Nanoscale modulation of the density of states at the conducting interface between LaAlO3 and SrTiO3 band insulators

The appearance of high-mobility electrons at the LaAlO3/SrTiO3 interface has raised strong interest in the material science community and a lively debate on the origin of the phenomenon. A possible explanation is an electronic reconstruction, realizing a transfer of electrons to SrTiO3 at the interface, thereby avoiding the build-up of excessive Coulomb energy as described by the "polarization catastrophe" associated with the alternating polar layers of the LaAlO3 film. Theoretical models predict that electrons are transferred into titanium 3d(xy) interface states and, in the presence of strong correlations, generate a charge and orbital order. Here we provide experimental evidence that at room temperature the local density of states of the LaAlO3/SrTiO3 conducting interface is modulated at the nanoscale in a short-range quasiperiodic pattern, which is consistent with the appearance of an orbital (short-range) order. This result, together with the splitting of the 3d states, confirms that an electronic reconstruction drives the functional properties of the LaAlO3/SrTiO3 oxide interface. The short-range superstructure does not fully agree with the theoretical predictions. Thus, further experimental and theoretical investigations are required to understand the electronic properties of the 2D electron system realised at the LaAlO3/SrTiO3 interface.