Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3quantum wells

Abstract

The electrical and structural characteristics of SmTiO 3 /SrTiO 3 /SmTiO 3 and GdTiO 3 /SrTiO 3 /GdTiO 3 heterostructures are compared. Both types of structures contain narrow SrTiO 3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO 3 quantum wells embedded in GdTiO 3 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 SmTiO 3 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 SmTiO 3 or in GdTiO 3 . We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state.3

Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO3/SrTiO3/SmTiO3 Quantum Well Structures

“…The SmTO/STO/SmTO heterostructures used in these studies were grown by hybrid molecular beam epitaxy (MBE) on LSAT substrates as described elsewhere 8,16,19 (also see the…”

“…Those studies, performed using structures with lateral dimensions of ~100 µm 2 to 1 cm 2 , 8,[15][16][17] …”

“…1,2 The electron gas at this interface exhibits strong electronic correlations, resulting in competing orders that can be tuned by various external parameters (e.g., temperature, magnetic field, pressure, chemical doping, and electrostatic gating). [3][4][5][6][7][8] Techniques for growing high-quality films and atomically sharp interfaces of these transition metal oxides (TMOs) and their characterization are central to many ongoing research efforts. 9 The possibility for rich interplay between charge, spin, and orbital ordering makes oxide heterostructure systems excellent tools in the study of correlation phenomena with wide-ranging implications, from understanding high-T c superconductivity 10 to engineering materials with desirable functional properties.…”

“…Those studies, performed using structures with lateral dimensions of ~100 µm 2 to 1 cm 2 , 8,[15][16][17] demonstrated that the transport properties of the SmTO/STO QWs can be tuned from a regime of seemingly Fermi liquid (FL) behavior to a nonFermi liquid (NFL) regime as the STO thickness is reduced below a critical value of 5 SrO layers, as inferred from the temperature dependence of the resistance. Fitting with the functional form = !…”

Oxide 2D electron gases as a route for high carrier densities on (001) Si

“…4b). For RTO-STO superlattices, the electron density scales with the number of interfaces regardless of STO thickness 10,26 with an increase in mobility being observed in other work for 2DEGs grown on oxide substrates. 5 The carrier densities measured here are lower than those reported by…”