6,755 research outputs found
Exchange bias effect and intragranular magnetoresistance in Nd$_{0.84}Sr_{0.16}CoO_3
Electrical transport properties as a function of magnetic field and time have
been investigated in polycrystalline, Nd_{0.84}Sr_{0.16}CoO_3. A strong
exchange bias (EB) effect is observed associated with the fairly large
intragranular magnetoresistance (MR). The EB effect observed in the MR curve is
compared with the EB effect manifested in magnetic hysteresis loop. Training
effect, described as the decrease of EB effect when the sample is successively
field-cycled at a particular temperature, has been observed in the shift of the
MR curve. Training effect could be analysed by the successful models. The EB
effect, MR and a considerable time dependence in MR are attributed to the
intrinsic nanostructure giving rise to the varieties of magnetic interfaces in
the grain interior
Long-range correlation of thermal radiation
A general theory is presented for the spatial correlations in the intensity
of the radiation emitted by a random medium in thermal equilibrium. We find
that a non-zero correlation persists over distances large compared to the
transverse coherence length of the thermal radiation. This long-range
correlation vanishes in the limit of an ideal black body. We analyze two types
of systems (a disordered waveguide and an optical cavity with chaotic
scattering) where it should be observable.Comment: 4 pages, 3 figure
Photon shot noise
A recent theory is reviewed for the shot noise of coherent radiation
propagating through a random medium. The Fano factor P/I (the ratio of the
noise power and the mean transmitted current) is related to the scattering
matrix of the medium. This is the optical analogue of Buttiker's formula for
electronic shot noise. Scattering by itself has no effect on the Fano factor,
which remains equal to 1 (as for a Poisson process). Absorption and
amplification both increase the Fano factor above the Poisson value. For strong
absorption P/I has the universal limit 1+3f/2 with f the Bose-Einstein function
at the frequency of the incident radiation. This is the optical analogue of the
one-third reduction factor of electronic shot noise in diffusive conductors. In
the amplifying case the Fano factor diverges at the laser threshold, while the
signal-to-noise ratio I^2/P reaches a finite, universal limit.Comment: 11 pages, 4 figures (caption to figure 3 corrected
Excess noise for coherent radiation propagating through amplifying random media
A general theory is presented for the photodetection statistics of coherent
radiation that has been amplified by a disordered medium. The beating of the
coherent radiation with the spontaneous emission increases the noise above the
shot-noise level. The excess noise is expressed in terms of the transmission
and reflection matrices of the medium, and evaluated using the methods of
random-matrix theory. Inter-mode scattering between propagating modes
increases the noise figure by up to a factor of , as one approaches the
laser threshold. Results are contrasted with those for an absorbing medium.Comment: 8 pages, 6 figure
No-go theorems for \psi-epistemic models based on a continuity assumption
The quantum state \psi is a mathematical object used to determine the
probabilities of different outcomes when measuring a physical system. Its
fundamental nature has been the subject of discussions since the inception of
quantum theory: is it ontic, that is, does it correspond to a real property of
the physical system? Or is it epistemic, that is, does it merely represent our
knowledge about the system? Assuming a natural continuity assumption and a weak
separability assumption, we show here that epistemic interpretations of the
quantum state are in contradiction with quantum theory. Our argument is
different from the recent proof of Pusey, Barrett, and Rudolph and it already
yields a non-trivial constraint on \psi-epistemic models using a single copy of
the system in question.Comment: Version 1 contains both theory and an illustrative experiment.
Version 2 contains only the theory (the experiment with expanded discussion
will be posted separatly at a later date). The main novelty of Version 2 is a
detailed comparison in appendix 2 with L. Hardy arXiv:1205.14396. Version 2
is 6 pages of text and 1 figure; v3: minor change
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