2,409 research outputs found
Microstructure and Fe-vacancy ordering in the KFexSe2 superconducting system
Structural investigations by means of transmission electron microscopy (TEM)
on KFexSe2 with 1.5 \leq x \leq 1.8 have revealed a rich variety of
microstructure phenomena, the KFe1.5Se2 crystal often shows a superstructure
modulation along the [310] zone-axis direction, this superstructure can be well
interpreted by the Fe-vacancy order within the a-b plane. Increase of
Fe-concentration in the KFexSe2 materials could not only result in the
appearance of superconductivity but also yield clear alternations of
microstructure. Structural inhomogeneity, the complex superstructures and
defect structures in the superconducting KFe1.8Se2 sample have been
investigated based on the high-resolution TEM.Comment: 13 pages, 4 figure
Room temperature antiferromagnetic order in superconducting X_yFe_{2-x}Se_ 2, (X= Rb, K): a powder neutron diffraction study
Magnetic and crystal structures of superconducting X yFe 2-xSe 2 (X= Rb and K
with Tc=31.5 K and 29.5 K) have been studied by neutron powder diffraction at
room temperature. Both crystals show ordered iron vacancy pattern and the
crystal structure is well described in the I4/m space group with the lattice
constants a=8.799, c=14.576 and a=8.730, c=14.115 A, and the refined
stoichiometry x=0.30(1), y=0.83(2) and x=0.34(1), y=0.83(1) for Rb- and
K-crystals respectively. The structure contains one fully occupied iron
position and one almost empty vacancy position. Assuming that the iron moment
is ordered only on the fully occupied site we have sorted out all eight
irreducible representations (irreps) for the propagation vector k=0 and have
found that irreps tau_2 and tau_7 well fit the experimental data with the
moments along c-axis. The moment amplitudes amounted to 2.15(3) mu_B, 2.55(3)
mu_B for tau_2 and 2.08(6) mu_B, 2.57(3) mu_B for tau_7 for Rb- and K-crystals
respectively. Irrep tau_2 corresponds to the Shubnikov group I4/m' and gives a
constant moment antiferromagnetic configuration, whereas tau_7 does not have
Shubnikov counterpart and allows two different magnetic moments in the
structure.Comment: 5 pages, 1 table, 4 figure
Measurements of decays into Vector- Tensor final states
Decays of the into vector plus tensor meson final states have been
studied with 14 million events collected with the BESII detector.
Branching fractions of \psi(2S) \rt \omega f_{2}(1270), ,
and are
determined. They improve upon previous BESI results and confirm the violation
of the "12%" rule for decays to VT channels with higher precision.Comment: 7 pages, 7 figures and 2 table
\psi(2S) Decays into \J plus Two Photons
Using \gamma \gamma J/\psi, J/\psi \ra e^+ e^- and events
from a sample of \psip decays collected with the BESII
detector, the branching fractions for \psip\ra \pi^0\J, \eta\J, and
\psi(2S)\ar\gamma\chi_{c1},\gamma\chi_{c2}\ar\gamma\gamma\jpsi are measured
to be B(\psip\ra \pi^0\J) = (1.43\pm0.14\pm0.13)\times 10^{-3}, B(\psip\ra
\eta\J) = (2.98\pm0.09\pm0.23)%,
B(\psi(2S)\ar\gamma\chi_{c1}\ar\gamma\gamma\jpsi) = (2.81\pm0.05\pm 0.23)%,
and B(\psi(2S)\ar\gamma\chi_{c2}\ar\gamma\gamma\jpsi) = (1.62\pm0.04\pm
0.12)%.Comment: 7 pages, 6 figures. submitted to Phys. Rev.
Construction and On-site Performance of the LHAASO WFCTA Camera
The focal plane camera is the core component of the Wide Field-of-view
Cherenkov/fluorescence Telescope Array (WFCTA) of the Large High-Altitude Air
Shower Observatory (LHAASO). Because of the capability of working under
moonlight without aging, silicon photomultipliers (SiPM) have been proven to be
not only an alternative but also an improvement to conventional photomultiplier
tubes (PMT) in this application. Eighteen SiPM-based cameras with square light
funnels have been built for WFCTA. The telescopes have collected more than 100
million cosmic ray events and preliminary results indicate that these cameras
are capable of working under moonlight. The characteristics of the light
funnels and SiPMs pose challenges (e.g. dynamic range, dark count rate,
assembly techniques). In this paper, we present the design features,
manufacturing techniques and performances of these cameras. Finally, the test
facilities, the test methods and results of SiPMs in the cameras are reported
here.Comment: 45 pages, 21 figures, articl
Observation of a near-threshold enhancement in th p pbar mass spectrum from radiative J/psi-->gamma p pbar decays
We observe a narrow enhancement near 2mp in the invariant mass spectrum of
ppbar pairs from radiative J/psi-->gamma ppbar decays. The enhancement can be
fit with either an S- or P-wave Breit Wigner fuction. In the case of the S-wave
fit, the peak mass is below the 2mp threshold and the full width is less than
30 MeV. These mass and width values are not consistent with the properties of
any known meson resonance.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
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