shot noise in mesoscopic conductors - Low Temperature Laboratory

96 Ya.M. Blanter, M. Bu( ttiker / Physics Reports 336 (2000) 1}166
Fig. 28. Experimental results of Jehl et al. [181]. Solid and dotted lines are theoretical curves corresponding to the
e!ective charges 2e and e, respectively. Copyright 1999 by the American Physical Society.
interface, than with the F" prediction for normal systems, though the agreement is far from
perfect. The experimental results are shown in Fig. 28.
A clear experimental demonstration of the shot noise doubling with clean NS interfaces remains
to be performed.
4.2. Noise of Josephson junctions
Josephson junctions are contacts which separate two superconducting bulk electrodes by an
insulating barrier. We brie#y describe here noise properties for the case when the transmission of
this barrier is quite low; other, more interesting, cases are addressed in the next subsection.
The transport properties of Josephson junctions can be summarized as follows. First, at zero
voltage a Josephson current may #ow across the junction, I"I sin , where is the di!erence of
the phases of the superconducting order parameter between the two electrodes. In addition, for
"nite voltage tunneling of quasiparticles between the electrodes is possible. For zero temperature
this quasiparticle current only exists when the voltage exceeds 2/e; for "nite temperature an
(exponentially small) quasiparticle current #ows at any voltage.
The Josephson current is a property of the ground state of the junction, and therefore it does not
#uctuate. Hence, shot noise in Josephson junctions is due to the quasiparticle current, and
basically coincides with the corresponding shot noise properties of normal tunnel barriers. For
zero temperature, there is no shot noise for voltages below 2/e. Thermal and shot noise in
Josephson junctions are analyzed in detail by Rogovin and Scalapino [26], and have been
measured by Kanter and Vernon, Jr. [183,184].
As stated by Likharev in his 1979 review [182], `the most important results of all the theories of #uctuations in the
Josephson e!ect is that the only intrinsic source of #uctuations is the normal current rather than the supercurrent of the
junctiona.