Competition between polar and nonpolar lattice distortions in oxide quantum wells: new critical thickness at polar interfaces

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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 the 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. By combining scanning transmission electron microscopy (STEM) and first-principles-based models, we find a crossover between a bulklike LaAlO₃ structure where AFD rotations prevail, to a strongly polar state with no AFD tilts at a thickness of approximately three unit cells; therefore, in addition to the celebrated electronic reconstruction, our work unveils a second critical thickness, related not to the electronic properties but to the structural ones. 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 material properties.
© 2017 American Physical Society. This work was supported by the Spanish MA2014- 56063-C2-1-RT, Severo Ochoa SEV-2015-0496 grant, the RyC-2012–11709 contract of J. G., and the Generalitat de Catalunya (2014 SGR 734) project. Electron microscopy observations at ORNL were supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. M. S. acknowledges the support of MINECO through Grants No. MAT2016-77100-C2-2-P and No. FIS2013- 48668-C2-2-P and of Generalitat de Catalunya (Grant No. 2014 SGR301). Calculations were performed at the Supercomputing Center of Galicia (CESGA). The authors are also grateful to Nico Dix for his collaboration in the growth of the films. Research at UCM sponsored by Fundación BBVA and Spanish MINECO MAT2015- 66888-C3-3-R.