5,016 research outputs found
Superconductivity and Magnetism in REFeAsO1-xFx (RE=Rare Earth Elements)
Fluoride-doped iron-based oxypnictides containing rare-earth gadolinium
(GdFeAsO0.8F0.2) and co-doping with yttrium (Gd0.8Y0.2FeAsO0.8F0.2) have been
prepared via conventional solid state reaction at ambient pressure. The
non-yttrium substituted oxypnictide show superconducting transition as high as
43.9 K from temperature dependent resistance measurements with the Meissner
effect observed at a lower temperature of 40.8 K from temperature dependent
magnetization measurements. By replacing a small amount of gadolinium with
yttrium Tc was observed to be lowered by 10 K which might be caused by a change
in the electronic or magnetic structures since the crystal structure was not
altered.Comment: 4 pages, 4 figures, Journal of Physics: Conference Series
(Proceedings in the LT25 Low Temperature Physics Conference) Submitte
Comment on ``A new efficient method for calculating perturbative energies using functions which are not square integrable'': regularization and justification
The method recently proposed by Skala and Cizek for calculating perturbation
energies in a strict sense is ambiguous because it is expressed as a ratio of
two quantities which are separately divergent. Even though this ratio comes out
finite and gives the correct perturbation energies, the calculational process
must be regularized to be justified. We examine one possible method of
regularization and show that the proposed method gives traditional quantum
mechanics results.Comment: 6 pages in REVTeX, no figure
Origins of ferromagnetism in transition-metal doped Si
We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis
Molecular structure of highly-excited resonant states in Mg and the corresponding Be+O and C+C decays
Exotic Be and C decays from high-lying resonances in Mg are
analyzed in terms of a cluster model. The calculated quantities agree well with
the corresponding experimental data. It is found that the calculated decay
widths are very sensitive to the angular momentum carried by the outgoing
cluster. It is shown that this property makes cluster decay a powerful tool to
determine the spin as well as the molecular structures of the resonances.Comment: 17 pages, no figur
Upper critical fields and critical current density of BaFe2(As0.68P0.32)2 single crystal
The transport properties, upper critical fields, superconducting anisotropy,
and critical current density in an iso-valent phosphorus-doped BaFe2As2 single
crystal close to optimum doping are investigated in this report. Temperature
dependent resistivity and susceptibility show a superconducting transition
temperature, Tc, just below 31 K both with sharp transitions. The upper
critical field parallel to the ab-plane, Hc2^ab, is above 77 Tesla while that
along the c-axis direction, Hc2^c, is just above 36 Tesla, yielding a low
superconducting anisotropy ratio ~ 2. The estimated inter-plane coherence
length based on the Ginzburg-Landau (G-L) theory indicates BaFe2(As0.68P0.32)2
is still above the point for a dimensional crossover inferring the
superconducting layers are not weakly-coupled in this system. The critical
current density at 5 K obtained from magnetization loops measurement show a
modest Jc as high as 10^5 A/cm2.Comment: 13 pages, 5 figure
Noise Enhanced Stability in Fluctuating Metastable States
We derive general equations for the nonlinear relaxation time of Brownian
diffusion in randomly switching potential with a sink. For piece-wise linear
dichotomously fluctuating potential with metastable state, we obtain the exact
average lifetime as a function of the potential parameters and the noise
intensity. Our result is valid for arbitrary white noise intensity and for
arbitrary fluctuation rate of the potential. We find noise enhanced stability
phenomenon in the system investigated: the average lifetime of the metastable
state is greater than the time obtained in the absence of additive white noise.
We obtain the parameter region of the fluctuating potential where the effect
can be observed. The system investigated also exhibits a maximum of the
lifetime as a function of the fluctuation rate of the potential.Comment: 7 pages, 5 figures, to appear in Phys. Rev. E vol. 69 (6),200
Large low-temperature magnetoresistance in SrFe2As2 single crystals
We present the first report on a large low-temperature magnetoresistance (MR)
of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca
ion-implanted SrFe2As2 single crystal that occurs before the emergence of
crystallographic strain-induced bulk superconductivity arising from a sample
aging effect. In accordance to band structure calculations from literature,
which consitently show more than 2 bands are involved in the transport, we have
modeled this large MR at high fields using a 3-carrier scenario rather than
solely on quantum linear MR model generally used to explain the MR in
iron-pnictides. At and below 20 K the large MR may be due to 3-carrier
transport in an inhomogeneous state where there are superconducting and
metallic regions.Comment: 6 pages, 4 figure
Improving the economic value of photographic screening for optical coherence tomography-detectable macular oedema : a prospective, multicentre, UK study
Peer reviewedPublisher PD
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