997 research outputs found
Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation
The role of orbital differentiation on the emergence of superconductivity in
the Fe-based superconductors remains an open question to the scientific
community. In this investigation, we employ a suitable microscopic spin probe
technique, namely Electron Spin Resonance (ESR), to investigate this issue on
selected chemically substituted BaFeAs single crystals. As the
spin-density wave (SDW) phase is suppressed, we observe a clear increase of the
Fe 3 bands anisotropy along with their localization at the FeAs plane. Such
an increase of the planar orbital content interestingly occurs independently on
the chemical substitution responsible for suppressing the SDW phase. As a
consequence, the magnetic fluctuations combined with the resultant particular
symmetry of the Fe 3 bands are propitious ingredients to the emergence of
superconductivity in this class of materials.Comment: 6 pages, 5 figure
Quantum Oscillations in EuFe2As2 single crystals
Quantum oscillation measurements can provide important information about the
Fermi surface (FS) properties of strongly correlated metals. Here, we report a
Shubnikov-de Haas (SdH) effect study on the pnictide parent compounds
EuFeAs (Eu122) and BaFeAs (Ba122) grown by In-flux.
Although both members are isovalent compounds with approximately the same
density of states at the Fermi level, our results reveal subtle changes in
their fermiology. Eu122 displays a complex pattern in the Fourier spectrum,
with band splitting, magnetic breakdown orbits, and effective masses
sistematically larger when compared to Ba122, indicating that the former is a
more correlated metal. Moreover, the observed pockets in Eu122 are more
isotropic and 3D-like, suggesting an equal contribution from the Fe
orbitals to the FS. We speculate that these FS changes may be responsible for
the higher spin-density wave ordering temperature in Eu122.Comment: 5 pages, 4 figure
New Physics in the Rayleigh-Jeans Tail of the CMB
We show that despite stringent constraints on the shape of the main part of
the CMB spectrum, there is considerable room for its modification within its
Rayleigh-Jeans (RJ) end, . We construct explicit New
Physics models that give an order one (or larger) increase of photon count in
the RJ tail, which can be tested by existing and upcoming experiments aiming to
detect the cosmological 21 cm emission/absorption signal. This class of models
stipulates the decay of unstable particles to dark photons, , that have a
small mass, eV, non-vanishing mixing angle
with electromagnetism, and energies much smaller than .
The non-thermal number density of dark photons can be many orders of magnitude
above the number density of CMB photons, and even a small probability of oscillations, for values of as small as , can
significantly increase the number of RJ photons. In particular, we show that
resonant oscillations of dark photons into regular photons in the interval of
redshifts can be invoked as an explanation of the recent
tentative observation of a stronger-than-expected absorption signal of 21 cm
photons. We present a model that realizes this possibility, where milli-eV mass
dark matter decays to dark photons, with a lifetime longer than the age of the
Universe.Comment: 6 pages, 4 figures; version published in PR
Possible unconventional superconductivity in substituted BaFeAs revealed by magnetic pair-breaking studies
The possible existence of a sign-changing gap symmetry in
BaFeAs-derived superconductors (SC) has been an exciting topic of
research in the last few years. To further investigate this subject we combine
Electron Spin Resonance (ESR) and pressure-dependent transport measurements to
investigate magnetic pair-breaking effects on BaFeAs (
Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence
of localized magnetic moments, is observed only for Cu and Mn compounds,
which display very low SC transition temperature () and no SC,
respectively. From the ESR analysis assuming the absence of bottleneck effects,
the microscopic parameters are extracted to show that this reduction of
cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a
sign-preserving gap function. Our results reveal an unconventional spin- and
pressure-dependent pair-breaking effect and impose strong constraints on the
pairing symmetry of these materials
Initial State Radiation in Majorana Dark Matter Annihilations
The cross section for a Majorana Dark Matter particle annihilating into light
fermions is helicity suppressed. We show that, if the Dark Matter is the
neutral Majorana component of a multiplet which is charged under the
electroweak interactions of the Standard Model, the emission of gauge bosons
from the initial state lifts the suppression and allows an s-wave annihilation.
The resulting energy spectra of stable Standard Model particles are importantly
affected. This has an impact on indirect searches for Dark Matter.Comment: 9 pages, 3 figure
Distinct high-T transitions in underdoped BaKFeAs
In contrast to the simultaneous structural and magnetic first order phase
transition previously reported, our detailed investigation on an
underdoped BaKFeAs single crystal unambiguously
revealed that the transitions are not concomitant. The tetragonal (:
I4/mmm) - orthorhombic (: Fmmm) structural transition occurs at
110 K, followed by an adjacent antiferromagnetic (AFM) transition
at 102 K. Hysteresis and coexistence of the and
phases over a finite temperature range observed in our NMR
experiments confirm the first order character of the structural transition and
provide evidence that both and are strongly correlated. Our
data also show that superconductivity (SC) develops in the phase
below = 20 K and coexists with long range AFM. This new observation,
, firmly establishes another similarity between the hole-doped
BaFeAs via K substitution and the electron-doped iron-arsenide
superconductors.Comment: 4 pages, 3 figure
Guidance on the use of MRI for treatment planning in radiotherapy clinical trials
The aim of this article is to propose meaningful guidance covering the technical and safety issues involved when designing or conducting radiotherapy clinical trials that use MRI for treatment planning. The complexity of imaging requirements will depend on the trial aims, design and MRI methods used.The use of MRI within the RT pathway is becoming more prevalent and clinically appropriate as access to MRI increases, treatment planning systems become more versatile and potential indications for MRI-planning in RT are documented. Novel MRI-planning opportunities are often initiated and validated within clinical trials.The guidance in this document is intended to assist researchers designing RT clinical trials involving MRI, so that they may provide sufficient information about the appropriate methods to be used for image acquisition, post-processing and quality assurance such that participating sites complete MRI to consistent standards. It has been produced in collaboration with the National Radiotherapy Trials Quality Assurance Group (RTTQA).As the use of MRI in RT is developed, it is highly recommended for researchers writing clinical trial protocols to include imaging guidance as part of their clinical trial documentation covering the trial-specific requirements for MRI procedures. Many of the considerations and recommendations in this guidance may well apply to MR-guided treatment machines, where clinical trials will be crucial. Similarly, many of these recommendations will apply to the general use of MRI in RT, outside of clinical trials.This document contains a large number of recommendations, not all of which will be relevant to any particular trial. Designers of RT clinical trials must therefore take this into account. They must also use their own judgement as to the appropriate compromise between accessibility of the trial and its technical rigour
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