Tuning Correlations in a 2D Electron Liquid

Abstract

Viewpoint: Tuning Correlations in a 2D Electron Liquid Johann Kroha, Physikalisches Institut and Bethe Center for Theoretical Physics, Universität Bonn, Nussallee, 12, D-53115 Bonn, Germany Published December 12, 2011 | Physics 4, 106 (2011) | DOI: 10.1103/Physics.4.106 Simulating part of a perovskite heterostructure with a polarizable gel reveals the instrinsic properties of a two-dimensional electron liquid. Electrolyte Gate-Controlled Kondo Effect in SrTiO3 Menyoung Lee, J. R. Williams, Sipei Zhang, C. Daniel Frisbie, and D. Goldhaber-Gordon Phys. Rev. Lett. 107, 256601 (2011) Published December 12, 2011 | PDF (free) +Enlarge image Figure 1 APS/Alan Stonebraker Figure 1 (a) Lattice structure of the STO-LAO interface, showing the unpoled SrO and TiO2 planes in STO as well as the LaO+ and AlO2- planes with alternating polarity in LAO. (b) Simulation of the LAO overlayer by an electrolytic gel with frozen-in polarization, allowing for controlled tuning of the charge-carrier density in the two-dimensional electron gas. The diagrams to the right of (a) and (b) show the potential energy of the LAO overlayer and of the polarized gel, respectively, increasing linearly with layer thickness. 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=80K [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).