Marco
reviewed a number of experimental results on LAO/STO and argued that they indicat
a high degree of inhomogeneity in the superconducting state. He emphasized two
key observations:
1. The
temperature range where resistance starts deviating from the metallic value --
but without vanishing completely -- is much larger than T_c itself. This
behavior is typical for granular superconducting materials.
2.
The resistance curves exhibit long low-temperature tails which are
characteristic for systems exhibiting a percolation transition.
Based
on these observation, Marco and collaborators proposed that in the “underdoped”
side of the superconducting dome, LAO/STO is in a phase separated state. Here,
nanoscale superconducting puddles are embedded in a metallic background. As a
consequence, below the percolation threshold there is no long-range phase
coherence, which is suggested as an explanation for the experimentally observed
pseudogap behavior.
Marco
then explained that the phase separated state may originate from an intrinsic
mechanism. The charge carriers at the interface (electrons) and the outer
surface (holes) arise from electronic reconstruction due to the polarity of the
system. If the system forms puddles of higher and lower charge carrier
concentration, then it is electrostatically favorable for the electrons
to mirror the hole puddles. Moreover, the hole density determines the depth of
the potential well experienced by the electrons, and correspondingly the
strength of the spin-orbit coupling. The key result was that such a phase
separated state may be energetically favorable over a homogeneous state, as
indicated by a negative compressibility.
Marco
predicted that within this scenario the superconducting fluctuations exhibit an
anomalous dynamical critical exponent. This affects a number of observables and
may offer a route to experimentally testing the theory.
Blogged
by Karen Michaeli
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