82 research outputs found
Doping Dependence of the Chemical Potential in Cuprate High- Superconductors I: LaSrCuO
The names of the authors, which were inadvertedly lacking in the tex-file
submitted two days ago, have been added.Comment: 10 pages, figures on request. Revtex, version 2, Materials Science
Center Internal Report Number VSGD.94.6.
Evidence for Supercurrent Connectivity in Conglomerate Particles in NdFeAsO1-d
Here we use global and local magnetometry and Hall probe imaging to
investigate the electromagnetic connectivity of the superconducting current
path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High
resolution transmission electron microscopy and scanning electron microscopy
show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around
individual particles, and second phase neodymium oxide which may be responsible
for the large paramagnetic background at high field and at high temperatures.
From global magnetometry and electrical transport measurements it is clear
that there is a small supercurrent flowing on macroscopic sample dimensions
(mm), with a lower bound for the average (over this length scale) critical
current density of the order of 103 A/cm2. From magnetometry of powder samples
and local Hall probe imaging of a single large conglomerate particle ~120
microns it is clear that on smaller scales, there is better current
connectivity with a critical current density of the order of 5 x 104 A/cm2. We
find enhanced flux creep around the second peak anomaly in the magnetisation
curve and an irreversibility line significantly below Hc2(T) as determined by
ac calorimetry.Comment: 11 pages, 4 figure
Phase transitions in LaFeAsO: structural, magnetic, elastic, and transport properties, heat capacity and Mossbauer spectra
We present results from a detailed experimental investigation of LaFeAsO, the
parent material in the series of "FeAs" based oxypnictide superconductors. Upon
cooling this material undergoes a tetragonal-orthorhombic crystallographic
phase transition at ~160 K followed closely by an antiferromagnetic ordering
near 145 K. Analysis of these phase transitions using temperature dependent
powder X-ray and neutron diffraction measurements is presented. A magnetic
moment of ~0.35 Bohr magnetons per iron is derived from Mossbauer spectra in
the low temperature phase. Evidence of the structural transition is observed at
temperatures well above the structural transition (up to near 200 K) in the
diffraction data as well as the polycrystalline elastic moduli probed by
resonant ultrasound spectroscopy measurements. The effects of the two phase
transitions on the transport properties (resistivity, thermal conductivity,
Seebeck coefficient, Hall coefficient), heat capacity, and magnetization of
LaFeAsO are also reported, including a dramatic increase in the magnitude of
the Hall coefficient below 160 K. The results suggest that the structural
distortion leads to a localization of carriers on Fe, producing small local
magnetic moments which subsequently order antiferromagnetically upon further
cooling. Evidence of strong electron-phonon interactions in the
high-temperature tetragonal phase is also observed.Comment: Revised and expanded magnetization and Mossbauer spectroscopy
section. Clarified sample preparation description. This paper contains some
results from arXiv:0804.0796. 10 figure
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
WACCM-D Whole Atmosphere Community Climate Model with D-region ion chemistry
Energetic particle precipitation (EPP) and ion chemistry affect the neutral composition of the polar middle atmosphere. For example, production of odd nitrogen and odd hydrogen during strong events can decrease ozone by tens of percent. However, the standard ion chemistry parameterization used in atmospheric models neglects the effects on some important species, such as nitric acid. We present WACCM-D, a variant of the Whole Atmosphere Community Climate Model, which includes a set of lower ionosphere (D-region) chemistry: 307 reactions of 20 positive ions and 21 negative ions. We consider realistic ionization scenarios and compare the WACCM-D results to those from the Sodankylä Ion and Neutral Chemistry (SIC), a state-of-the-art 1-D model of the D-region chemistry. We show that WACCM-D produces well the main characteristics of the D-region ionosphere, as well as the overall proportion of important ion groups, in agreement with SIC. Comparison of ion concentrations shows that the WACCM-D bias is typically within ±10% or less below 70 km. At 70–90 km, when strong altitude gradients in ionization rates and/or ion concentrations exist, the bias can be larger for some groups but is still within tens of percent. Based on the good agreement overall and the fact that part of the differences are caused by different model setups, WACCM-D provides a state-of-the-art global representation of D-region ion chemistry and is therefore expected to improve EPP modeling considerably. These improvements are demonstrated in a companion paper by Andersson et al
Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides ReFeAsO1-delta (Re = rare earth metal) without F-Doping
Here we report a new class of superconductors prepared by high pressure
synthesis in the quaternary family ReFeAsO1-delta (Re = Sm, Nd, Pr, Ce, La)
without fluorine doping. The onset superconducting critical temperature (Tc) in
these compounds increases with the reduction of Re atom size, and the highest
Tc obtained so far is 55 K in SmFeAsO1-delta. For the NdFeAsO1-delta system
with different oxygen concentration a dome-shaped phase diagram was found
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