Origin of metallic states at the heterointerface between the band insulators LaAlO3 and SrTiO3

We have studied the electronic structure at the heterointerface between the band insulators LaAlO(3) and SrTiO(3) using in situ photoemission spectroscopy. Our experimental results clearly reveal the formation of a notched structure on the SrTiO(3) side due to band bending at the metallic LaAlO(3)/TiO(2)-SrTiO(3) interface. The structure, however, is absent at the insulating LaAlO(3)/SrO-SrTiO(3) interface. The present results indicate that the metallic states originate not from the charge transfer through the interface on a short-range scale but from the accumulation of carriers on a long-range scale.