539 research outputs found
Comment on ``Reduction of static field equation of Faddeev model to first order PDE'', arXiv:0707.2207
The authors of the article Phys. Lett. B 652 (2007) 384, (arXiv:0707.2207),
propose an interesting method to solve the Faddeev model by reducing it to a
set of first order PDEs. They first construct a vectorial quantity , depending on the original field and its first derivatives, in terms of which
the field equations reduce to a linear first order equation. Then they find
vectors and which identically obey this linear
first order equation. The last step consists in the identification of the with the original as a function of the original field.
Unfortunately, the derivation of this last step in the paper cited above
contains an error which invalidates most of its results
Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors
We report resistivity and upper critical field B_c2(T) data for disordered
(As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field
range up to 60 T. These samples exhibit a slightly enhanced superconducting
transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4
T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K
above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0)
approximately 63 T. We compare our results with B_c2(T)-data from the
literature for clean and disordered samples. Whereas clean samples show almost
no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced
flattening of B_c2(T) for applied fields H II ab observed for several
disordered closely related systems is interpreted also as a manifestation of
PLB. Consequences are discussed in terms of disorder effects within the frames
of (un)conventional superconductivity, respectively.Comment: 2 pages, 3 figures, submitted to M2S Tokyo 0
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DC superconducting quantum interference devices fabricated using bicrystal grain boundary junctions in Co-doped BaFe2As2 epitaxial films
DC superconducting quantum interference devices (dc-SQUIDs) were fabricated
in Co-doped BaFe2As2 epitaxial films on (La, Sr)(Al, Ta)O3 bicrystal substrates
with 30deg misorientation angles. The 18 x 8 micro-meter^2 SQUID loop with an
estimated inductance of 13 pH contained two 3 micro-meter wide grain boundary
junctions. The voltage-flux characteristics clearly exhibited periodic
modulations with deltaV = 1.4 micro-volt at 14 K, while the intrinsic flux
noise of dc-SQUIDs was 7.8 x 10^-5 fai0/Hz^1/2 above 20 Hz. The rather high
flux noise is mainly attributed to the small voltage modulation depth which
results from the superconductor-normal metal-superconductor junction nature of
the bicrystal grain boundary
Ti-Al composite wires with high specific strength
An alternative deformation technique was applied to a composite made of titanium and an aluminium alloy in order to achieve severe plastic deformation. This involves accumulative swaging and bundling. Furthermore, it allows uniform deformation of a composite material while producing a wire which can be further used easily. Detailed analysis concerning the control of the deformation process, mesostructural and microstructural features and tensile testing was carried out on the as produced wires. A strong grain refinement to a grain size of 250–500 nm accompanied by a decrease in h111i fibre texture component and a change from low angle to high angle grain boundary characteristics is observed in the Al alloy. A strong increase in the mechanical properties in terms of ultimate tensile strength ranging from 600 to 930 MPa being equivalent to a specific strength of up to 223 MPa/g/cm3 was achieved
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