Electronic localization in CaVO3 films via bandwidth control.

Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO₃, a correlated metal in its bulk form that has only a single electron in its V⁴⁺ 3d manifold, as a representative example. We find that thick films and ultrathin films (≤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10 u.c. CaVO₃ film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V³⁺ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10 u.c. CaVO₃ film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3d–O 2p hybridization. Correlated metals: A thickness-dependent metal–insulator transition A sharp thickness-dependent metal–insulator transition has been observed in CaVO₃, which is a correlated metal in its bulk form, but an insulator in its ultrathin film form. Milan Radovic and Thorsten Schmitt from the Paul Scherrer Institute in Switzerland, and colleagues, used X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study CaVO₃ films deposited on a SrTiO₃ substrate. They found that with a growing number of layers the electronic bandwidth changed continuously by up to 40%, affecting the degree of electron localization and eventually resulting in a thermal metal–insulator transition at a thickness of 10 unit cells. The ability to control this phase transition is potentially useful for applications, for example in transparent conductors and field-effect transistors, while understanding the role of reduced dimensionality is a necessary step towards the design of functional quantum materials. [ABSTRACT FROM AUTHOR]