478 research outputs found
Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs
Since the discovery of high transition-temperature (Tc) superconductivity in
copper oxides two decades ago, continuous efforts have been devoted to
searching for similar phenomenon in other compounds. With the exception of MgB2
(Tc =39 K), however, Tc is generally far lower than desired. Recently,
breakthrough has been made in a new class of oxypnictide compounds. Following
the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc
onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile,
unravelling the nature of the energy associated with the formation of
current-carrying pairs (Cooper pairs), referred to as the superconducting
energy gap, is the first and vital step towards understanding why the
superconductivity occurs at such high temperature and is also important for
finding superconductors with still higher Tc. Here we show that, on the basis
of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc
=45 K), the Cooper pair is in the spin-singlet state (two spins are
anti-paralleled), with two energy gaps opening below Tc. The results strongly
suggest the existence of nodes (zeros) in the gap. None of superconductors
known to date has such unique gap features, although copper-oxides and MgB2
share part of them.Comment: submitted on May 1
Nernst effect of the new iron-based superconductor LaOFFeAs
We report the first Nernst effect measurement on the new iron-based
superconductor LaOFFeAs . In the normal state, the
Nernst signal is negative and very small. Below a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
Electron-hole Asymmetry and Quantum Critical Point in Hole-doped BaFeAs
We show, from first-principles calculations, that the hole-doped side of
FeAs-based compounds is different from its electron-doped counterparts. The
electron side is characterized as Fermi surface nesting, and SDW-to-NM quantum
critical point (QCP) is realized by doping. For the hole-doped side, on the
other hand, orbital-selective partial orbital ordering develops together with
checkboard antiferromagnetic (AF) ordering without lattice distortion. A unique
SDW-to-AF QCP is achieved, and = criteria (in the approximate
J_1&J_2 model) is satisfied. The observed superconductivity is located in the
vicinity of QCP for both sides.Comment: 4 page
Search for an invisible muon philic scalar or vector via decay at BESIII
A light scalar or vector particles have been introduced as a
possible explanation for the anomaly and dark matter phenomena.
Using \jpsi events collected by the BESIII
detector, we search for a light muon philic scalar or vector in
the processes with invisible decays. No
obvious signal is found, and the upper limits on the coupling
between the muon and the particles are set to be between
and for the mass in the range
of ~MeV at 90 confidence level.Comment: 9 pages 7 figure
Updated measurements of the M1 transition with
Based on a data sample of events
collected with the BESIII detector at the BEPCII collider, the M1 transition
with is
studied, where is or
. The mass and width of the are
measured to be MeV/
and MeV, respectively. The
product branching fraction is determined to be . Using , we obtain the branching fraction of the
radiative transition to be , where the third uncertainty is due to the quoted
First Observation of a Three-Resonance Structure in {non-open} Charm Hadrons
We report the measurement of the cross sections for
{nOCH} (nOCH stands for non-open charm hadrons) with
improved precision at center-of-mass energies from 3.645 to 3.871 GeV. We
observe for the first time a three-resonance structure in the energy-dependent
lineshape of the cross sections, which are , and with significances of ,
, and , respectively. The is observed
for the first time. We found two solutions in analysis of the cross sections.
For solution I [solution II], we measure the mass, the total width and the
product of electronic width and nOCH decay branching fraction to be [] MeV/, [] MeV, and [] eV for the , respectively. In addition, we
measure the branching fractions {nOCH} for the first time, and {nOCH}. Moreover, we determine the open-charm (OC) branching fraction
{OC}, which supports the interpretation of as an OC pair molecular state, but contained a simple four-quark state
component. The first uncertainties are from fits to the cross sections, and the
second are systematic
Study of the doubly Cabibbo-suppressed decays and
Based on 7.33 fb of collision data collected at
center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector,
the experimental studies of the doubly Cabibbo-suppressed decays and are reported. We determine the
absolute branching fraction of to be
() . No
significant signal of is observed and the upper
limit on its decay branching fraction at 90\% confidence level is set to be
.Comment: 10 pages, 4 figures, 4 table
Measurement of the cross sections from 2.000 to 3.080 GeV
Based on collision data collected at center-of-mass energies
from 2.000 to 3.080 GeV by the BESIII detector at the BEPCII collider, a
partial wave analysis is performed for the process . The results allow the Born cross sections of the process
, as well as its subprocesses
and to be
measured. The Born cross sections for are consistent with previous measurements by BaBar and SND,
but with substantially improved precision. The Born cross section lineshape of
the process is consistent with a vector
meson state around 2.2 GeV with a statistical significance of 3.2. A
Breit-Wigner fit determines its mass as
and its width as
, where the first uncertainties are
statistical and the second ones are systematic, respectively
A coupled-channel analysis of the lineshape with BESIII data
We perform a study of the lineshape using the data samples of
, and collected with the BESIII detector. The effects of the coupled-channels
and the off-shell are included in the parameterization of the
lineshape. The lineshape mass parameter is obtained to be MeV. Two poles are found on the first and second Riemann
sheets corresponding to the branch cut. The pole location on
the first sheet is much closer to the threshold than the
other, and is determined to be MeV above the
threshold with an imaginary part
MeV
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