• corrente Josephson
• fuori equilibrio
• giunzione tunnel
• nonequilibrio
• refrigerazione
• superconductivity
• superconduttività
• supercorrente

Since the liquefaction of helium by Kramerlingh Onnes in 1911 and the birth of
low temperature physics, superconducting systems have brought to the attention
of the scientific community a number of new and interesting physical phenomena.
Recently, out-of-equilibrium superconducting systems have gained much attention
from both the theoretical and experimental points of view and allowed the realization
of controllable Josephson junctions through injection of an electric current.
The first proposal for the use of out-of-equilibrium phenomena in superconductors
was put forward by Parmenter in 1961. The first demonstration of enhancement
of superconductivity by quasiparticles extraction was performed by Chi and Clarke
in 1979 on a three-layer stack of aluminum separated by tunnel barriers. In 1992
Blamire et al. showed superconductivity enhancement in a Nb-Al-Nb tunnel junction.
In both these experiments gap-enhancement was demonstrated by observing
features at finite energies in the current-voltage characteristics. More recently
(1999) Manninen et al. demonstrated electron-cooling effects by extraction of quasiparticles
in an array of Al-AlO-Ti junctions. The zero bias resistance of an additional
junction was exploited for thermometry. It was also experimentally demonstrated
that the Josephson current in superconductor-normal metal-superconductor (SNS)
junctions can be controlled by injection of a current through the normal metal, even
producing phase reversal of the supercurrent (pi-junction).
It is in this framework that this work originates with the purpose of designing
and fabricating superconducting junctions controllable by tunneling extraction of
quasiparticles in order to investigate the behavior of the Josephson current in out-of-
equilibrium superconductors.
To do this I realized a device consisting of a superconducting titanium island of
1.5 micron x 600 nm connected to two aluminum superconducting reservoirs through
tunnel junctions. These reservoirs are DC biased to extract quasiparticles from the
island and in this way drive it out of equilibrium. Two additional aluminum tunnel
contacts are used to measure the Josephson current as the bias voltage of the
reservoirs is varied.
The devices were fabricated at the Scuola Normale NEST Laboratory and measured
at Helsinki University of Technology. I was able to demonstrate heating and
cooling of the titanium island inducing an out-of-equilibrium distribution by varying
the voltage bias of the control line. This led to the observation of supercurrent at
lattice temperatures up to about twice the critical temperature of titanium, demonstrating
an electron-cooling effect at bath temperatures about 350-400 mK. This
phenomenology was consistently reproduced in all devices measured.
About the processing phase, some useful results on the behavior of aluminum
oxide for the case of interfaces with titanium deserve mention. Refinements in the shadow-evaporation technique allowed to
reproducibly fabricate aluminum-titanium tunneling junctions with a good flexibility
in terms of specific resistance and geometry.Protocols were optimized to consistently produce superconducting Al-AlxOx-Ti junctions with specific resistance down
to 100 Ohm*micron^2, and sizes down to 150 micron x 150 micron or smaller.
Experimental results on supercurrent values in out-of-equilibrium all-superconducting
structures were analyzed within the present quasiequilibrium theory. Some
unexpected features in the voltage-current characteristics of superconducting junctions
will also be reported and discussed by extending the available quasiequilibrium
model. Good qualitative agreement with the major features of the full currentvoltage
characteristics and with the observed behavior of the supercurrent versus
different injection voltages will be shown.