2,259 research outputs found
Positron potential and wavefunction in LaFeAsO
We report calculations of the positron potential and wavefunction in LaFeAsO.
These calculations show that the positron wavefunction does sample the entire
unit cell although it is largest in the interstices of the La layer adjacent to
As atoms. The implication is that angular correlation of annihilation radiation
(ACAR) is a viable probe of the Fermi surfaces in this material. The results
also apply to positive muons, and indicate that these will be localized in the
La layer adjacent to As
Stokes Parameters as a Minkowskian Four-vector
It is noted that the Jones-matrix formalism for polarization optics is a
six-parameter two-by-two representation of the Lorentz group. It is shown that
the four independent Stokes parameters form a Minkowskian four-vector, just
like the energy-momentum four-vector in special relativity. The optical filters
are represented by four-by-four Lorentz-transformation matrices. This
four-by-four formalism can deal with partial coherence described by the Stokes
parameters. A four-by-four matrix formulation is given for decoherence effects
on the Stokes parameters, and a possible experiment is proposed. It is shown
also that this Lorentz-group formalism leads to optical filters with a symmetry
property corresponding to that of two-dimensional Euclidean transformations.Comment: RevTeX, 22 pages, no figures, submitted to Phys. Rev.
Anomalous Behavior Of The Complex Conductivity Of Y_{1-x}Pr_xBa_2Cu_3O_7 Observed With THz Spectroscopy
We have measured the electrodynamic properties of Y_{1-x}Pr_xBa_2Cu_3O_7
single crystal thin films as a function of temperature using coherent
THz-time-domain spectroscopy. We obtain directly the complex conductivity
, the London penetration depth , the
plasma frequency , and the quasiparticle scattering rate . We
find that drops exponentially rapidly with below the critical
temperature in {\em all} the superconducting samples, implying that this
behavior is a {\em signature} of high- superconductivity. The plasma
frequency decreases with increasing Pr content, providing evidence that Pr
depletes carriers, leaving the CuO planes {\em underdoped}. Both the
conductivity in the THz region and the dc resistivity yield evidence for the
opening of a spin gap {\em above} .Comment: 9 pages, REVTEX 3.
SONOLYTICAL PRODUCTION OF BIO-DIESEL FUEL FROM NON-EDIBLE VEGETABLE OIL
Joint Research on Environmental Science and Technology for the Eart
Coherence, incoherence and scaling along the c axis of YBa_2Cu_3O_{6+x}
The optical properties of single crystals of YBa_2Cu_3O_{6+x} have been
examined along the c axis above and below the critical temperature (T_c) for a
wide range of oxygen dopings. The temperature dependence of the
optically-determined value of the dc conductivity (\sigma_{dc}) in the normal
state suggests a crossover from incoherent (hopping-type) transport at lower
oxygen dopings (x \lesssim 0.9) to more coherent anisotropic three-dimensional
behavior in the overdoped (x \approx 0.99) material at temperatures close to
T_c. The assumption that superconductivity occurs along the c axis through the
Josephson effect yields a scaling relation between the strength of the
superconducting condensate (\rho_{s,c}, a measure of the number of
superconducting carriers), the critical temperature, and the normal-state
c-axis value for \sigma_{dc} just above T_c; \rho_{s,c} \propto \sigma_{dc}
T_c. This scaling relation is observed along the c axis for all oxygen dopings,
as well as several other cuprate materials. However, the agreement with the
Josephson coupling model does not necessarily imply incoherent transport,
suggesting that these materials may indeed be tending towards coherent behavior
at the higher oxygen dopings.Comment: Six pages with four figures and one tabl
Localization Transition in Multilayered Disordered Systems
The Anderson delocalization-localization transition is studied in
multilayered systems with randomly placed interlayer bonds of density and
strength . In the absence of diagonal disorder (W=0), following an
appropriate perturbation expansion, we estimate the mean free paths in the main
directions and verify by scaling of the conductance that the states remain
extended for any finite , despite the interlayer disorder. In the presence
of additional diagonal disorder () we obtain an Anderson transition with
critical disorder and localization length exponent independently of
the direction. The critical conductance distribution varies,
however, for the parallel and the perpendicular directions. The results are
discussed in connection to disordered anisotropic materials.Comment: 10 pages, Revtex file, 8 postscript files, minor change
High-transition-temperature superconductivity in the absence of the magnetic-resonance mode
The fundamental mechanism that gives rise to high-transition-temperature
(high-Tc) superconductivity in the copper oxide materials has been debated
since the discovery of the phenomenon. Recent work has focussed on a sharp
'kink' in the kinetic energy spectra of the electrons as a possible signature
of the force that creates the superconducting state. The kink has been related
to a magnetic resonance and also to phonons. Here we report that infrared
spectra of Bi2Sr2CaCu2O(8+d), (Bi-2212) show that this sharp feature can be
separated from a broad background and, interestingly, weakens with doping
before disappearing completely at a critical doping level of 0.23 holes per
copper atom. Superconductivity is still strong in terms of the transition
temperature (Tc approx 55 K), so our results rule out both the magnetic
resonance peak and phonons as the principal cause of high-Tc superconductivity.
The broad background, on the other hand, is a universal property of the copper
oxygen plane and a good candidate for the 'glue' that binds the electrons.Comment: 4 pages, 3 figure
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