Tuning Correlations in a 2D Electron Liquid

Undoped strontium titanate, SrTiO3 (STO), is a widely studied ceramic material with a huge dielectric constant ε, ranging from 300 at room temperature up to 5000 at T = 80 K[1]. The large values of ε are a macroscopic signature that the internal charges are easily displaced by an external electric field, while the material remains an excellent insulator. These properties have made STO an interesting material for high-performance capacitors [2]. More fundamentally, this apparently paradoxical behavior is an indication that our conventional picture—of single electrons determining the dielectric properties—fails here, and strong electronic correlations play an important role. Bearing STO’s strongly insulating behavior in mind, it was a big surprise when it was discovered that, at the interface of the two insulators SrTiO3 and LaAlO3 (LAO), there exists a two-dimensional electron liquid (2DEL) with high mobility of the electrons parallel to the interface [3]. Later findings that this electron liquid can have magnetic order [4], superconduct [5], and even exhibit a coexistence of these two phases [6, 7] raised even more excitement (see also 25 July 2011 Viewpoint).