Engineering the sub-band electronic structure in transition metal oxide quantum wells

Quantum confinement is an essential tool both for modern technologies as devices become scaled down to only a few atoms thick, and for exploring the fundamental physics of 2D electron systems. In the simplest picture, confinement in the out-of-plane direction results in quantized, two-dimensional sub-bands. In nearly all quantum well systems that have been investigated to date (e.g. semiconductors, noble metals), the in-plane effective mass is nearly independent of the sub-band index. Yet it would be desirable to deliberately and deterministically engineer the effective mass of the sub-bands for technological applications such as quantum cascade lasers, tunnel diodes, and photocatalysis. Here, we demonstrate the ability to deterministically enhance sub-band effective masses by a factor of 5 in atomically thin films of the transition metal oxide IrO₂ grown by oxide molecular beam epitaxy (MBE) and studied by angle-resolved photoemission spectroscopy (ARPES). We show that the sub-band effective masses can be deliberately engineered through consideration of the significant long-range, out-of-plane hopping matrix elements, and this approach can be broadly applied to a wide class of other functional electronic materials.