Stefano starts
his talk by a general introduction on the fascinating properties of the
LaAlO3/SrTiO3 (LAO/STO) interface, namely superconductivity, strong spin-orbit
coupling and magnetism. In this interface between a non-polar material (STO)
and a polar material (LAO), an electronic reconstruction takes place to avoid
the divergence of the electrostatic potential as the LAO thickness increases.
As a result, electrons are transferred from the surface of the LAO to the STO
where Ti ions can have a mixed-valence ionic state. A 2DEG is formed at the
interface and extends in the STO on a typical thickness of 10 nm. We will get
back to this point latter. One key question that Stefano addressed is whether
or not this electronic reconstruction is specific to the LAO/STO interface. The
answer is...no and several other oxides interfaces have been found to be
conducting and even superconducting with similar transition temperatures than
the one of LAO/STO. But of course, one always needs the same ingredient : a
polar discontinuity. Let's look at a few examples. If you replace the LaAlO3
layer by a LaGaO3 layer, another wide gap and polar material, you also obtain a
2DEG which is superconducting below 300mK. Not very surprising since Ga has the
same valence state than Al (just one row below in the periodic table). A 2DEG
is also observed with LaTiO3 and LaVO3, two Mott insulators where the
transition metal ion (Ti or V) can take many different valence states. The
LaTiO3/SrTiO3 interface is also superconducting but so far there is no report
of superconductivity in the LaVO3/SrTiO3 one. Good, but these are all STO based
heterostructures. Can we use
another non-polar material to replace STO ? The answer is yes. H. Hwang and
collaborators have shown that under proper atomic boundary conditions, the
TiO2/LaAlO3 interface is conducting with a high-mobility.
Then Stefano
reported a measurement of both the perpendicular and the parallel critical
magnetic field of superconducting LAO/STO interfaces over the entire phase
diagram (i.e. as a function of electrostatic back gating). By checking
carefully the temperature dependence of these two critical fields, it is in
principle possible to discriminate between single-gap and two-gap
superconductivity. That's an important point for the LAO/STO interface since,
back in the early days, two gaps have been measured in bulk doped STO by
tunneling spectroscopy. As far as we can say from the experimental data of
Stefano, it seems that there is only one superconducting gap in the LAO/STO
interface.
Now let's go
back to the extension of the 2DEG in the STO substrate. From the perpendicular
critical magnetic field, Stefano extracted the GL coherence length has a
function of back gate voltage or
equivalently as a function of the sheet conductance. As expected, the coherence length takes a minimum value when
the Tc is maximum (for a conductance of approximately 1 mS) and follows an
inversed dome shape. From the parallel critical field, Stefano extracted the
thickness of the superconducting 2DEG using again a simple GL picture. For low
conductance (negative gate voltage), the extension of the 2DEG in the STO is
about 10 nm and it increases up to 30 nm for the highest conductance (positive
gate voltage). This result is consistent with the electrostatic filling of the
highest subbands in the interfacial quantum well, which delocalize deeper in
the STO substrate. This is a beautiful result, which should help us to better
understand the superconducting properties of these interfaces as a function of
electrostatic gating. However, it also raises a very fundamental question. This
analysis being entirely based on a simple GL picture, how can we explain that
the parallel critical field exceeds by far the Pauli limit? Everybody agrees
that the strong Rashba spin-orbit coupling probably plays a role into this but
this key point still needs to be clarified.
Blogged by Nicolas
Bergeal
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