A Laser-ARPES view on the 2D electron systems at LaAlO₃/SrTiO₃ and Al/SrTiO₃ interfaces

We have measured the electronic structure of the two-dimensional electron system (2DES) found at the Al/SrTiO₃ (Al/STO) and LaAlO₃/SrTiO₃ (LAO/STO) interfaces by means of laser angle resolved photoemission spectroscopy (ARPES) taking advantage of the large photoelectron escape depth at low photon energy. We demonstrate the possibility of tuning the electronic density in Al/STO by varying the Al layer thickness and show that the electronic structure evolution is well described by self-consistent tight binding supercell calculations but differs qualitatively from a rigid band shift model. We show that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in related STO based 2DES. Tuning the electronic density in Al/STO to match that of LAO/STO allows to discuss similarities and differences between both systems, we estimate that the intrinsic LAO/STO 2DES has a bandwidth of ≈ 50 meV and a carrier density of ≈ 6 10¹³ cm^-2.