Strain control of electronic structure in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$

Introducing biaxial strain into complex oxide thin films by epitaxial growth on lattice mismatched substrates is a powerful approach to engineering electronic and magnetic properties not attainable in bulk materials. Due to the strong many-body interactions characteristic of transition metal oxides, a microscopic understanding of the mechanisms underlying strain-driven phase transitions remains unclear. Here we utilize an integrated oxide molecular-beam epitaxy and angle-resolved photoelectron spectroscopy system to directly measure the electronic structure of colossal magnetoresistive La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ on four substrates, spanning -2.3\% to +1.6\% biaxial strain and two strain driven metal-insulator transitions. Contrary to conventional expectations of a bandwidth driven metal-insulator transition in strongly correlated systems, we find widely dispersive states in both insulating phases with finite weight at the Fermi level under compressive strain and a narrow gap under tensile strain. Our results point to two distinct mechanisms behind the metal-insulator transitions, and highlight the importance of phase coexistence and charge or orbital ordering in oxide thin films.