We investigate correlation physics in high-density, two-dimensional electron liquids that reside in narrow SrTiO 3 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.

Quantum phase transitions are driven by quantum fluctuations that alter the nature of the electronic quasiparticles, resulting in phenomena such as non-Fermi liquid behaviour. Oxide heterostructures offer fundamentally new ways of manipulating quantum criticality. Here, we report on non-Fermi liquid behaviour in thin SrTiO 3 quantum wells that are embedded in insulating, antiferromagnetic SmTiO 3 , as a function of temperature, quantum well thickness and SmTiO 3 layer thickness in superlattices. Such quantum wells contain very high sheet carrier densities on the order of one electron per pseudocubic planar unit cell. We show that the quantum well thickness is a tuning parameter for non-Fermi liquid behaviour. Increasing the thickness by a single atomic layer and coupling in superlattices recover the Fermi liquid behaviour. The critical exponents, the symmetry of the order parameter, the role of carrier densities and symmetry-lowering distortions are discussed, and the results are compared with those of quantum wells embedded in ferrimagnetic GdTiO 3 .

Pulsed laser atom probe tomography (APT) has enabled the investigation of semiconducting materials at sub‐nm length scales and 10 ppm chemical sensitivity. This has enabled APT to be the best technique for nanoscale detection of dopant distributions and low levels of chemical segregation at interfaces, which are both important for semiconductor processing; however, the accuracy of measured interfacial profiles is typically compromised by aberrations. Interfacial profiles in APT data will vary with respect to different interfacial combinations, especially when the evaporation field between two materials is drastically different. Here, the ability of APT to measure SiGe/Si/SiGe interfacial profiles is tested with an 8 nm Si well embedded in SiGe. The APT measurements are compared to those measured using scanning transmission electron microscopy (STEM) to evaluate reconstruction and post‐reconstruction processing methods to appropriately measure interfacial profiles using APT. Without post‐APT reconstruction processing, the measured Si/SiGe interfacial widths between APT and STEM match poorly, but after applying the z‐redistribution algorithm, the interfacial profiles are in good agreement. These results indicate that APT can be used to accurately identify SiGe/Si/SiGe interfacial profiles after application of the z‐redistribution algorithm, which will greatly impact the synergy between growth and characterization of semiconductor devices using Si/SiGe interfaces.

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

We investigate the angular dependence of the magnetoresistance of thin (< 1 nm), metallic SrTiO 3 quantum wells epitaxially embedded in insulating, ferrimagnetic GdTiO 3 and insulating, antiferromagnetic SmTiO 3 , respectively. The SrTiO 3 quantum wells contain a high density of mobile electrons (~7 ×10 14 cm −2 ). We show that the longitudinal and transverse magnetoresistance in the structures with GdTiO 3 are consistent with anisotropic magnetoresistance, and thus indicative of induced ferromagnetism in the SrTiO 3 , rather than a nonequilibrium proximity effect. Comparison with the structures with antiferromagnetic SmTiO 3 shows that the properties of thin SrTiO 3 quantum wells can be tuned to obtain magnetic states that do not exist in the bulk material.