Domain walls separating regions of ferroelectric material with polarization oriented in different directions are crucial for applications of ferroelectrics. Rational design of ferroelectric materials requires the development of a theory describing how compositional and environmental changes affect domain walls. To model domain wall systems, a discrete microscopic Landau-Ginzburg-Devonshire (dmLGD) approach with A- and B-site cation displacements serving as order parameters is developed. Application of dmLGD to the classic BaTiO₃ , KNbO_3, and PbTiO₃ ferroelectrics shows that A-B cation repulsion is the key interaction that couples the polarization in neighboring unit cells of the material. dmLGD decomposition of the total energy of the system into the contributions of the individual cations and their interactions enables the prediction of different properties for a wide range of ferroelectric perovskites based on the results obtained for BaTiO₃ , KNbO_3, and PbTiO₃ only. It is found that the information necessary to estimate the structure and energy of domain-wall "defects" can be extracted from single-domain 5-atom first-principles calculations, and that "defect-like" domain walls offer a simple model system that sheds light on the relative stabilities of the ferroelectric, antiferroelectric, and paraelectric bulk phases. The dmLGD approach provides a general theoretical framework for understanding and designing ferroelectric perovskite oxides.

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.

Ordering of ferroelectric polarization¹ and its trajectory in response to an electric field² are essential for the operation of non-volatile memories³, transducers⁴ and electro-optic devices⁵. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field⁶. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature⁷, where tunability is compromised. Therefore, there is an unavoidable trade-off between the requirements of high tunability and low loss in tunable dielectric devices, which leads to severe limitations on their figure of merit. Here we show that domain structure can in fact be exploited to obtain ultralow loss and exceptional frequency selectivity without piezoelectric resonance. We use intrinsically tunable materials with properties that are defined not only by their chemical composition, but also by the proximity and accessibility of thermodynamically predicted strain-induced, ferroelectric domain-wall variants⁸. The resulting gigahertz microwave tunability and dielectric loss are better than those of the best film devices by one to two orders of magnitude and comparable to those of bulk single crystals. The measured quality factors exceed the theoretically predicted zero-field intrinsic limit owing to domain-wall fluctuations, rather than field-induced piezoelectric oscillations, which are usually associated with resonance. Resonant frequency tuning across the entire L, S and C microwave bands (1-8 gigahertz) is achieved in an individual device-a range about 100 times larger than that of the best intrinsically tunable material. These results point to a rich phase space of possible nanometre-scale domain structures that can be used to surmount current limitations, and demonstrate a promising strategy for obtaining ultrahigh frequency agility and low-loss microwave devices.