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.

Response of the Lattice across the Filling-Controlled Mott Metal-Insulator Transition of a Rare Earth Titanate.

The lattice response of a prototype Mott insulator, SmTiO_{3}, to hole doping is investigated with atomic-scale spatial resolution. SmTiO_{3} films are doped with Sr on the Sm site with concentrations that span the insulating and metallic sides of the filling-controlled Mott metal-insulator transition (MIT). The GdFeO_{3}-type distortions are investigated using an atomic resolution scanning transmission electron microscopy technique that can resolve small lattice distortions with picometer precision. We show that these distortions are gradually and uniformly reduced as the Sr concentration is increased without any phase separation. Significant distortions persist into the metallic state. The results present a new picture of the physics of this prototype filling-controlled MIT, which is discussed.

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 T 2 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.

Pseudogaps and Emergence of Coherence in Two-Dimensional Electron Liquids in SrTiO_{3}.

Using tunneling spectroscopy, we show that pseudogaps emerge in strongly correlated, two-dimensional electron liquids in SrTiO_{3} quantum wells that are tuned near a quantum critical point. Coherence peaks emerge at low temperatures in quantum wells embedded in antiferromagnetic SmTiO_{3} that remain itinerant to the lowest thickness. Quantum wells embedded in ferrimagnetic GdTiO_{3} that become ferromagnetic at low temperatures show no indication of quasiparticle coherence. They undergo a symmetry-lowering metal-to-insulator transition at the lowest thicknesses that coincides with a vanishing single-particle density of states (DOS) around the Fermi level. Both types of quantum wells show a power-law depletion of the DOS at high energies. The results show that the different pseudogap behaviors are closely correlated with the type of magnetism in the proximity of the quantum wells and thus provide insights into the microscopic mechanisms.

Carrier density independent scattering rate in SrTiO3-based electron liquids.

We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with T(n) (T is the temperature and n ≤ 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (EF). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.