Zoology of spin and orbital fluctuations in ultrathin oxide films

TL;DR

This study investigates how ultrathin SrVO3 films on SrTiO3 substrates exhibit complex magnetic and orbital fluctuations, revealing that surface termination significantly influences their electronic phases and transitions.

Contribution

The paper provides a detailed many-body simulation showing how surface termination affects the phase diagram and orbital ordering in monolayer SrVO3, highlighting the importance of surface effects in ultrathin oxide films.

Findings

Both SrO and VO2 terminations lead to Mott insulating behavior.

Doping and gating induce diverse magnetic and orbital instabilities.

Surface termination determines the sign of crystal-field splitting.

Abstract

Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit-cells thick. The microscopic origins of these thickness induced metal-to-insulator transitions however remain under dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal -- the strontium vanadate SrVO$_3$ -- deposited on a SrTiO$_3$ substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a VO$_2$ top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping -- chemically or through an applied gate voltage -- is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, as well as stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultra-thin films drive rich properties that are radically different from the material's bulk physics.