Direct observation of surface bandgap shrinkage and negative electronic compressibility in SrTiO3
In this work, we investigate and compare the electronic structures of SrTiO3 and KTaO3 under ultraviolet (UV) light induced electron doping. Using angle-resolved photoemission spectroscopy (ARPES), the evolution of the surface electronic structures of SrTiO3 and KTaO3 is systematically examined as a function of electron density. In contrast to KTaO3, SrTiO3 exhibits a pronounced shrinking of its surface bandgap by approximately 390 meV, accompanied by a counterintuitive shift of the valence band peak toward lower binding energies of up to 200 meV with increasing electron density. This anomalous behavior constitutes a spectroscopic signature of negative electronic compressibility (NEC). Density-functional-theory calculations provide qualitative support for the experimental observations. The calculations show that surface formation already reduces the apparent near-gap separation in SrTiO3, while additional electron accumulation further drives the slab toward a more metallic state; oxygen-vacancy models likewise produce strong bandgap reduction, identifying plausible mechanisms contributing to the observed surface bandgap shrinkage. These findings establish a direct spectroscopic link between bandgap engineering and the NEC effect at the SrTiO3 surface, highlighting the potential of SrTiO3 for next-generation oxide electronic, optoelectronic, and high-performance capacitive energy storage devices applications.