1,161 research outputs found
Phase diagram and neutron spin resonance of superconducting NaFe1âxCuxAs
We use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe1âxCuxAs single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near xâ2% with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near xâ50%. Using transport measurements, we demonstrate that the resistivity in NaFe1âxCuxAs exhibits non-Fermi-liquid behavior near xâ1.8%. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along c axis in NaFe0.98Cu0.02As. The resonance is high in energy relative to the superconducting transition temperature Tc but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite that the superconducting phase in NaFe1âxCuxAs is continuously connected to an antiferromagnetically ordered insulating phase near xâ50% with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe1âxCuxAs and other iron pnictides
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
Spectral functions in the sigma-channel near the critical end point
Spectral functions in the -channel are investigated near the chiral
critical end point (CEP), that is, the point where the chiral phase transition
ceases to be first-ordered in the -plane of the QCD phase diagram. At
that point the meson becomes massless in spite of explicit breaking of
the chiral symmetry. It is expected that experimental signatures peculiar to
CEP can be observed through spectral changes in the presence of abnormally
light mesons. As a candidate, the invariant-mass spectrum for diphoton
emission is estimated with the chiral quark model incorporated. The results
show the characteristic shape with a peak in the low energy region, which may
serve as a signal for CEP. However, we find that the diphoton multiplicity is
highly suppressed by infrared behaviors of the meson. Experimentally,
in such a low energy region below the threshold of two pions, photons from
are major sources of the background for the signal.Comment: 12 pages, 8 figures, 1 figure replaced, minor modification
Recommended from our members
Characterization of submicron particles influenced by mixed biogenic and anthropogenic emissions using high-resolution aerosol mass spectrometry: Results from CARES
An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative Effects Study (CARES) that took place in northern California in June 2010. We present results obtained at Cool (denoted as the T1 site of the project) in the foothills of the Sierra Nevada Mountains, where intense biogenic emissions are periodically mixed with urban outflow transported by daytime southwesterly winds from the Sacramento metropolitan area. During this study, the average mass loading of submicrometer particles (PM1) was 3.0 ÎŒg mâ3, dominated by organics (80%) and sulfate (9.9%). The organic aerosol (OA) had a nominal formula of C1H1.38N0.004OM0.44, thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Two distinct oxygenated OA factors were identified via Positive matrix factorization (PMF) of the high-resolution mass spectra of organics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a surrogate for secondary OA (SOA) influenced by biogenic emissions whereas the less oxidized LO-OOA (O/C = 0.42) was found to represent SOA formed in photochemically processed urban emissions. LO-OOA correlated strongly with ozone and MO-OOA correlated well with two 1st generation isoprene oxidation products (methacrolein and methyl vinyl ketone), indicating that both SOAs were relatively fresh. A hydrocarbon like OA (HOA) factor was also identified, representing primary emissions mainly due to local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for 91% of the total OA mass and 72% of the PM1 mass observed at Cool. Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool) were identified using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The average PM1 mass loading was considerably higher in urban plumes than in air masses dominated by biogenic SOA. The change in OA mass relative to CO (ÎOA/ÎCO) varied in the range of 5-196 ÎŒg mâ3 ppmâ1, reflecting large variability in SOA production. The highest ÎOA/ÎCO was reached when air masses were dominated by anthropogenic emissions in the presence of a high concentration of biogenic volatile organic compounds (BVOCs). This ratio, which was 97 ÎŒg mâ3 ppmâ1 on average, was much higher than when urban plumes arrived in a low BVOC environment (~36 ÎŒg mâ3 ppmâ1) or during other periods dominated by biogenic SOA (35 ÎŒg mâ3 ppmâ1). These results demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
Hybrid stars with the color dielectric and the MIT bag models
We study the hadron-quark phase transition in the interior of neutron stars
(NS). For the hadronic sector, we use a microscopic equation of state (EOS)
involving nucleons and hyperons derived within the Brueckner-Bethe-Goldstone
many-body theory, with realistic two-body and three-body forces. For the
description of quark matter, we employ both the MIT bag model with a density
dependent bag constant, and the color dielectric model. We calculate the
structure of NS interiors with the EOS comprising both phases, and we find that
the NS maximum masses are never larger than 1.7 solar masses, no matter the
model chosen for describing the pure quark phase.Comment: 11 pages, 5 figures, submitted to Phys. Rev.
Quantum Smoluchowski equation: Escape from a metastable state
We develop a quantum Smoluchowski equation in terms of a true probability
distribution function to describe quantum Brownian motion in configuration
space in large friction limit at arbitrary temperature and derive the rate of
barrier crossing and tunneling within an unified scheme. The present treatment
is independent of path integral formalism and is based on canonical
quantization procedure.Comment: 10 pages, To appear in the Proceedings of Statphys - Kolkata I
Interplay of Electron-Phonon Interaction and Electron Correlation in High Temperature Superconductivity
We study the electron-phonon interaction in the strongly correlated
superconducting cuprates. Two types of the electron-phonon interactions are
introduced in the model; the diagonal and off-diagonal interactions which
modify the formation energy of the Zhang-Rice singlet and its transfer
integral, respectively. The characteristic phonon-momentum and
electron-momentum dependence resulted from the off-diagonal coupling
can explain a variety of experiments. The vertex correction for the
electron-phonon interaction is formulated in the SU(2) slave-boson theory by
taking into account the collective modes in the superconducting ground states.
It is shown that the vertex correction enhances the attractive potential for
the d-wave paring mediated by phonon with around
which corresponds to the half-breathing mode of the oxygen
motion.Comment: 14 pages, 13 figure
Neutron star properties in the quark-meson coupling model
The effects of internal quark structure of baryons on the composition and
structure of neutron star matter with hyperons are investigated in the
quark-meson coupling (QMC) model. The QMC model is based on mean-field
description of nonoverlapping spherical bags bound by self-consistent exchange
of scalar and vector mesons. The predictions of this model are compared with
quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear
matter saturation properties. By employing a density dependent bag constant
through direct coupling to the scalar field, the QMC model is found to exhibit
identical properties as QHD near saturation density. Furthermore, this modified
QMC model provides well-behaved and continuous solutions at high densities
relevant to the core of neutron stars. Two additional strange mesons are
introduced which couple only to the strange quark in the QMC model and to the
hyperons in the QHD model. The constitution and structure of stars with
hyperons in the QMC and QHD models reveal interesting differences. This
suggests the importance of quark structure effects in the baryons at high
densities.Comment: 28 pages, 10 figures, to appear in Physical Review
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