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Simultaneous manipulation of multiple boundary conditions in nanoscale heterostructures offers a versatile route to stabilizing unusual structures and emergent phases. Here, we show that a stable supercrystal phase comprising a three-dimensional ordering of nanoscale domains with tailored periodicities can be engineered in PbTiO3–SrRuO3 ferroelectric–metal superlattices. A combination of laboratory and synchrotron X-ray diffraction, piezoresponse force microscopy, scanning transmission electron microscopy and phase-field simulations reveals a complex hierarchical domain structure that forms to minimize the elastic and electrostatic energy. Large local deformations of the ferroelectric lattice are accommodated by periodic lattice modulations of the metallic SrRuO3 layers with curvatures up to 107 m−1. Our results show that multidomain ferroelectric systems can be exploited as versatile templates to induce large curvatures in correlated materials, and present a route for engineering correlated materials with modulated structural and electronic properties that can be controlled using electric fields. Ferroelectric superlattices can present a rich variety of phenomena. Here, in PbTiO3/SrRuO3 superlattices, it is shown that a complex and stable hierarchical supercrystal can form, with the correlated metal of the SrRuO3 layers showing large curvatures.

Ferroelectrics subject to suitable electric boundary conditions present a steady negative capacitance response1,2. When the ferroelectric is in a heterostructure, this behaviour yields a voltage amplification in the other elements, which experience a potential difference larger than the one applied, holding promise for low-power electronics3. So far research has focused on verifying this effect and little is known about how to optimize it. Here, we describe an electrostatic theory of ferroelectric/dielectric superlattices, convenient model systems4,5, and show the relationship between the negative permittivity of the ferroelectric layers and the voltage amplification in the dielectric ones. Then, we run simulations of PbTiO3/SrTiO3 superlattices to reveal the factors most strongly affecting the amplification. In particular, we find that giant effects (up to tenfold increases) can be obtained when PbTiO3 is brought close to the so-called ‘incipient ferroelectric’ state.

The capacitor is a key element of electronic devices and is characterized by positive capacitance. However, a negative capacitance (NC) behaviour may occur in certain cases and implies a local voltage drop opposed to the overall applied bias. Therefore, a local NC response results in voltage enhancement across the rest of the circuit. Within a suitably designed heterostructure, ferroelectrics display such an NC effect, and various ferroelectric-based microelectronic and nanoelectronic devices have been developed, showing improved performance attributed to NC. However, the exact physical nature of the NC response and direct experimental evidence remain elusive or controversial thus far. In this Review, we discuss the physical mechanisms responsible for ferroelectric NC, tackling static and transient NC responses. We examine ferroelectric responses to voltage and charge, as well as ferroelectric switching, and discuss proof-of-concept experiments and possibilities for device implementation. Finally, we highlight different approaches for the optimization of the intrinsic NC response to maximize voltage amplification. Ferroelectrics-based materials can display a negative capacitance (NC) effect, providing an opportunity to implement NC in electronic circuits to improve their performance. In this Review, the authors discuss static and transient NC responses in ferroelectrics and highlight proof-of-concept experiments and possibilities for device implementation.

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activate new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out-of-plane polarized to an in-plane polarized state is observed when a PbTiO3 /SrTiO3 superlattice with a SrRuO3 bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO3 layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron-based scanning X-ray diffraction imaging, Raman scattering, piezoresponse force microscopy and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation. This article is protected by copyright. All rights reserved.

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO

Polydomain superlattices composed of ferroelectric PbTiO3 and paraelectric SrTiO3 were investigated by means of temperature-dependent X-ray diffraction and electrical measurements. The ferroelectric nanodomains were found to have an almost isotropic distribution of domain wall alignments and to be highly responsive to applied electric fields, giving rise to large enhancements in the overall dielectric response of the structure. Electrical measurements revealed a dramatic dependence of the dielectric permittivity, tunability, and polarization-voltage characteristics on the superlattice period. For short-period superlattices, double hysteresis loops were observed.

A summary is presented of recent developments in ferroelectric nanotubes, nanowires and nanodots, their electrical characterization and related theories of their structures, including the possibility of toroidal ordering. Also summarized is recent work on ultra-thin single crystals and a status report on thin films, particularly in [3D] configurations for DRAM or FRAM trenched capacitors.

A review is given of ceramic and single-crystal thin film ferroelectric oxides, emphasizing perovskite phases, together with some new developments on hafnia films. It is shown that single-crystal barium titanate films behave as bulk down to at least 77 nm, with no finite size effects, no phase transition temperature shifts, and no dielectric peak broadening or change from first- to second-order transitions, suggesting that the gradient defect model of Bratkovsky and Levanyuk correctly describes such effects as extrinsic in experimental studies of equally thin ceramic thin films. In ceramic barium–strontium titanate (BST) thin films, it is shown that there is also no intrinsic broadening or shifts in phase transitions, with sharp, unshifted, bulk-like transitions observed only as re-entrant upon warming from cryogenic temperatures; this shows that phase transitions in ceramic thin films are dominated by kinetics and not thermodynamics and are definitely not equilibrium measurements. At high fields (>1 GV/m), the films exhibit space charge-limited conduction; no variable-range hopping is observed, contrary to recent studies on SrTiO3. Some novel, unconventional switching processes are discussed, comparing the “perimeter effect” (non-equilibrium, ballistic) with Molotskii's equilibrium model. Theory and experiment are described for [3D] nanotubes, nanorods, and nano-ribbons (or micro-ribbons). The layered-structure-perovskite–pyrochlore conversion in bismuth titanate is described together with the PbO+TiO2 phase separation in lead zirconate titanate during electrical breakdown, as are novel HfO2 precursors that demonstrate enhanced temperature crystallization from the amorphous state and hence commercial advantages for front-end processing.

International audience; Nickelates are known for their metal to insulator transition (MIT) and an unusual magnetic ordering, occurring at T=T Néel. Here, we investigate thin films of SmNiO 3 subjected to different levels of epitaxial strain. We find that the original bulk behavior (T Néel

The effects of space charges on hysteresis loops and field distributions in ferroelectrics have been investigated numerically using the phenomenological Landau-Ginzburg-Devonshire theory. Cases with the ferroelectric fully and partially depleted have been considered. In general, increasing the number of charged impurities results in a lowering of the polarization and coercive field values. Squarer loops were observed in the partially depleted cases and a method was proposed to identify fully depleted samples experimentally from dielectric and polarization measurements alone. Unusual field distributions found for higher dopant concentrations have some interesting implications for leakage mechanisms and limit the range of validity of usual semiconductor equations for carrier transport., Comment: 23 pages, 11 figures