Hard x-ray angle-resolved photoemission from a buried high-mobility electron system

Novel two-dimensional electron systems at the interfaces and surfaces of transition-metal oxides recently have attracted much attention as they display tunable, intriguing properties that can be exploited in future electronic devices. Here we show that a high-mobility quasi-two-dimensional electron system with strong spin-orbit coupling can be induced at the surface of a KTaO$_3$ (001) crystal by pulsed laser deposition of a disordered LaAlO$_3$ film. The momentum-resolved electronic structure of the buried electron system is mapped out by hard x-ray angle-resolved photoelectron spectroscopy. From a comparison to calculations, it is found that the band structure deviates from that of electron-doped bulk KTaO$_3$ due to the confinement to the interface. Fermi surface mapping shows a three-dimensional, periodic intensity pattern consistent with electron pockets of quantum well states centered around the Γ points and the expectations from a Fourier analysis-based description of photoemission on confined electron systems. From the k broadening of the Fermi surface and core-level depth profiling, we estimate the extension of the electron system to be at least 1 nm but not much larger than 2 nm, respectively