Interface Science

Electronic and/or atomic reconstruction. In heterostructures there is always some mismatch between the two constituents – crystallographic (different lattice constants), electrostatic (violation of local charge
neutrality) or dynamic (difference in chemical potentials). Consequences are numerous and profound. The atomic structure can be strained and modified; electronic and/or atomic reconstruction may occur, including formation of oxygen and/or cation vacancies as well as large atomic displacements.

Metastability. Most heterostructures are not thermodynamically stable; the synthesis is at least in part kinetically controlled and the atoms are frozen in one out of many nearly-degenerate metastable states. The
actual atomic structure at the interface is thus basically impossible to predict. To determine it experimentally, new tools and techniques for study of buried interfaces are required, and being developed fast.

2D quantum confinement. Digital synthesis of complex oxides – one-unit-cell or even one-atomic-layer at a time – yields ultrathin layers with atomically sharp interfaces. Electrons can be extremely confined in one direction, while propagating with high mobility in-plane. Ultrathin metals, superconductors, ferromagnets or ferroelectrics host new phenomena, such as massive critical fluctuations, thermal or quantum.

Proximity effects. Interesting new physics occurs also when the two materials exhibit different broken symmetries and order parameters. Competing instabilities, if finely balanced, can result in extreme susceptibility and colossal responses to small perturbations. These may find applications in sensing, ultrafast non-volatile switching, etc., and is hoped to eventually beget new Oxide Electronics.

In this lecture, a number of simple examples will be given, largely drawn from my own practice with atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) of high-Tc cuprate superconductors, but intended to illustrate the more general concepts listed above.