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Register a new userFerroelectricity driven magnetism at domain walls in LaAlO$_3$/PbTiO$_3$ superlattices
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Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO$_3$/PbTiO$_3$ superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO$_3$ in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electrical-control of magnetism in nanoscale heterostructures.
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Recent experiments have shown that transition metal oxide heterostructures such as SrTiO$_3$-based interfaces, exhibit large, gate tunable, spintronic responses. Our theoretical study showcases key factors controlling the magnitude of the conversion, measured by the inverse Edelstein and Spin Hall effects, and their evolution with respect to an electrostatic doping. The origin of the response can be linked to spin-orbital textures. These stem from the broken inversion symmetry at the interface which produces an unusual form of the interfacial spin-orbit coupling, provided a bulk atomic spin-orbit contribution is present. The amplitudes and variations of these observables are direct consequences of the multi-orbital subband structure of these materials, featuring avoided and topological crossings. Interband contributions to the coefficients lead to enhanced responses and non-monotonic evolution with doping. We highlight these effects using analytical approaches and low energy modeling.
The paradigm of electrons interacting with a periodic lattice potential is central to solid-state physics. Semiconductor heterostructures and ultracold neutral atomic lattices capture many of the essential properties of 1D electronic systems. However, fully one-dimensional superlattices are highly challenging to fabricate in the solid state due to the inherently small length scales involved. Conductive atomic-force microscope (c-AFM) lithography has recently been demonstrated to create ballistic few-mode electron waveguides with highly quantized conductance and strongly attractive electron-electron interactions. Here we show that artificial Kronig-Penney-like superlattice potentials can be imposed on such waveguides, introducing a new superlattice spacing that can be made comparable to the mean separation between electrons. The imposed superlattice potential fractures the electronic subbands into a manifold of new subbands with magnetically-tunable fractional conductance (in units of $e^2/h$). The lowest $G=2e^2/h$ plateau, associated with ballistic transport of spin-singlet electron pairs, is stable against de-pairing up to the highest magnetic fields explored ($|B|=16$ T). A 1D model of the system suggests that an engineered spin-orbit interaction in the superlattice contributes to the enhanced pairing observed in the devices. These findings represent an important advance in the ability to design new families of quantum materials with emergent properties, and mark a milestone in the development of a solid-state 1D quantum simulation platform.
PbTiO$_3$ is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [$001$]-orientated films. There are few studies on PbTiO$_3$ films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO$_3$ films strained by SrTiO$_3$ and LaAlO$_3$ substrates. Our results show that the polarization of PbTiO$_3$ films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO$_3$ mainly depends on the tetragonality of the lattice, instead of the simple strain.
The emergence of magnetic reconstructions at the interfaces of oxide heterostructures are often explained via subtle modifications in the electronic densities, exchange couplings, or strain. Here an additional possible route for induced magnetism is studied in the context of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_n$ superlattices using a hybrid tight-binding model. In the LaNiO$_3$ region, the induced magnetizations decouple from the intensity of charge leakage from Mn to Ni, but originate from the spin-filtered quantum confinement present in these nanostructures. In general, the induced magnetization is the largest for the (111)-stacking and the weakest for the (001)-stacking superlattices, results compatible with the exchange bias effects reported by Gibert et al. Nat. Mater. 11, 195 (2012).
Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. In ultra thin film form however the polar crystals are expected to remain stable without necessitating surface reconstructions, yet the built-in potential has eluded observation. Here we present evidence of a built-in potential across polar lao ~thin films grown on sto ~substrates, a system well known for the electron gas that forms at the interface. By performing electron tunneling measurements between the electron gas and a metallic gate on lao ~we measure a built-in electric field across lao ~of 93 meV/AA. Additionally, capacitance measurements reveal the presence of an induced dipole moment near the interface in sto, illuminating a unique property of sto ~substrates. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.
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