an order of magnitude gain in flux now allowing for intensity demanding options to be fully exploited at FLEXX. In this article, we describe the layout and design of two newly available FLEXX instrument options in detail. The new Heusler analyzer gives an increase of the detected polarized neutron flux due to its superior focusing properties, significantly improving the feasibility of future polarized and neutron resonance spin echo experiments. The MultiFLEXX option provides simultaneous access to large regions in wavevector and energy space for inelastic excitations thus adding mapping capabilities to the spectrometer
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