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

¹ CNR-SPIN, Complesso MonteSantangelo - Via Cinthia, I-80126 Napoli, Italy, EU
² Dipartimento S.pe.S., Università degli Studi del Molise - Via De Sanctis, I-86100 Campobasso, Italy, EU
³ CNR-SPIN and Dipartimento di Fisica, Politecnico di Milano - Piazza Leonardo da Vinci 32, I-20133 Milano, Italy, EU
⁴ European Synchrotron Radiation Facility - 6 Rue Jules Horowitz, B.P. 220, F-38043 Grenoble Cedex, France, EU
⁵ Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg - D-86135 Augsburg, Germany, EU

^a salluzzo@na.infn.it

Received: 18 October 2010
Accepted: 17 December 2010

Abstract

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 quasi-periodic 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.