3,725 research outputs found
Electronic structure of heavily electron-doped BaFeCoAs studied by angle-resolved photoemission
We have performed high-resolution angle-resolved photoemission spectroscopy
on heavily electron-doped non-superconducting (SC)
BaFeCoAs. We find that the two hole Fermi surface pockets
at the zone center observed in the hole-doped superconducting
BaKFeAs are absent or very small in this compound,
while the two electron pockets at the M point significantly expand due to
electron doping by the Co substitution. Comparison of the Fermi surface between
non-SC and SC samples indicates that the coexistence of hole and electron
pockets connected via the antiferromagnetic wave vector is essential in
realizing the mechanism of superconductivity in the iron-based superconductors.Comment: 5 pages, 4 figure
An Intra-pulse feedforward algorithm for improving pulsed microwave stability
During the pulsed operation of the linear accelerator in DCLS (Dalian
Coherent Light Source), we found a strong correlation between the klystron
modulator's high voltage and the klystron output microwave, with noticeable
jitter among adjacent microwaves. Therefore, we propose an intra-pulse
feedforward algorithm and implement it in LLRF (Low-Level Radiofrequency)
systems. This algorithm assumes that the transfer model of the microwave system
is linear within a small range of work points and measures the transfer
coefficient of the microwave between the LLRF and klystron. For each pulsed
microwave of the klystron output, the LLRF system first calculates the vector
deviation between the initial measurement within its pulse and the target. The
deviation will be compensated in the LLRF excitation so that the jitter in the
later part of the pulsed microwave is suppressed. Experiments have shown that
this algorithm can effectively suppress the jitter among adjacent microwaves,
e.g., improving the amplitude and phase stability (RMS) from 0.11%/0.2{\deg} to
0.1%/0.05{\deg}. This algorithm can also be applied to other accelerators
operating in pulsed modes.Comment: Talk presented at LLRF Workshop 2023 (LLRF2023, arXiv: 2310.03199
The microwave amplitude and phase setting based on event timing for the DCLS
The primary accelerator of DCLS (Dalian Coherent Light Source) operates at a
repetition rate of 20 Hz now, and the beam is divided at the end of the linear
accelera-tor through Kicker to make two 10 Hz beamlines work simultaneously.
For the simultaneous emission FEL of two beamlines, the beam energy of the two
beamlines is required to be controlled independently, so we need to set the
amplitude and phase of each beamline. This paper implements a microwave
amplitude and phase setting function based on event timing. We upgraded the
EVG/EVR event timing system and LLRF (Low-Level Radiofrequency) system. Two
special event codes and a repetition rate division of 10 Hz are added to the
event timing system, and we can set the microwave amplitude and phase by
judging the event code in LLRF. We ulti-mately perform the microwave triggering
at a repetition rate of 10 Hz for each beamline and validate this function
through beam experiments.Comment: Poster presented at LLRF Workshop 2023 (LLRF2023, arXiv: 2310.03199
Implementation of microwave with arbitrary amplitude and phase for the DCLS
In many experiments, the simultaneous emission of multiple wavelengths of FEL
(Free-Electron Laser) is significant. For the pulsed-mode FEL facility, we must
accelerate multiple electron beams in one microwave pulse, and they may be in
different amplitudes and phases in the acceleration field. Therefore, we
implement a microwave excitation, whose amplitude and phase have arbitrary
shapes in the LLRF (Low-Level Radiofrequency) system. We generate a microwave
pulse with step-shaped amplitude and phase for dual beam operation in DCLS
(Dalian Coherent Light Source). The microwave system of the primary accelerator
has four pulsed LLRF devices, which output excitation to drive four solid-state
amplifiers and then excite two 50 MW and two 80 MW klystrons, respectively.
Preliminary experiments have shown that this step-shaped microwave can be used
for the DCLS twin-bunch operation.Comment: Poster presented at LLRF Workshop 2023 (LLRF2023, arXiv: 2310.03199
Cationic vacancy induced room-temperature ferromagnetism in transparent conducting anatase Ti_{1-x}Ta_xO_2 (x~0.05) thin films
We report room-temperature ferromagnetism in highly conducting transparent
anatase Ti1-xTaxO2 (x~0.05) thin films grown by pulsed laser deposition on
LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), x-ray
diffraction (XRD), proton induced x-ray emission (PIXE), x-ray absorption
spectroscopy (XAS) and time-of-flight secondary ion mass spectrometry
(TOF-SIMS) indicated negligible magnetic contaminants in the films. The
presence of ferromagnetism with concomitant large carrier densities was
determined by a combination of superconducting quantum interference device
(SQUID) magnetometry, electrical transport measurements, soft x-ray magnetic
circular dichroism (SXMCD), XAS, and optical magnetic circular dichroism (OMCD)
and was supported by first-principle calculations. SXMCD and XAS measurements
revealed a 90% contribution to ferromagnetism from the Ti ions and a 10%
contribution from the O ions. RBS/channelling measurements show complete Ta
substitution in the Ti sites though carrier activation was only 50% at 5% Ta
concentration implying compensation by cationic defects. The role of Ti vacancy
and Ti3+ was studied via XAS and x-ray photoemission spectroscopy (XPS)
respectively. It was found that in films with strong ferromagnetism, the Ti
vacancy signal was strong while Ti3+ signal was absent. We propose (in the
absence of any obvious exchange mechanisms) that the localised magnetic
moments, Ti vacancy sites, are ferromagnetically ordered by itinerant carriers.
Cationic-defect-induced magnetism is an alternative route to ferromagnetism in
wide-band-gap semiconducting oxides without any magnetic elements.Comment: 21 pages, 10 figures, to appear in Philosophical Transaction - Royal
Soc.
Competitions of magnetism and superconductivity in FeAs-based materials
Using the numerical unrestricted Hartree-Fock approach, we study the ground
state of a two-orbital model describing newly discovered FeAs-based
superconductors. We observe the competition of a mode spin-density
wave and the superconductivity as the doping concentration changes. There might
be a small region in the electron-doping side where the magnetism and
superconductivity coexist. The superconducting pairing is found to be spin
singlet, orbital even, and mixed s + d wave (even
parity).Comment: 5 pages, 3 figure
Ultrahigh Surface Area Three-Dimensional Porous Graphitic Carbon from Conjugated Polymeric Molecular Framework
Porous graphitic carbon is essential for many applications such as energy storage devices, catalysts, and sorbents. However, current graphitic carbons are limited by low conductivity, low surface area, and ineffective pore structure. Here we report a scalable synthesis of porous graphitic carbons using a conjugated polymeric molecular framework as precursor. The multivalent cross-linker and rigid conjugated framework help to maintain micro- and mesoporous structures, while promoting graphitization during carbonization and chemical activation. The above unique design results in a class of highly graphitic carbons at temperature as low as 800 ??C with record-high surface area (4073 m2 g-1), large pore volume (2.26 cm-3), and hierarchical pore architecture. Such carbons simultaneously exhibit electrical conductivity >3 times more than activated carbons, very high electrochemical activity at high mass loading, and high stability, as demonstrated by supercapacitors and lithium-sulfur batteries with excellent performance. Moreover, the synthesis can be readily tuned to make a broad range of graphitic carbons with desired structures and compositions for many applications.clos
Spin-Gap Proximity Effect Mechanism of High Temperature Superconductivity
When holes are doped into an antiferromagnetic insulator they form a slowly
fluctuating array of ``topological defects'' (metallic stripes) in which the
motion of the holes exhibits a self-organized quasi one-dimensional electronic
character. The accompanying lateral confinement of the intervening
Mott-insulating regions induces a spin gap or pseudogap in the environment of
the stripes. We present a theory of underdoped high temperature superconductors
and show that there is a {\it local} separation of spin and charge, and that
the mobile holes on an individual stripe acquire a spin gap via pair hopping
between the stripe and its environment; i.e. via a magnetic analog of the usual
superconducting proximity effect. In this way a high pairing scale without a
large mass renormalization is established despite the strong Coulomb repulsion
between the holes. Thus the {\it mechanism} of pairing is the generation of a
spin gap in spatially-confined {\it Mott-insulating} regions of the material in
the proximity of the metallic stripes. At non-vanishing stripe densities,
Josephson coupling between stripes produces a dimensional crossover to a state
with long-range superconducting phase coherence. This picture is established by
obtaining exact and well-controlled approximate solutions of a model of a
one-dimensional electron gas in an active environment. An extended discussion
of the experimental evidence supporting the relevance of these results to the
cuprate superconductors is given.Comment: 30 pages, 2 figure
Observation of in
Using a sample of events recorded with
the BESIII detector at the symmetric electron positron collider BEPCII, we
report the observation of the decay of the charmonium state
into a pair of mesons in the process
. The branching fraction is measured for the first
time to be , where the first uncertainty is
statistical, the second systematic and the third is from the uncertainty of
. The mass and width of the are
determined as MeV/ and
MeV.Comment: 13 pages, 6 figure
Measurement of proton electromagnetic form factors in in the energy region 2.00-3.08 GeV
The process of is studied at 22 center-of-mass
energy points () from 2.00 to 3.08 GeV, exploiting 688.5~pb of
data collected with the BESIII detector operating at the BEPCII collider. The
Born cross section~() of is
measured with the energy-scan technique and it is found to be consistent with
previously published data, but with much improved accuracy. In addition, the
electromagnetic form-factor ratio () and the value of the
effective (), electric () and magnetic () form
factors are measured by studying the helicity angle of the proton at 16
center-of-mass energy points. and are determined with
high accuracy, providing uncertainties comparable to data in the space-like
region, and is measured for the first time. We reach unprecedented
accuracy, and precision results in the time-like region provide information to
improve our understanding of the proton inner structure and to test theoretical
models which depend on non-perturbative Quantum Chromodynamics
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