352 research outputs found
SPHERES, J\"ulich's High-Flux Neutron Backscattering Spectrometer at FRM II
SPHERES (SPectrometer with High Energy RESolution) is a third-generation
neutron backscattering spectrometer, located at the 20 MW German neutron source
FRM II and operated by the Juelich Centre for Neutron Science. It offers an
energy resolution (fwhm) better than 0.65 micro-eV, a dynamic range of +-31
micro-eV, and a signal-to-noise ratio of up to 1750:1.Comment: 12 pages, 7 figures, 2 tables. Supplemental material consists of 3
pages, 2 figures, 2 table
The squeezed dark nuclear spin state in lead halide perovskites
Coherent many-body states are highly promising for robust quantum information processing. While far-reaching theoretical predictions have been made for various implementations, direct experimental evidence of their appealing properties can be challenging. Here, we demonstrate optical manipulation of the nuclear spin ensemble in the lead halide perovskite semiconductor FAPbBr3 (FA = formamidinium), targeting a long-postulated collective dark state that is insensitive to optical pumping after its build-up. Via optical orientation of localized hole spins we drive the nuclear many-body system into this entangled state, requiring a weak magnetic field of only a few milli-Tesla strength at cryogenic temperatures. During its fast establishment, the nuclear polarization along the optical axis remains small, while the transverse nuclear spin fluctuations are strongly reduced, corresponding to spin squeezing as evidenced by a strong violation of the generalized nuclear squeezing-inequality with ξs < 0.5. The dark state corresponds to an ~35-body entanglement between the nuclei. Dark nuclear spin states can be exploited to store quantum information benefiting from their long-lived many-body coherence and to perform quantum measurements with a precision beyond the standard limit
Genetically engineered minipigs model the major clinical features of human neurofibromatosis type 1.
Neurofibromatosis Type 1 (NF1) is a genetic disease caused by mutations in Neurofibromin 1 (NF1). NF1 patients present with a variety of clinical manifestations and are predisposed to cancer development. Many NF1 animal models have been developed, yet none display the spectrum of disease seen in patients and the translational impact of these models has been limited. We describe a minipig model that exhibits clinical hallmarks of NF1, including café au lait macules, neurofibromas, and optic pathway glioma. Spontaneous loss of heterozygosity is observed in this model, a phenomenon also described in NF1 patients. Oral administration of a mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor suppresses Ras signaling. To our knowledge, this model provides an unprecedented opportunity to study the complex biology and natural history of NF1 and could prove indispensable for development of imaging methods, biomarkers, and evaluation of safety and efficacy of NF1-targeted therapies
Evaluation of a method for time-of-flight, wavelength and distance calibration for neutron scattering instruments by means of a mini-chopper and standard neutron monitors
Accurate conversion of neutron time-of-flight (TOF) to wavelength, and its
uncertainty, is of fundamental importance to neutron scattering measurements.
Especially in cases where instruments are highly configurable, the
determination of the absolute wavelength after any change must always be
performed. Inspired by the manner with which neutron spectrometers determine
the absolute wavelength, we evaluate for the first time, in the author's
knowledge, a commonly used method for converting TOF to neutron wavelength, the
distance of a monitor from the source of neutrons and we analytically calculate
the uncertainty contributions that limit the precision of the conversion. The
method was evaluated at the V20 test beamline at the Helmholtz Zentrum Berlin
(HZB), emulating the ESS source with a pulse of 2.86 ms length and 14 Hz
repetition rate, by using a mini-chopper operated at 140 Hz, beam monitors
(BMs) and data acquisition infrastructure. The mini-chopper created
well-defined neutron pulses and the BM was placed at two positions, enabling
the average wavelength of each of the pulses created to be determined. The used
experimental setup resulted in absolute wavelength determination at the monitor
positions with a of 1.8% for
\r{A}. With a modest increase of the distance between the
reference monitor positions a of below
0.5% can be achieved. Further improvements are possible by using a thinner
monitor, smaller chopper disc openings and a higher rotational speed chopper.
The method requires only two neutron measurements and doesn't necessitate the
use of crystals or complex fitting, and could constitute a suitable addition to
imaging, diffraction, reflectometers and small angle neutron scattering
instruments, at spallation sources, that do not normally utilise fast choppers
Tailoring the electron and hole Land\'e factors in lead halide perovskite nanocrystals by quantum confinement and halide exchange
The tunability of the optical properties of lead halide perovskite
nanocrystals makes them highly appealing for applications. Both, halide anion
exchange and quantum confinement pave the way for tailoring their band gap
energy. For spintronics applications, the Land\'e g-factors of electrons and
hole are of great importance. By means of the empirical tight-binding and
methods, we calculate them for nanocrystals of the
class of all-inorganic lead halide perovskites CsPb (). The hole g-factor as function of the band
gap follows the universal dependence found for bulk perovskites, while for the
electrons a considerable modification is predicted. Based on the
analysis we conclude that this difference arises
from the interaction of the bottom conduction band with the spin-orbit split
electron states. The model predictions are confirmed by experimental data for
the electron and hole g-factors in CsPbI3 nanocrystals placed in a glass
matrix, measured by time-resolved Faraday ellipticity in a magnetic field at
cryogenic temperatures
"m=1" coatings for neutron guides
A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section
"m=1" coatings for neutron guides
A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section
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