Disorder versus two transport lifetimes in a strongly correlated electron liquid

We report on angle-dependent measurements of the sheet resistances and Hall coefficients of electron liquids in SmTiO3/SrTiO3/SmTiO3 quantum well structures, which were grown by molecular beam epitaxy on (001) DyScO3. We compare their transport properties with those of similar structures grown on LSAT [(La0.3Sr0.7)(Al0.65Ta0.35)O3]. On DyScO3, planar defects normal to the quantum wells lead to a strong in-plane anisotropy in the transport properties. This allows for quantifying the role of defects in transport. In particular, we investigate differences in the longitudinal and Hall scattering rates, which is a non-Fermi liquid phenomenon known as lifetime separation. The residuals in both the longitudinal resistance and Hall angle were found to depend on the relative orientations of the transport direction to the planar defects. The Hall angle exhibited a robust T2 temperature dependence along all directions, whereas no simple power law could describe the temperature dependence of the longitudinal resistances. Remarkably, the degree of the carrier lifetime separation, as manifested in the distinctly different temperature dependences and diverging residuals near a critical quantum well thickness, was completely insensitive to disorder. The results allow for a clear distinction between disorder-induced contributions to the transport and intrinsic, non-Fermi liquid phenomena, which includes the lifetime separation.Comment: In press, Sci. Re

Controlling a Van Hove singularity and Fermi surface topology at a complex oxide heterostructure interface.

The emergence of saddle-point Van Hove singularities (VHSs) in the density of states, accompanied by a change in Fermi surface topology, Lifshitz transition, constitutes an ideal ground for the emergence of different electronic phenomena, such as superconductivity, pseudo-gap, magnetism, and density waves. However, in most materials the Fermi level, [Formula: see text], is too far from the VHS where the change of electronic topology takes place, making it difficult to reach with standard chemical doping or gating techniques. Here, we demonstrate that this scenario can be realized at the interface between a Mott insulator and a band insulator as a result of quantum confinement and correlation enhancement, and easily tuned by fine control of layer thickness and orbital occupancy. These results provide a tunable pathway for Fermi surface topology and VHS engineering of electronic phases

Unveiling the Origin of Charge Transport in SrTiO_3 Beyond the Quasiparticle Regime

In materials with strong electron-phonon (e-ph) interactions, the electrons carry a phonon cloud during their motion, forming quasiparticles known as polarons. Charge transport and its temperature dependence in the polaron regime remain poorly understood. Here, we present first-principles calculations of charge transport in a prototypical material with large polarons, SrTiO_3. Using a cumulant diagram-resummation technique that can capture the strong e-ph interactions, our calculations can accurately predict the experimental electron mobility in SrTiO_3 between 150−300 K. They further reveal that for increasing temperature the charge transport mechanism transitions from band-like conduction, in which the scattering of renormalized quasiparticles is dominant, to an incoherent transport regime governed by dynamical interactions between the electrons and their phonon cloud. Our work reveals long-sought microscopic details of charge transport in SrTiO_3, and provides a broadly applicable method for predicting charge transport in materials with strong e-ph interactions and polarons

Improving Transport in Complex Oxide Heterostructures: High Density 2DELs and High Mobility Stannates

Perovskite oxides present a wide range of electronic and magnetic properties that set them apart from traditional semiconductors like Si or III--V compounds. However, such properties are yet to be fully harvested in developing new electronic devices with advanced functionalities. The discovery of high density, two-dimensional electron liquids (2DELs) in oxide thin films has shown promise for the advancement of oxide electronics. Yet, many fundamental properties of these electron liquids remain unclear. Another major impediment in developing functional oxide devices is poor electrical conductivities in most perovskite oxide semiconductors. To that end, this work utilizes oxide molecular beam epitaxy (MBE) to develop new geometries in titanate heterostructures that improve our fundamental understanding of electrical transport in high density oxide 2DELs. This work also pursues growth of a new class of perovskite thin films, alkali-earth stannates, for increased room temperature electrical conductivities in complex oxides.In this thesis, I discuss the origin and underlying transport mechanisms in 2DELs at (001) and (111) SrTiO3/RTiO3 (R: rare earth atom) interfaces. By utilizing new titanate devices, we study the sub-band characteristics of 2DELs in SrTiO3. The high density 2DELs are also utilized to answer long-standing fundamental questions about ferroelectricity and its coexistence with metallicity in BaTiO3. Finally, we present approaches in oxide MBE used to develop high quality films of BaSnO3 with improved electrical transport characteristics, featuring room temperature mobilities exceeding 170 cm2/Vs at carrier densities > 5E19 cm3. Growth of high quality SrSnO3 and BaxSr1-xSnO3 films and their structural and optical properties will also be discussed. It will be shown that a combination of the different techniques developed in this thesis has lead to new avenues of research in advancing the goal of functional oxide electronics