183 research outputs found
Temperature dependence of the resonance and low energy spin excitations in superconducting FeTeSe
We use inelastic neutron scattering to study the temperature dependence of
the low-energy spin excitations in single crystals of superconducting
FeTeSe ( K). In the low-temperature superconducting
state, the imaginary part of the dynamic susceptibility at the electron and
hole Fermi surfaces nesting wave vector ,
, has a small spin gap, a two-dimensional
neutron spin resonance above the spin gap, and increases linearly with
increasing for energies above the resonance. While the intensity
of the resonance decreases like an order parameter with increasing temperature
and disappears at temperature slightly above , the energy of the mode is
weakly temperature dependent and vanishes concurrently above . This
suggests that in spite of its similarities with the resonance in electron-doped
superconducting BaFe(Co,Ni)As, the mode in
FeTeSe is not directly associated with the superconducting
electronic gap.Comment: 7 pages, 6 figure
Electron doping evolution of the magnetic excitations in NaFeCoAs
We use time-of-flight (ToF) inelastic neutron scattering (INS) spectroscopy
to investigate the doping dependence of magnetic excitations across the phase
diagram of NaFeCoAs with and .
The effect of electron-doping by partially substituting Fe by Co is to form
resonances that couple with superconductivity, broaden and suppress low energy
( meV) spin excitations compared with spin waves in undoped NaFeAs.
However, high energy ( meV) spin excitations are weakly Co-doping
dependent. Integration of the local spin dynamic susceptibility
of NaFeCoAs reveals a total
fluctuating moment of 3.6 /Fe and a small but systematic reduction
with electron doping. The presence of a large spin gap in the Co-overdoped
nonsuperconducting NaFeCoAs suggests that Fermi surface
nesting is responsible for low-energy spin excitations. These results parallel
Ni-doping evolution of spin excitations in BaFeNiAs, confirming
the notion that low-energy spin excitations coupling with itinerant electrons
are important for superconductivity, while weakly doping dependent high-energy
spin excitations result from localized moments.Comment: 14 pages, 16 figure
Electron doping evolution of the neutron spin resonance in NaFeCoAs
Neutron spin resonance, a collective magnetic excitation coupled to
superconductivity, is one of the most prominent features shared by a broad
family of unconventional superconductors including copper oxides, iron
pnictides, and heavy fermions. In this work, we study the doping evolution of
the resonances in NaFeCoAs covering the entire superconducting
dome. For the underdoped compositions, two resonance modes coexist. As doping
increases, the low-energy resonance gradually loses its spectral weight to the
high-energy one but remains at the same energy. By contrast, in the overdoped
regime we only find one single resonance, which acquires a broader width in
both energy and momentum, but retains approximately the same peak position even
when drops by nearly a half compared to optimal doping. These results
suggest that the energy of the resonance in electron overdoped
NaFeCoAs is neither simply proportional to nor the
superconducting gap, but is controlled by the multi-orbital character of the
system and doped impurity scattering effect.Comment: accepted by PR
Short-range cluster spin glass near optimal superconductivity in BaFeNiAs
High-temperature superconductivity in iron pnictides occurs when electrons
are doped into their antiferromagnetic (AF) parent compounds. In addition to
inducing superconductivity, electron-doping also changes the static
commensurate AF order in the undoped parent compounds into short-range
incommensurate AF order near optimal superconductivity. Here we use neutron
scattering to demonstrate that the incommensurate AF order in
BaFeNiAs is not a spin-density-wave arising from the
itinerant electrons in nested Fermi surfaces, but consistent with a cluster
spin glass in the matrix of the superconducting phase. Therefore, optimal
superconductivity in iron pnictides coexists and competes with a mesoscopically
separated cluster spin glass phase, much different from the homogeneous
coexisting AF and superconducting phases in the underdoped regime.Comment: 4 figure
Phase Separation, Competition, and Volume Fraction Control in NaFeCoAs
We report a detailed nuclear magnetic resonance (NMR) study by combined
Na and As measurements over a broad range of doping to map the
phase diagram of NaFeCoAs. In the underdoped regime (
0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order,
which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and
possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction \% is approximately constant. Near optimal doping, at , we
observe a phase separation between static antiferromagnetism related to the
{\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The
volume fraction of AFM phase increases upon cooling, but both the N{\'e}el
temperature and the volume fraction can be suppressed systematically by
applying a -axis magnetic field. On cooling below , superconductivity
occupies the PM region and its volume fraction grows at the expense of the AFM
phase, demonstrating a phase separation of the two types of order based on
volume exclusion. At higher dopings, static antiferromagnetism and even
critical AFM fluctuations are completely suppressed by superconductivity. Thus
the phase diagram we establish contains two distinct types of phase separation
and reflects a strong competition between AFM and superconducting phases both
in real space and in momentum space. We suggest that both this strict mutual
exclusion and the robustness of superconductivity against magnetism are
consequences of the extreme two-dimensionality of NaFeAs.Comment: 12 pages, 6 figure
Distinguishing and electron pairing symmetries by neutron spin resonance in superconducting NaFeCoAs
A determination of the superconducting (SC) electron pairing symmetry forms
the basis for establishing a microscopic mechansim for superconductivity. For
iron pnictide superconductors, the -pairing symmetry theory predicts the
presence of a sharp neutron spin resonance at an energy below the sum of hole
and electron SC gap energies () below . On the other hand,
the -pairing symmetry expects a broad spin excitation enhancement at an
energy above below . Although the resonance has been observed in
iron pnictide superconductors at an energy below consistent with the
-pairing symmetry, the mode has also be interpreted as arising from the
-pairing symmetry with due to its broad energy width and
the large uncertainty in determining the SC gaps. Here we use inelastic neutron
scattering to reveal a sharp resonance at E=7 meV in SC
NaFeCoAs ( K). On warming towards , the mode
energy hardly softens while its energy width increases rapidly. By comparing
with calculated spin-excitations spectra within the and
-pairing symmetries, we conclude that the ground-state resonance in
NaFeCoAs is only consistent with the -pairing, and
is inconsistent with the -pairing symmetry.Comment: 9 pages, 8 figures. submitted to PR
Crosstalk between MLO-Y4 osteocytes and C2C12 muscle cells is mediated by the Wnt/β-catenin pathway
We examined the effects of osteocyte secreted factors on myogenesis and muscle function. MLO-Y4 osteocyte-like cell conditioned media (CM) (10%) increased ex vivo soleus muscle contractile force by ∼25%. MLO-Y4 and primary osteocyte CM (1-10%) stimulated myogenic differentiation of C2C12 myoblasts, but 10% osteoblast CMs did not enhance C2C12 cell differentiation. Since WNT3a and WNT1 are secreted by osteocytes, and the expression level of Wnt3a is increased in MLO-Y4 cells by fluid flow shear stress, both were compared, showing WNT3a more potent than WNT1 in inducing myogenesis. Treatment of C2C12 myoblasts with WNT3a at concentrations as low as 0.5ng/mL mirrored the effects of both primary osteocyte and MLO-Y4 CM by inducing nuclear translocation of β-catenin with myogenic differentiation, suggesting that Wnts might be potential factors secreted by osteocytes that signal to muscle cells. Knocking down Wnt3a in MLO-Y4 osteocytes inhibited the effect of CM on C2C12 myogenic differentiation. Sclerostin (100ng/mL) inhibited both the effects of MLO-Y4 CM and WNT3a on C2C12 cell differentiation. RT-PCR array results supported the activation of the Wnt/β-catenin pathway by MLO-Y4 CM and WNT3a. These results were confirmed by qPCR showing up-regulation of myogenic markers and two Wnt/β-catenin downstream genes, Numb and Flh1. We postulated that MLO-Y4 CM/WNT3a could modulate intracellular calcium homeostasis as the trigger mechanism for the enhanced myogenesis and contractile force. MLO-Y4 CM and WNT3a increased caffeine-induced Ca2+ release from the sarcoplasmic reticulum (SR) of C2C12 myotubes and the expression of genes directly associated with intracellular Ca2+ signaling and homeostasis. Together, these data show that in vitro and ex vivo, osteocytes can stimulate myogenesis and enhance muscle contractile function and suggest that Wnts could be mediators of bone to muscle signaling, likely via modulation of intracellular Ca2+ signaling and the Wnt/ β-Catenin pathway
I4U Submission to NIST SRE 2018: Leveraging from a Decade of Shared Experiences
The I4U consortium was established to facilitate a joint entry to NIST
speaker recognition evaluations (SRE). The latest edition of such joint
submission was in SRE 2018, in which the I4U submission was among the
best-performing systems. SRE'18 also marks the 10-year anniversary of I4U
consortium into NIST SRE series of evaluation. The primary objective of the
current paper is to summarize the results and lessons learned based on the
twelve sub-systems and their fusion submitted to SRE'18. It is also our
intention to present a shared view on the advancements, progresses, and major
paradigm shifts that we have witnessed as an SRE participant in the past decade
from SRE'08 to SRE'18. In this regard, we have seen, among others, a paradigm
shift from supervector representation to deep speaker embedding, and a switch
of research challenge from channel compensation to domain adaptation.Comment: 5 page
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