Electronic properties of rare earth vanadate heterostructures

Abstract

RVO3 compounds are a class of strongly correlated Mott insulators that harbour a complex interplay of spin, orbital and lattice degrees of freedom, which gives rise to an equally intricate phase diagram. They display two distinct types of spin ordering and orbital ordering (C-OO/G-SO and G-OO/C-SO) phases as a function of the rare-earth cation size and temperature. Due to the competing nature of its two ground states, these compounds present an ideal playground for exploring the prospects of heterostructuring to manipulate their properties. In this thesis we study different aspects of heteroepitaxy on RVO3 (R=Y, La), focusing mainly on YVO3, which sits at the edge of the phase boundary of the two spin-orbital phases, and exhibits both phases as a function of temperature. We synthesized films and superlattices of RVO3 using pulsed laser deposition. In the first part, we used resonant reflectometry to investigate YVO3-LaAlO3 superlattices and found an inversion of orbital polarization of the xz and yz orbitals between the interface and central layers of the YVO3 slab, which is stable down to 30 K. We explain these results based on epitaxial strain and spatial confinement by LaAlO3, at the interface. Further, we delve deeper into the relationship between the structural and electronic properties using a combination of DFT+U calculations and extensive structural analysis. The results reveal that the substrate facet can be used to obtain the desired orientation for YVO3 heterostructures, which, together with the thickness of the YVO3 layers and the presence of spacer layers (such as LaAlO3) in a superlattice, govern the resulting orbital polarization. In the second part, we used polarized Raman spectroscopy and ellipsometry to explore the effect of strain on the orbital ordering patterns of YVO3 and LaVO3 films. We found that the c-axis compression for (110)-oriented films stabilizes the G-OO phase, whereas c-axis elongation for (001)-oriented films favours the C-OO phase. By isolating the influences of the sign of strain, degree of strain and orientation of the unit cell, we attribute these results to strain-induced bond-length changes which in turn alter the superexchange interactions and lattice effects that are known to compete in bulk RVO3. Our results reveal that strain and interface engineering are a promising route to substantially modify the properties of these compounds and thus illustrate the diverse prospects of heteroepitaxy to tailor the properties of strongly-correlated transition metal oxides.