Complex oxides are record holder materials for many phenomena, including ferroelectricity, piezoelectricity, superconductivity and multiferroicity. Complex oxides often have competing ground states with energies slightly higher than that of the true ground state. This competition is fortuitous because thermodynamic variables (for example, temperature, electric field, magnetic field, stress and chemical potentials) can access these metastable phases that are usually hidden but emerge as the energetic landscape is reshaped by adjusting the thermodynamic variables. Epitaxial superlattices are a platform for imposing thermodynamic boundary conditions to unleash the properties of hidden phases by altering the delicate balance between competing spin, charge, orbital and lattice degrees of freedom. Additionally, a feature of complex oxides with large responses (large property coefficients) is the coexistence of phases on the nanoscale. New phases can emerge at the heterointerfaces of oxide superlattices, and X-ray, electron, neutron and proximal probes as well as ab initio theoretical studies can provide insights into these emergent phenomena. Oxide superlattices reveal emergent phenomena if the balance between competing degrees of freedom is altered. In this Review, synthetic approaches to tap the properties of competing ground states are described, focusing on two examples — one example yielding a room-temperature multiferroic and a second producing polarization skyrmions.

Creating emergent phenomena in oxide superlattices

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsfs.2016.0026

Electroactive polymers for sensing

We present data on the electro‐optics and the display performance of liquid‐crystal devices based on the distorted helix ferroelectric (DHF) effect which demonstrate TV switching rates and low driving voltages. The DHF effect is based on S*c ferroelectric liquid crystals with very short pitch, which form in suitable cells a bookshelf arrangement of the smectic layers with a helical axis parallel to the plane of the display. The distortion of this helix by an applied electric field is responsible for the electro‐optical effect: If the pitch is shorter than the wavelength, the distortion leads to a change of the effective refractive index. Unwinding of the helix is one of the limitations of these devices. We show that standard cell preparation techniques and driving schemes—including active matrix addressing—minimize helix unwinding and lead to highly multiplexible displays with short response times in the 10‐μs region. Black–white contrast ratios >12:1 at driving voltages <2 V and grey scales are reported.

Fast responding and highly multiplexible distorted helix ferroelectric liquid‐crystal displays

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

The requirements for ferroelectric liquid crystals (FLC) for an application in fast-switching electro-optic devices are summarized. Typical molecular structures for FLC-mixtures are discussed. New non-chiral SmC-substances as well as some chiral dopants are shown. Important material parameters which are presented include spontaneous polarization, tilt angle, helical pitch in the smectic and cholesteric phase, rotational viscosity, response time, optical anisotropy and dielectric anisotropy. New developed broad range FLC-mixtures with SmC*-phases, SmI*-phases or SmA*-phases at room temperature are introduced.

Ferroelectric mixtures and their physico-chemical properties

Fast-switching displays with memory properties are provided, which can display digits, symbols, and images, and which contain as chemically and thermally stable ferroelectric liquid crystalline substances, according to the invention, liquid crystalline derivatives which contain a chiral alkyl halide residue corresponding to the general formula ##STR1## are used in mixtures of one another as well as in conjunction with other liquid crystalline or non-crystalline liquid substances.

Carsten Tschirske

Papers

Ferroelectric liquid crystals