Nature of the charge carriers in LaAlO₃-SrTiO₃ oxide heterostructures probed using hard X-ray photoelectron spectroscopy

A proper investigation of the valence band electronic structure is the essential first step towards understanding the intriguing co-existence of several exotic electronic phases like ferromagnetism and superconductivity, in LaAlO₃-SrTiO₃ heterostructures. In order to comprehend the electronic structure across the LaAlO₃-SrTiO₃ oxide interface, a detailed valence band investigation has been carried out using variable energy hard X-ray photoelectron spectroscopy and signatures of both low-energy coherent and high-energy features have been observed in the valence band spectra. Our combined experimental and theoretical study suggests that the charge carriers at the interface are weakly correlated and the high-energy feature does not arise from the lower Hubbard band. Instead, this high-energy feature is attributed to in-gap states induced by oxygen vacancies and possible polaron formation.