During this
workshop, we have heard about Cooper pairs being exposed to a lot of weird
conditions (as compared to the original BCS scenario) such as strong Rashba
Spin Orbit coupling, extreme surface confinement, and above all, disorder of
all kinds that one can think of. Then came Daniela Stornaiuolo, who proposed to
make them coexist with a ferromagnetic order in new hetero-structures, by
including a thin EuTiO3 layer within the regular LAO/STO interface. But the
hard time for the Cooper pairs arrived when Mark forced them to cross a
ferromagnetic barrier.
Indeed, Mark
exposed us very beautiful experiments where Josephson and tunneling junctions
are made of a thin GdN barrier sandwiched between two NbN superconducting ones.
At low thickness, GdN is not magnetic, but for thickness beyond a few
nanometers, a ferromagnetic behavior is observed, with a weak coercive field.
In that case, the junction acts as a spin filter for 80 to 90% of the
electrons. I-V characteristics shows Josephson coupling, weaker when GdN is
magnetic as expected because of the strong depairing effect at play. More
interesting, the Ic x Rn product (critical current times the normal state
resistance) of the magnetic Josephson Junctions (JJ) deviates from the regular
Ambegaokar-Baratoff law at low temperature. Finally, the Fraunhofer pattern,
that is the modulation of Ic with an applied magnetic field, reveals the spin
filtering effect. Indeed, it appears to be first highly asymmetric in field,
and second, to shift according to the ramping direction of the magnetic field.
The magnetization hysteresis loop of the GdN layer accounts for the latter
behavior, since the total magnetic flux that goes through the JJ controls the
critical current. However, the analysis of the asymmetry shows that, when the
barrier becomes magnetic, the first order term in the current-phase
relationship within the JJ is weakened, and that the second order one
dominates. This is not completely understood yet, even if recent theoretical
papers made interesting propositions to explain this behavior.
Then, Mark
described experiments where one of the electrode of the junction is normal, in
order to measure the density of states of a superconductor through a spin
filtering barrier, in the spirit of the pioneer work of Tedrow and Meservey a
long time ago. In that case, Zeeman splitting shifts the superconducting gap,
and an offset in energy is observed in the tunneling conductance curve,
corresponding to an internal field in the barrier of 1.5 T. The sign of the
asymmetry tells that the magnetism acts more on one side of the junction. This
offset has also been seen in JJ, indicating that there is an intrinsic
asymmetry in the junctions, that could account for the disappearance of the
first order term in the current-phase relation. Growth considerations and
intermixing at the interfaces might explain this asymmetry.
But the story is
not over, and Mark made a very nice teasing by showing us the low temperature
tunneling conductance curves of the magnetic junctions, where a big zero-bias
conductance peak builds up at zero energy. As Jim Valles mentioned, this
immediately reminded me the old days of High Tc superconductors, when I was discovering such an anomaly in
YBCO/Pb junctions, which have been later on attributed to a zero-energy Andreev
state at the (110) surface of a d-wave superconductor. This is obviously a
different situation, but that may be a bound state of some sort.
Mark let the
discussion open about this peak ... and the Cooper pairs again in a strange
situation ...
Blogged by
Jerome Lesueur
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