Polarization-driven resistive switching in ferroelectric tunnel junctions (FTJs)-structures composed of two electrodes separated by an ultrathin ferroelectric barrier-offers new physics and materials functionalities, as well as exciting opportunities for the next generation of non-volatile memories and logic devices. Performance of FTJs is highly sensitive to the electrical boundary conditions, which can be controlled by electrode material and/or interface engineering. Here, we demonstrate the use of graphene as electrodes in FTJs that allows control of interface properties for significant enhancement of device performance. Ferroelectric polarization stability and resistive switching are strongly affected by a molecular layer at the graphene/BaTiO 3 interface. For the FTJ with the interfacial ammonia layer we find an enhanced tunnelling electroresistance (TER) effect of 6 Â 10 5 %. The obtained results demonstrate a new approach based on using graphene electrodes for interface-facilitated polarization stability and enhancement of the TER effect, which can be exploited in the FTJbased devices.

The discovery of a two-dimensional electron gas (2DEG) at the LaAlO/SrTiO interface has resulted in the observation of many properties not present in conventional semiconductor heterostructures, and so become a focal point for device applications. Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed , the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO/LaAlO/SrTiO heterostructures. Using electrical transport measurements and in-line electron holography, we provide direct evidence of a 2DHG that coexists with a 2DEG at complementary heterointerfaces in the same structure. First-principles calculations, coherent Bragg rod analysis and depth-resolved cathodoluminescence spectroscopy consistently support our finding that to eliminate ionic point defects is key to realizing a 2DHG. The coexistence of a 2DEG and a 2DHG in a single oxide heterostructure provides a platform for the exciting physics of confined electron-hole systems and for developing applications.

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The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO3 thin films. Theoretical calculations predict the key role of the BaTiO3/PrScO3$${({{{{{\boldsymbol{110}}}}}})}_{{{{{{\bf{O}}}}}}}$$ ( 110 ) O substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

We use a combination of global back-gating and local top-gating to define nanoscale devices in the two-dimensional electron gas at the LaAlO3-SrTiO3 interface, demonstrating an efficient way for much finer spatial control over the properties of the interface, as compared to back-gating alone. The devices show indications of an inhomogenous superconducting weak link. The variation of critical current with perpendicular magnetic field shows evidence of oscillations, which hints at Josephson coupling. The variation of the critical current and zero bias resistance with temperature is consistent with short, overdamped weak links. We show that the applied top-gate voltage provides a strong handle on the properties of these weak links. This technique can be an important tool to define a variety of device structures in this system, allowing us to probe the nature of superconductivity in the LaAlO3-SrTiO3 interface system in different ways.