Emergence of Improper Electronic Ferroelectricity and Flat Band in Twisted Bilayer Tl2S.

Two-dimensional (2D) ferroelectrics possessing out-of-plane (OP) polarization are highly desirable for applications and fundamental physics. Here, by first-principles calculations, we reveal that large-angle interlayer twisting can efficiently stabilize an unexpected ordering of sizable electric dipoles, producing OP polarization out of the centrosymmetric ground-state structure of Tl2S, in great contrast to the recently proposed interlayer-sliding ferroelectricity. The ferroelectricity originates from a nonlinear coupling between a polar order dominantly contributed by electrons and an unstable phonon mode associated with a commensurate k point (1/3, 1/3, 0) in the two constituent monolayers, therefore indicating an improper and electronic ferroelectric nature. More interestingly, a flat band and a van Hove singularity occur in its electronic structures just below the Fermi level in the large-angle twisted bilayer Tl2S. The unusual coexistence of improper electronic ferroelectricity, a flat band, and a van Hove singularity in one 2D material offers exceptional opportunities for exploring novel physics and applications.

Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4 O12.

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi4 O12 . This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below TN ≈12 K, Mn2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice.

Charged Ferroelectric Domain Walls for Deterministic ac Signal Control at the Nanoscale.

The direct current (dc) conductivity and emergent functionalities at ferroelectric domain walls are closely linked to the local polarization charges. Depending on the charge state, the walls can exhibit unusual dc conduction ranging from insulating to metallic-like, which is leveraged in domain-wall-based memory, multilevel data storage, and synaptic devices. In contrast to the functional dc behaviors at charged walls, their response to alternating currents (ac) remains to be resolved. Here, we reveal ac characteristics at positively and negatively charged walls in ErMnO3, distinctly different from the response of the surrounding domains. By combining voltage-dependent spectroscopic measurements on macroscopic and local scales, we demonstrate a pronounced nonlinear response at the electrode-wall junction, which correlates with the domain-wall charge state. The dependence on the ac drive voltage enables reversible switching between uni- and bipolar output signals, providing conceptually new opportunities for the application of charged walls as functional nanoelements in ac circuitry.

Critical Role of Sc Substitution in Modulating Ferroelectricity in Multiferroic LuFeO3.

Understanding how individual dopants or substitutional atoms interact with host lattices enables us to manipulate, control, and improve the functionality of materials. However, because of the intimate coupling among various degrees of freedom in multiferroics, the atomic-scale influence of individual foreign atoms has remained elusive. Here, we unravel the critical roles of individual Sc substitutional atoms in modulating ferroelectricity at the atomic scale of typical multiferroics, Lu1-xScxFeO3, by combining advanced microscopy and theoretical studies. Atomic variations in polar displacement of intriguing topological vortex domains stabilized by Sc substitution are directly correlated with Sc atom-mediated local chemical and electronic fluctuations. The local FeO5 trimerization magnitude and Lu/Sc-O hybridization strength are found to be significantly reinforced by Sc, clarifying the origin of the strong dependence of improper ferroelectricity on Sc content. This study could pave the way for correlating dopant-regulated atomic-scale local structures with global properties to engineer emergent functionalities of numerous chemically doped functional materials.

Origin of the Critical Thickness in Improper Ferroelectric Thin Films.

Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.

An Electronically Driven Improper Ferroelectric: Tungsten Bronzes as Microstructural Analogs for the Hexagonal Manganites.

Since the observation that the properties of ferroic domain walls (DWs) can differ significantly from the bulk materials in which they are formed, it has been realized that domain wall engineering offers exciting new opportunities for nanoelectronics and nanodevice architectures. Here, a novel improper ferroelectric, CsNbW2 O9 , with the hexagonal tungsten bronze structure, is reported. Powder neutron diffraction and symmetry mode analysis indicate that the improper transition (TC = 1100 K) involves unit cell tripling, reminiscent of the hexagonal rare earth manganites. However, in contrast to the manganites, the symmetry breaking in CsNbW2 O9 is electronically driven (i.e., purely displacive) via the second-order Jahn-Teller effect in contrast to the geometrically driven tilt mechanism of the manganites. Nevertheless CsNbW2 O9 displays the same kinds of domain microstructure as those found in the manganites, such as the characteristic six-domain "cloverleaf" vertices and DW sections with polar discontinuities. The discovery of a completely new material system, with domain patterns already known to generate interesting functionality in the manganites, is important for the emerging field of DW nanoelectronics.

A group-theoretical approach to enumerating magnetoelectric and multiferroic couplings in perovskites.

A group-theoretical approach is used to enumerate the possible couplings between magnetism and ferroelectric polarization in the parent Pm{\overline 3}m perovskite structure. It is shown that third-order magnetoelectric coupling terms must always involve magnetic ordering at the A and B sites which either transforms both as R-point or both as X-point time-odd irreducible representations (irreps). For fourth-order couplings it is demonstrated that this criterion may be relaxed allowing couplings involving irreps at X-, M- and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be odd with respect to inversion symmetry. It is possible to show that the time-even irreps in this triad must transform as one of: X1+, M3,5- or R5+, corresponding to A-site cation order, A-site antipolar displacements or anion rocksalt ordering, respectively. This greatly reduces the search space for type-II multiferroic perovskites. Similar arguments are used to demonstrate how weak ferromagnetism may be engineered and a variety of schemes are proposed for coupling this to ferroelectric polarization. The approach is illustrated with density functional theory calculations on magnetoelectric couplings and, by considering the literature, suggestions are given of which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.

Multiferroic Hysteresis Loop.

Multiferroics, showing both ferroelectric and magnetic order, are promising candidates for future electronic devices. Especially, the fundamental understanding of ferroelectric switching is of key relevance for further improvements, which however is rarely reported in literature. On a prime example for a spin-driven multiferroic, LiCuVO4, we present an extensive study of the ferroelectric order and the switching behavior as functions of external electric and magnetic fields. From frequency-dependent polarization switching and using the Ishibashi-Orihara theory, we deduce the existence of ferroelectric domains and domain-walls. These have to be related to counterclockwise and clockwise spin-spirals leading to the formation of multiferroic domains. A novel measurement-multiferroic hysteresis loop-is established to analyze the electrical polarization simultaneously as a function of electrical and magnetic fields. This technique allows characterizing the complex coupling between ferroelectric and magnetic order in multiferroic LiCuVO4.