Session E46: Complex Oxide Interfaces & Heterostructures -- Ferroelectrics/multiferroics, polar metals and strange metals

Investigation of LaVO3/PrVO3 superlattices as geometric ferroelectrics

Structural distortions in orthorhombic perovskite structures have been suggested as a source of ferroelectricity in layer-by-layer grown heterostructures of transition metal oxides. Here we explore the idea to achieve a multiferroic state by epitaxial growth in superlattices of LaVO₃ and PrVO₃. Heterostructures were synthesised by pulsed laser deposition and investigated by X-ray diffraction in order to determine the pattern of the oxygen octahedra. Cation polar distortions were measured by scanning tunnelling electron microscopy in order to confirm the prediction that superlattices with odd repetitions of LaVO₃ and PrVO₃ layers present a macroscopic ferroelectric state.
  

Domain-wall ferroelectric tunnel junction with triple resistance states

The tunneling electroresistance (TER) effect in ferroelectric tunnel junctions (FTJs) has attracted interest due to potential applications in non-volatile memory devices. TER represents dramatic change in resistance of FTJ upon polarization reversal in ferroelectric barrier. A new type of TER's proposed, utilizing a head-to-head ferroelectric domain wall in La_1-xSr_xMnO₃(LSMO)/BaTiO₃(BTO)/LSMO FTJ with the wall parallel to the plane. Domain wall state has higher conductance than uniformly polarized states, but is metastable and thus not reversible. This work, using first-principles DFT calculations, explores stability of the wall in this FTJ, by considering different La_1-xSr_xO/TiO₂ terminations at LSMO/BTO interfaces. Due to polar nature of the interface, appearance of the wall depends on doping level x. For x<0.4 domain wall state becomes global minimum, and DFT calculations demonstrate a triple-well energy profile with respect to net polarization. Using an electric field, the FTJ can be switched reversibly between domain wall and two uniformly polarized states. Using a phenomenological model based on Landau-Devonshire free energy and parameters obtained from DFT calculations, we analyze the hysteresis loops of this FTJ demonstrating the emergence of the triple resistance states.   

Design of a new polar metal via 6s lone pair electrons

Polar metals are extremely rare because mobile electrons
within a metal eliminate internal dipoles that may arise owing to
asymmetric charge distributions. In this work we combine
first-principles calculations and crystal structural search (CALYPSO) method
to design a new polar metal. We show that an ordered double perovskite
oxide BaBiTi₂O₆ undergoes a continuous transition from a
high-temperature centrosymmetric P4/mmm structure to a low-temperature
non-centrosymmetric Pmm2 structure. The material is conducting
throughout the structural phase transition. The underlying mechanism
is that the 6s lone-pair electrons in Bi³⁺ leads to polar
instability and simultaneously donotes electrons to empty Ti-d states,
which results in conduction. Our work demonstrates that the lone-pair
electrons are effective to induce polar instability in metals as they
induce ferroelectric instabilities in insulators. This provides a robust
design principle for new polar metals.   

Dynamical Multiferroicity

An appealing mechanism for inducing multiferroicity in complex oxides is the generation of electric polarization by a spatially varying magnetization that is coupled to the lattice through the spin-orbit interaction. Here, I will describe the reciprocal effect, in which a time-dependent electric polarization induces a magnetization, even in a material composed only of non-magnetic ions [1]. Two predicted consequences of dynamical multiferroicity will be illustrated using the example of strontium titanate as a model material: A Phonon Zeeman Effect, in which previously degenerate optical phonons split in the presence of a magnetic field, and a Quantum Critical Multiferroicity, manifesting as an anomalous magnetic susceptibility in the vicinity of the ferroelectric quantum critical point [2]. Finally, I will discuss possibilities for engineering and optimizing the behaviors using heteroepitaxial strain.

[1] Dynamical multiferroicity, D. M. Juraschek, M. Fechner, A. V. Balatsky and N. A. Spaldin, Phys. Rev. Materials 1, 014401 (2017)
[2] Dynamic multiferroicity of a ferroelectric quantum critical point, K. Dunnett, J.-X. Zhu, N. A. Spaldin, V. Juricic, and A. V. Balatsky, arXiv:1808.05509   

Interface induced polar metals by multi-mode coupling

Multi-mode coupling between various phonon modes in ABO₃ perovskite oxides can be tuned by forming heterostructures, which can lead to precise control of their functionalities [1]. Considering the interest in ferroelectric properties in a metal i.e., in polar metals [2], an efficient way to induce ferroelectric-like (FEL) displacements in the centrosymmetric phase is by interface-induced coupling [3]. Here, using first-principles density functional theory (DFT) and DFT+U (static d-d Coulomb interaction) we have investigated (BaTiO₃)_m/(Metal)_p/(BaTiO₃)_m' heterostructures [(m/p/m')= 6/3/7, 7/2/7, 7/1/8, m+p+m'= 16] to explore a new family of polar metals. We have chosen various metal blocks in a way that can represent a BTO/3dⁿ/BTO system, where n= 1-6. We found that a FEL phase coupled with in-plane rotation of BO₆ octahedra (a⁰a⁰c⁺) can be induced in the centrosymmetric 'metal' block when BaTiO₃ is in the ferroelectric phase. The properties are found to depend strongly on the relative polarization direction of BTO units. [1]. S. Ghosh et. al. Phys. Rev. B. 92, 184112, (2015), [2] S. Ghosh et. al. Phys. Rev. Lett. 119, 177603 (2017), [3] H. Guo, PNAS, 114, E5062-E5069 (2017).   

Electrically-Tuned Strain Fields in a Ferroelectric/Ferromagnetic (PZT/LSMO) Heterostructure

Manipulating the magnetic properties of complex materials can be achieved by varying the temperature or composition (doping), but alternative methods like strain-induced modification of magnetic domains has been less explored. Nanostructures can accommodate larger values of strain, and can be applied using various epitaxial growth methods and dynamically altered by an external stimulus. Therefore, lattice strain can be used as a tuning parameter to modify magnetic properties on the nanoscale. To explore this possibility, we investigated the response from multiferroric materials that uses an intermediary strain field as a coupling mechanism. Specifically, we performed x-ray nanodiffraction to spatially resolve the coupled strain field between PZT and LSMO, a ferroelectric/ferromagnetic heterostructure, as a function of an applied electric field in the ferroelectric PZT layer. Our results reveal this interfacial strain transfer with 30nm spatial resolution and uncover the details of confined size effects associated with nanoscale structural domains.   

Tunnel Electroresistance in Superconducting/Ferroelectric Junctions

The term tunnel electroresistance (TER) was coined to describe a non-volatile resistive switching observed in ferroelectric tunnel junctions, in which the application of an electric-field pulse drastically changes the electrons’ tunneling rate. This effect can be explained by various mechanisms related to the nonvolatile reversal of the ferroelectric polarization. Using superconducting junctions we demonstrate that the same resistance switching effects can be obtained via an unrelated mechanism: oxygen electro-migration and the resulting modification of the electrodes’ ground-state at the junction interfaces. This mechanism allows simplifying the architecture of the TER-like devices, making unnecessary the presence of a ferroelectric barrier. Furthermore, we demonstrate that in the case of quasiparticles (electrons with superconducting correlations the tunnel resistance switching is over one order-of-magnitude larger than for normal electrons.   

Meeting a strange metal in heterostructures and thin films of LaNiO3

The transition metal oxides are host to several ‘strange’ metals whose properties cannot be described within the paradigm of conventional Fermi liquids. Heterostructures incorporating such metals also have properties that are distinct from those found in similar structures using conventional metals. In this talk, I will present our work on epitaxial thin films of LaNiO₃, and superlattices of La_2/3Sr_1/3MnO₃/LaNiO₃ grown using ozone-assisted molecular beam epitaxy. LaNiO₃ is widely believed to be a paramagnetic metal down to the lowest temperatures, the only exception amongst the ReNiO₃ rare earth nickelates. I will describe new insights from transport measurements in LaNiO₃ about the nature of this strange metal. In addition, when LaNiO₃ is placed in proximity with La_2/3Sr_1/3MnO₃, interfacial charge transfer and exchange interactions give rise to striking phenomena, including a non-collinear helical magnetic structure in LaNiO₃. These findings will be situated in the context of our current understanding of LaNiO₃.   

Antiferromagnetic Domains in NdNiO3-based Heterostructures

Correlated oxides often exhibit the presence of intertwined phases of long-range order. The rare-earth nickelates(RNiO₃) serve as a model transition metal oxide platform to understand and engineer the electronic structure of such phases. For bulk NdNiO₃(NNO), the material undergoes a phase transition from a paramagnetic metallic phase to a charge-ordered antiferromagnetic(AF) insulating phase. Unique electronic and magnetic phase transitions exist in atomically layered NNO–based heterostructures resulting from the influences of dimensional confinement and interfacial coupling. We focus on the AF ground state in (NNO)_m-(NdAlO₃)₄ heterostructures: enhancement of AF domain dynamics and spin fluctuations in reduced dimensions appear to hinder spin ordering. We employ x-ray photon correlation spectroscopy(XPCS) measurements via coherent x-ray scattering of the heterostructures to elucidate dimensionality-driven spin dynamics. Speckle patterns arising from AF domains are investigated for different thickness of confined nickelate layers and their dynamics are studied across the charge and spin ordering transitions. This study demonstrates an approach to characterize dimensional effects on long-range order and the ability to control the AF domain configurations in oxide heterostructures.
  

Comparing Bulk and Layer Confined RNiO3 to Disentangle Lattice and Electronic Effects in the Metal-Insulator Transition

The metal-insulator transition (MIT) of transition metal oxides is often associated with a simultaneous lattice and electronic symmetry breaking. We disentangle the effects of the electronic and lattice structure in the MIT of rare earth nickelates (RNiO₃) by comparing bulk and layer-confined NdNiO₃, using a combination of electronic structure and many-body methods[1,2,3]. We find that electronic confinement leads to an increase in the relative role of local interactions that favor an insulating symmetry-broken state. However, heterostructuring with another material leads to an increase in the lattice stiffness with respect to structural disproportionation caused by the interfacial octahedral coupling. Our work explains why nickelate heterostructures can have both a higher MIT temperature despite a weaker structural signature of the insulating state[4]. These results are of general relevance to the physics of transition-metal oxides.
[1]AB Georgescu et al arXiv:1810.00480
[2]OE. Peil et al arXiv:1809:03720
[3]Q Han and AJ Millis PRL 121, 067601(2018)
[4]A Disa et al PRM 1,024410(2017)