7,138 research outputs found
Friedmann limits of rotating hypersurface-homogeneous dust models
The existence of Friedmann limits is systematically investigated for all the
hypersurface-homogeneous rotating dust models, presented in previous papers by
this author. Limiting transitions that involve a change of the Bianchi type are
included. Except for stationary models that obviously do not allow it, the
Friedmann limit expected for a given Bianchi type exists in all cases. Each of
the 3 Friedmann models has parents in the rotating class; the k = +1 model has
just one parent class, the other two each have several parent classes. The type
IX class is the one investigated in 1951 by Goedel. For each model, the
consecutive limits of zero rotation, zero tilt, zero shear and spatial isotropy
are explicitly calculated.Comment: 39 pages, LaTeX, 1 postscript figure. Subjects: General relativity,
exact solutions, cosmolog
Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations
By use of ultrasound studies we resolved magneto-acoustic quantum oscillation
deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C.
Below the upper critical field, only a very weak additional damping appears
that can be well explained by the field inhomogeneity caused by the flux-line
lattice in the mixed state. This is clear evidence for no or a vanishingly
small gap for one of the bands, namely, the spheroidal alpha band. This
contrasts de Haas--van Alphen data obtained by use of torque magnetometry for
the same sample, with a rapidly vanishing oscillation signal in the mixed
state. This points to a strongly distorted flux-line lattice in the latter case
that, in general, can hamper a reliable extraction of gap parameters by use of
such techniques.Comment: 6 pages, 6 figure
Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study
We present zero field and transverse field \muSR experiments on the recently
discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero
field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples
rule out any static magnetic order above 1.6 K in these superconducting
samples. From transverse field experiments in the vortex phase we deduce the
temperature and field dependence of the superfluid density. Whereas the
temperature dependence is consistent with a weak coupling BCS s-wave or a dirty
d-wave gap function scenario, the field dependence strongly evidences
unconventional superconductivity. We obtain the in-plane penetration depth of
\lambda_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and \lambda_{ab} (0) =
364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventional
superconductivity is provided by the ratio of T_c versus the superfluid
density, which is close to the Uemura line of hole doped high-T_c cuprates.Comment: 5 pages, 5 figure
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Cost Comparison Of Timber, Steel, And Prestressed Concrete Bridges
This study was undertaken to investigate the economics of timber bridge superstructures versus traditional steel/concrete and prestressed concrete alternatives in the short-span range of 20 to 60 ft (6.1-18.3 m). Only superstructure costs were considered because substructure and abutment costs are highly site-specific. A lack of definitive data regarding service lives and maintenance costs precluded a life-cycle cost study; thus, only initial costs were compared. Representative superstructure designs were obtained for timber, steel/concrete, and prestressed concrete bridges at 20-, 40-, and 60-ft spans. Five to six northern New England general contractors performed cost estimates on these designs. Also, nine timber bridge designs, three at each span length, received cost estimates from three timber bridge suppliers. Results from general contractors indicated that timber was cost competitive with steel/concrete and was less expensive than prestressed concrete. Results from timber bridge suppliers showed more impressive distinct initial cost advantages for timber over both steel/concrete and prestressed concrete. The study indicated the initial cost effectiveness of modern, short-span timber bridges in northern New England. © ASCE
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