28. Competition between polar and nonpolar lattice distortions

Competition between polar and nonpolar lattice distortions in oxide quantum wells: New critical thickness at polar interfaces.
J. Gazquez, M. Stengel, R. Mishra, M. Scigaj, M. Varela, M. A. Roldan, J. Fontcuberta, F. Sánchez, and G. Herranz.  Physical Review Letters 119, 106102 (2017) Editors’ Suggestion

Two basic lattice distortions permeate the structural phase diagram of oxide perovskites: antiferrodistortive (AFD) rotations and tilts of the oxygen octahedral network and polar ferroelectric modes. With some notable exceptions, these two order parameters rarely coexist in a bulk crystal, and understanding their competition is a lively area of active research. Here we demonstrate, by using LaAlO3/SrTiO3 system as a test case, that quantum confinement can be a viable tool to shift the balance between AFD and polar modes, and selectively stabilize one of the two phases. In particular, by combining scanning transmission electron microscopy and first-principles based models, we find a crossover between a bulk-like LaAlO3 structure to a strongly polar state with no AFD tilts at a thickness of approximately three unit cells. We discuss the implications of these findings, both for the specifics of the LaAlO3/SrTiO3 system, and for the general quest towards nanoscale control of materials properties.