Following Fu and Kane
suggestion (PRL 2010), he looks for the realization of such a intriguing state
in Josephson Junctions (JJ) where, through the proximity effect,
superconductivity is induced in surface states of a 3D topological insulator.
He makes the point that if one makes SNS JJ where N is a topological insulator,
Majorana fermions can form at zero energy, even if a finite barrier exists
between the S and N parts of the junctions. Is that dream became true?
Well, listening to
this nice talk, we have some hope, but the whole story is not complete yet. In
a first set of experiments, Alex showed that a sizable critical current can be
measured in Nb/Bi2Te3/Nb junctions, where a flake of Bi2Te3 is the 3D
topological insulator (Nat Mat 2012). A nice JJ behavior is observed but no
clear sign of topological superconductivity was observed. To be sure that the
bulk of the material is not conducting at all, and that only protected
topological surface states contribute to the proximity effect, Alex used an
alloy such as BiSbTeSe (with appropriate stoechiometry) to form JJ. But the
disorder is so high, that the proximity effect is very weak, and survive only
over a few hundreds of nanometers, which make the experiments really tricky for
his poor students. Bravely, he decided to grow his own Bi2Te3 thin films, only
a few unit cell thick, to make more controlled experiments. Using SrTiO3 as a
substrate, he was able to make nice JJ with tunable properties through
back-gating. Now, he just needs to make 1D devices for the the Majorana
fermions to show up … suspens …
Meanwhile, Alex
wanted to directly study the superconducting order parameter of such
topological superconductors, which are believed to have a sizable p-wave
component. He tried to make side junctions with 2D superconducting gases which
appear at the interface between oxides which are known to have a strong Rashba
spin orbit coupling (and are therefore candidates to topological
superconductivity), but unfortunately, superconductivity was destroyed when the
samples were fabricated. He made the same kind of experiments on BiSbTeSe with
the hope that tunneling spectra would show a signature of unconventional
pairing. The good news is that a notch in the dI/dV curve is seen at the Nb gap
value, which is a spectroscopic sign of coupling to the topological insulator.
A mini gap is also seen at low energy, whose energy range could correspond to
the Thouless energy in this diffusive system. To go further, and to evidence a
clear p-wave component in the order parameter, one has to compare these results
to some model, such as Usadel equations including p-wave superconductivity and
finite size effect, which does not exists yet.
Is this result a
smoking gun of unconventional superconductivity? May be, but there are several
issues raised by the audience that have to be addressed. P-wave states are very
sensitive to disorder. How can we handle this problem when making interfaces
and junctions with disordered materials such as an alloy? The 1D geometry of
the JJ is also a very stringent parameter, since the Majorana states can be
washed out by the scattering at the interfaces. Moreover, a parallel magnetic
field is required to observe these states, which may impair the JJ behavior
which is very sensitive to magnetic field.
Alex is setting up
nice building blocks to pave the way of the observation of topological
superconductivity, and to its specific spectroscopic signatures … but “long is
the road” …
Written by Jérôme Lesueur
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