3,524 research outputs found
The "normal" state of superconducting cuprates might really be normal after all
High magnetic field studies of cuprate superconductors revealed a non-BCS
temperature dependence of the upper critical field determined
resistively by several groups.
These determinations caused some doubts on the grounds of both the
contrasting effect of the magnetic field on the in-plane and out-of-plane
resistances reported for large Bi2212 sample and the large Nernst signal
\emph{well above} .
Here we present both and of tiny Bi2212 crystals
in magnetic fields up to 50 Tesla.
None of our measurements revealed a situation when on the field increase
reaches its maximum while remains very small if not zero.
The resistive %upper critical fields estimated from the in-plane and
out-of-plane estimated from and are
approximately the same. Our results support any theory of cuprates that
describes the state above the resistive phase transition as perfectly normal
with a zero off-diagonal order parameter. In particular, the anomalous Nernst
effect above the resistive phase transition in high- cuprates can be
described quantitatively as a normal state phenomenon in a model with itinerant
and localised fermions and/or charged bosons
A possibility to measure elastic photon--photon scattering in vacuum
Photon--photon scattering in vacuum due to the interaction with virtual
electron-positron pairs is a consequence of quantum electrodynamics. A way for
detecting this phenomenon has been devised based on interacting modes generated
in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo,
Phys. Rev. Lett. \textbf{87} 171801 (2001)]. Here we materialize these ideas,
suggest a concrete cavity geometry, make quantitative estimates and propose
experimental details. It is found that detection of photon-photon scattering
can be within the reach of present day technology.Comment: 7 pages, 3 figure
Nernst effect in poor conductors and the cuprate superconductors
We calculate the Nernst signal in disordered conductors with the chemical
potential near the mobility edge. The Nernst effect originates from
interference of itinerant and localised-carrier contributions to the
thermomagnetic transport. It reveals a strong temperature and magnetic field
dependence, which describes quantitatively the anomalous Nernst signal in
high-Tc cuprates.Comment: 4 pages, 2 figures, thermopower is discussed, Fig.1 change
How normal is the "normal" state of superconducting cuprates?
High magnetic field studies of the cuprate superconductors revealed a non-BCS
temperature dependence of the upper critical field determined
resistively by several groups. These determinations caused some doubts on the
grounds of the contrasting effect of the magnetic field on the in-plane,
, and out-of-plane, resistances reported for large sample
of Bi2212. Here we present careful measurements of both and
of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None
of our measurements revealed a situation when on field increase
reaches its maximum while remains very small if not zero. The
resistive estimated from and are
approximately the same. We also present a simple explanation of the unusual
Nernst signal in superconducting cuprates as a normal state phenomenon. Our
results support any theory of cuprates, which describes the state above the
resistive phase transition as perfectly 'normal' with a zero off-diagonal order
parameter
Detection of QED vacuum nonlinearities in Maxwell's equations by the use of waveguides
We present a novel method for detecting nonlinearities, due to quantum
electrodynamics through photon-photon scattering, in Maxwell's equation. The
photon-photon scattering gives rise to self-interaction terms, which are
similar to the nonlinearities due to the polarisation in nonlinear optics.
These self-interaction terms vanish in the limit of parallel propagating waves,
but if instead of parallel propagating waves the modes generated in wavesguides
are used, there will be a non-zero total effect. Based on this idea, we
calculate the nonlinear excitation of new modes and estimate the strength of
this effect. Furthermore, we suggest a principal experimental setup.Comment: 4 pages, REVTeX3. To appear in Phys. Rev. Let
C-axis negative magnetoresistance and upper critical field of Bi2Sr2CaCu2O8
The out-of-plane resistance and the resistive upper critical field of
BSCCO-2212 single crystals with Tc=91-93 K have been measured in magnetic
fields up to 50 T over a wide temperature range. The results are characterised
by a positive linear magnetoresistance in the superconducting state and a
negative linear magnetoresistance in the normal state. The zero field normal
state c-axis resistance, the negative linear normal state magnetoresistance,
and the divergent upper critical field Hc2(T)are explained in the framework of
the bipolaron theory of superconductivity.Comment: 4 pages (REVTeX), 4 figures, submitted to Physical Review Letters 6
April 1999, rejected in February 2000, accepted for publication in
Europhysics Letters on 31 May 200
The e-learning grid: integrating e-pedagogy with novel technologies
In this paper we present the approach taken by the European E-Learning Grid consortium in building learning Grids. We focus on combining collaborative and peer-to-peer approaches with the relevant pedagogical paradigms where we can arrive at the E-Learning Grid. We present a framework that supports the creation of multi-user collaborative sessions, allowing users to self-organise and communicate, share tasks, workloads, and content, and interact across multiple different computing platforms and are aiming for heterogeneity in terms of both network and operating system platforms centred on fundamental technologies
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