The discoverer
of conductivity at LAO/STO interfaces himself is giving us an update on
superconductivity in STO heterostructures, how neat is that!
STO is of course
so interesting because it is the lowest density bulk superconductor. The delta
doped systems under consideration here have a density of just a few percent of
an electron per unit cell. The technical trick to get these type of samples is
to use very high growth temperatures at low oxygen pressure. In this way as
many as possible oxygen vacancies appear, which enhances the Sr stoichiometry.
Oxygen can then be refilled later. By varying the thickness of a 1% Nb doped
STO interlayer, a 2D superconductor can be realized. For thin layers, the
magnetic field analysis of the superconductivity shows 2D behavior. If this
layer is thicker than about 100 nm a transition to 3D arises, consistent with a
coherence length of that order.
Decreasing the
thickness of the dopant plane greatly enhances the electron mobility. That is
very special! The more 2D the sample is, the cleaner the system, clean enough
to see beautiful quantum oscillations. Intriguingly, at low densities,
superconductivity disappears while the mobility stays large (2D metal).
Following a
Japanese prediction, Harold has been trying to pursue topological
superconductivity in a bilayer Rashba system by using two dopant layers.
Experimentally, it looks like a twoband superconductor, where the two bands
arise form quantum confinement. The coupling between bands can be played with.
To probe the
superconductivity in the low density doped STO layer with a tunnel probe, you
run into the issue of having a large Schottky barrier. Harold resolved this
challenge by including a LAO unit cell that acts as a compensating dipole.
Conductance increases by putting in an insulator! Isn’t this field great… A
superconducting tunnel spectrum with clear coherence peaks was obtained and
could be fitted very well with just thermal broadening.
Written by:
Alexander Brinkman
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