Toward an artificial Mott insulator: Correlations in confined, high-density electron liquids in SrTiO3

We investigate correlation physics in high-density, two-dimensional electron liquids that reside in narrow SrTiO3 quantum wells. The quantum wells are remotely doped via an interfacial polar discontinuity and the three-dimensional (3D) carrier density is modulated by changing the width of the quantum well. It is shown that even at 3D densities well below one electron per site, short-range Coulomb interactions become apparent in transport, and an insulating state emerges at a critical density. We also discuss the role of disorder in the insulating state.Comment: Accepted for publication in Physical Review B (Rapid Communication

Carrier-controlled ferromagnetism in SrTiO3

Magnetotransport and superconducting properties are investigated for uniformly La-doped SrTiO3 films and GdTiO3/SrTiO3 heterostructures, respectively. GdTiO3/SrTiO3 interfaces exhibit a high-density two-dimensional electron gas on the SrTiO3-side of the interface, while for the SrTiO3 films carriers are provided by the dopant atoms. Both types of samples exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. For the uniformly doped SrTiO3 films, the Curie temperature is found to increase with doping and to coexist with superconductivity for carrier concentrations on the high-density side of the superconducting dome. The Curie temperature of the GdTiO3/SrTiO3 heterostructures scales with the thickness of the SrTiO3 quantum well. The results are used to construct a stability diagram for the ferromagnetic and superconducting phases of SrTiO3.Comment: Revised version that is closer to the published version; Fig. 2 correcte

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3 quantum wells

The electrical and structural characteristics of SmTiO3/SrTiO3/SmTiO3 and GdTiO3/SrTiO3/GdTiO3 heterostructures are compared. Both types of structures contain narrow SrTiO3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO3 quantum wells embedded in GdTiO3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO3 or in GdTiO3. We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state.Comment: Accepted for publication as a Regular Article in Physical Review

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately 1/2 electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution and the influence of different electrostatic boundary conditions are obtained.Comment: The article has been accepted by Applied Physics Letters. After it is published, it will be found at http://apl.aip.org

Seebeck coefficient of a quantum confined, high-electron-density electron gas in SrTiO 3

We report on the Seebeck coefficient of quantum confined electron gases in GdTiO3/SrTiO3heterostructures. These structures contain two-dimensional electron gases with very highsheet-carrier concentrations on the SrTiO3-side of the interface due to intrinsic interface doping.While the sheet carrier concentrations are independent of the thickness of the SrTiO3layer, theSeebeck coefficient initially increases with SrTiO3thickness before saturating at a value of300 lK/V. A model of the Seebeck coefficient, based on thermally populated, self-consistent,tight binding subbands, captures in a semi-quantitative manner the observed thickness dependence