Scanning transmission electron microscopy (STEM) and first-principle models demonstrate the competition between structural phases intertwined with transport properties in a quantum-confined perovskite system https://doi.org/10.1103/PhysRevLett.119.106102
The quest for ultrathin ferroelectric capacitors (i.e. as thin as they can be made without sacrificing their defining property -- a switchable spontaneous polarization) has been occupying both materials scientists and engineers for decades. Yet, polar ferroelectric modes are just one small example of the many lattice (and electronic) degrees of freedom that make perovskites so attractive.
In this work, we address a long-standing question in the fundamental physics of perovskite ferroics: What is the critical thickness for a spontaneous lattice distortion to occur in a thin film?
Relevant Content
The broad family of perovskites hosts an extraordinary range of functional properties embracing, among others, magnetism, ferroelectricity or superconductivity. A clue to this astonishing versatility is the capability of perovskites to admit distortions from the ideal cubic reference structure, which leads to different deformations, such as buckling, tilting, rotations or elongations of the octahedral structural units. In turn, these lattice deformations modulate the ionic bonds between the constituent atomic species, fine-tuning the physical properties. Overall, 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. We analyzed such competition in a particular system, the LaAlO3/SrTiO3 interface.
What makes this system particularly interesting is that -unlike most perovskites- polar structures and AFD-distorted structures are very close in energy and there is a genuine competition between these lattice distortions. Interestingly, by exploiting quantum confinement we shifted the balance between AFD and polar modes and selectively stabilize one of the two phases. These results, confirmed by combining scanning transmission electron microscopy (STEM) and first-principles-based models, demonstrate the existence of a crossover between states in which AFD rotations prevail, to strongly polar states with no AFD tilts. Remarkably, these structural changes are concomitant with a metal-insulator transition that leads to the emergence of a highly conductive 2DEG. This observation beautifully illustrates how sensitive perovskite are to lattice distortions and provide plentiful insights towards nanoscale control of material properties
Researchers
J. Gazquez,1 M. Stengel,1,2 R. Mishra,3 M. Scigaj,1 M. Varela,4,5 M. A. Roldan,5 J. Fontcuberta,1 F. Sánchez,1 and G. Herranz1
1Institut de Ciència de Materials de Barcelona, Campus de la UAB, 08193 Bellaterra, Spain
2ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
3Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, USA
4Materials Science and Technology Division, Oak Ridge National Laboratory, Tennessee 37831-6071, USA
5Departamento de Física de Materiales and Instituto Pluridisciplinar, Universidad Complutense de Madrid, Madrid 28040, Spain