Probing 2D Crystallinity and Superconductivity in Oxide Heterostructures

Long-range order and phase transitions in two-dimensional (2D) systems have been important research topics for decades, with much current interest due to new materials systems for their investigation. Here we use oxide heterostructures to explore aspects of 2D crystalline order and superconductivity. For the first topic, we study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length reflects the Berezinskii-Kosterlitz-Thouless (BKT) transition, which has the unusual feature here of being driven by chemical bond breaking at the 2D layer – 3D bulk interface. For the second topic, we use dual electrostatic gatesto independently control the carrier density and disorder of the 2D superconductor at the LaAlO3/SrTiO3interface, enabling a comprehensively map of the phase diagram. We observe a sequence of transitions from an insulator, ‘metal’, through a pseudo-gap phase corresponding to precursor pairing, and finally the establishment of long-range phase coherence.