  The LaAlO3/SrTiO3 interface provides a unique platform for controlling the
electronic properties of the superconducting semiconductor SrTiO3. Prior
investigations have shown that two-dimensional superconductivity can be
produced at the LaAlO3/SrTiO3 interface and tuned electrostatically. The
recently demonstrated reversible control of the metal-insulator transition at
the same interface using conductive atomic force microscopy (c-AFM) raises the
question of whether this room-temperature technique can produce structures that
exhibit superconducting, normal metallic and insulating phases at sub-Kelvin
temperatures. Here we report low-temperature magnetotransport experiments on
conducting structures defined at an otherwise insulating LaAlO3/SrTiO3
interface. A quantum phase transition associated with the formation of Cooper
pairs is observed in these predefined structures at sub-Kelvin temperatures.
However, a finite resistance remains even at the lowest temperature. At higher
magnetic fields, interfaces with high mobility also exhibit strong Shubnikov-de
Haas oscillations as well as a larger Ginsburg-Landau coherence length. Cooper
pair localization, spin-orbit coupling, and finite-size effects may factor into
an explanation for some of the unusual properties observed.
