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Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. A unique platform for its manipulation is provided by transition metal oxide heterostructures, where engineering of emergent electrodynamics becomes possible at atomically sharp interfaces. We demonstrate that the Berry curvature and its corresponding vector potential can be manipulated by interface engineering of the correlated itinerant ferromagnet SrRuO$_3$ (SRO). Measurements of the AHE reveal the presence of two interface-tunable spin-polarized conduction channels. Using theoretical calculations, we show that the tunability of the AHE at SRO interfaces arises from the competition between two topologically non-trivial bands. Our results demonstrate how reconstructions at oxide interfaces can be used to control emergent electrodynamics on a nanometer-scale, opening new routes towards spintronics and topological electronics.
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Complex oxide interfaces have been one of the central focuses in condensed matter physics and material science. Over the past decade, aberration corrected scanning transmission electron microscopy and spectroscopy has proven to be invaluable to visualize and understand the emerging quantum phenomena at an interface. In this paper, we briefly review some recent progress in the utilization of electron microscopy to probe interfaces. Specifically, we discuss several important challenges for electron microscopy to advance our understanding on interface phenomena, from the perspective of variable temperature, magnetism, electron energy loss spectroscopy analysis, electronic symmetry, and defects probing.
Epitaxial growth of atomically-sharp interfaces serves as one of the main building blocks of nanofabrication. Such interfaces are crucial for the operation of various devices including transistors, photo-voltaic cells, and memory components. In order to avoid charge traps that may hamper the operation of such devices, it is critical for the layers to be atomically-sharp. Fabrication of atomically sharp interfaces normally requires ultra-high vacuum techniques and high substrate temperatures. We present here a new self-limiting wet chemical process for deposition of epitaxial layers from alkoxide precursors. This method is fast, cheap, and yields perfect interfaces as we validate by various analysis techniques. It allows the design of heterostructures with half-unit cell resolution. We demonstrate our method by designing hole-type oxide interfaces SrTiO3/BaO/LaAlO3. We show that transport through this interface exhibits properties of mixed electron-hole contributions with hole mobility exceeding that of electrons. Our method and results are an important step forward towards a controllable design of a p-type oxide interface.
Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO3/SrTiO3 at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays record-high negative values of the order of 10^4 - 10^5 microV/K, oscillating at regular intervals as a function of the gate voltage. The huge thermopower magnitude can be attributed to the phonon-drag contribution, while the oscillations map the progressive depletion and the Fermi level descent across a dense array of localized states lying at the bottom of the Ti 3d conduction band. This study is the first direct evidence of a localized Anderson tail in the two-dimensional (2D) electron liquid at the LaAlO3/SrTiO3 interface.
The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap insulator LaAlO3 with atomically precise layer sequences. Using optical ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as thin as two unit cells are shown to undergo a sequence of collective metalinsulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.
We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t2g and eg orbitals resulting in the occupation of the eg orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction depends strongly on the clustering of O vacancies and can be accompanied by a magnetic splitting between the local eg orbitals with lobes directed towards the vacancy and interface dxy orbitals. The reconstruction generates a two-dimensional interface magnetic state not observed in bulk SrTiO3. Using generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion (GGA+U), we find that this magnetic state is common for titanate surfaces and interfaces.
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