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A10: Finite size effects in atomically thin REO₃ single crystalline films

A novel route to room temperature superconductivity is via the metallization of ultrathin layers of antiferromagnetic (AF) insulating oxides. This consideration is based on the premise that magnetic interactions are at the heart of the superconducting coupling mechanism in high temperature superconductors that are based on cuprate oxides. The parent compounds in this family are known to be insulating antiferromagnets: these materials become metallic and then superconducting when the long-range AF interactions are suppressed, for example, by chemical doping or by pressure. Many studies suggest that short-range AF fluctuations nevertheless persist and that these could give rise to a superconducting Cooper pairing mechanism.

There are several families of oxide materials that exhibit AF states at ambient pressure: one of these are the rare earth (RE) oxides that exhibit high temperature antiferromagnetism: LaFeO₃ (LFO) has one of the highest known antiferromagnetic ordering temperatures with a Néel temperature of close to ~1,000 K. We have prepared high quality thin films of LFO by oxide molecular beam epitaxy. These films are insulating when prepared on SrTiO₃ (STO) substrates that have been both chemically cleaned and annealed at 1,000 °C in oxygen but exhibit what appears to be a two-dimensional electron gas (2DEG) when formed on similar substrates that are only chemically cleaned but not annealed. The only difference between the substrate surfaces is that the edges that define the terraces on the vicinal surfaces are smooth in the first case but diffuse in the second case. Cross-section transmission electron microscopy and electron energy loss spectroscopy (EELS) shows that the interface between the STO and LFO films are abrupt in both cases with roughness or intermixing on the single atomic layer scale, making the origin of the 2DEG difficult to comprehend. We have recently found that electrolyte gating can switch either one of these interfaces reversibly between metallic and insulating states, consistent with the formation or filling of oxygen vacancies, as we have recently shown in films of VO₂.

These results are pertinent to the debate concerning the formation of a 2DEG at the STO/LaAlO₃ (LAO) interface, which has been a subject of continuing debate for almost a decade: a majority group proposes an electronic origin arising from a polarization catastrophe and a minority group proposing various mechanisms arising from defects. To address this debate we propose a project to explore and engineer the metallization of ultrathin layers of oxides from the REO₃ family. This would require a combination of advanced thin film growth techniques, advanced structural and electronic characterization, advanced theoretical calculations and modeling. The goal would be the understanding and control of the metallization of these films with a view to finding novel, unconventional superconductors.

Principal Investigators

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