Modern dielectric materials with exceptional electromechanical response are based on perovskite solid solutions in the composition range of the morphotropic phase boundary (MPB). Despite their importance for applications, the explanation of their performance is elusive, as it is hard to access the real short-range atomic order experimentally and accurate calculations of finite-temperature properties with current models are challenging. We propose a comprehensive approach combining multiscale modeling and experiment. We engage first-principle techniques to design interatomic potentials fine-tuned for the atomic order in MPBs, which will be determined experimentally using the recently developed anomalous diffuse scattering method. Molecular-dynamics simulations of large enough structures will provide direct insight into the local structure-property relationship. Potentials for various cation species will enable to develop a high-throughput screening scheme and design of new, environmentally-friendly materials. The best candidates will be synthesized and characterized.

On the trail of novel spintronic and electronic devices via exotic light-matter interactions

Topological states are exotic phases of matter resistant to change. They give rise to phenomena such as crystals that insulate on the inside but conduct electricity on their surfaces (topological insulators) or chiral arrangements of some order parameters in real space. It is also possible to isolate individual topological entities and use them for specific tasks, especially for information technology purposes. The EU-funded TSAR project will investigate topological phenomena in 'unconventional' topological materials where the staggered orders (electric and magnetic) result in a macroscopic cancellation of their built-in fields. This will open new horizons in the manipulation of individual topological solitons at very fast speed.

With the end of Moore’s law in sight, new schemes must be devised to achieve energy efficient, high density and high-speed data storage and processing. One emerging concept in today’s condensed-matter physics that may fuel next-generation information technology is topology. Topological phenomena in real space can give rise to interesting objects (for instance magnetic skyrmions), which are topologically protected, i.e. endowed with an energy barrier associated with a change in their topology class. These solitonic objects have been found mainly in magnetic materials like ferromagnets and there are very recent reports that ferroelectrics may also be able to host them. Interestingly, antiferroic orders like antiferromagnetism or antiferroelectricity would provide extra properties, e.g. a faster motion or an increased robustness. In TSAR, we will design antiferroic systems based on oxide materials where spin and electric dipole textures will be nucleated. We will devise approaches to control these topological solitons using different stimuli, and in particular ultra-fast vortex light pulses carrying angular orbital momentum. Gathering a consortium with broad expertise comprising academic (experimental and theoretical groups) and industrial partners, strategies will be devised and applied starting from high quality materials to devices. The targeted breakthrough of our project is to realize the first proof-of-concept for agile, low-power, room-temperature spintronic and electronic devices based on antiferroic topological materials. Their intrinsic high speed operation and low-power consumption will help tackling present societal challenges. Success in these endeavors will establish topological antiferroic systems as a novel versatile platform for future energy-efficient nanoelectronics.

Domain walls in ferroelectrics are naturally formed 2D solitons with a defined, nm-thick polarization profile stable over macroscopic lateral dimensions. Strong coupling of the polarization gradient with strain drastically changes the material properties within the domain wall thickness. Increasing attention is paid to these mobile interfaces because the characterization tools have recently reached the desired nanoscale resolution, needed to uncover the rich spectrum of new phenomena expected there. We are convinced that some ferroelectrics can also host 1D analogues of domain walls, i.e. spontaneously formed ferroelectric line solitons, similar to the recently experimentally confirmed Bogdanov-Yablonskitype magnetic skyrmion lines. We wish to extend the explorations also to these interesting topological objects and to pave a path to the experimental discovery of the ferroelectric skyrmion phases, analogous to the vortex states in superconductors and skyrmion phases of chiral magnets.

Quantum coherence in systems with electron correlations will be studied by means of Green functions, renormalized many-body theory, and numerical simulations. We will extend an approximation earlier developed by us with a two-particle self-consistency from the reduced parquet equations qualitatively correctly describing the Kondo strong-coupling limit of the metallic dot. The general theory will be applied to a model of quantum dot attached to superconducting leads with the aim to explain and understand its behavior at the transition from the spin singlet to the spin doublet state (zero-pi transition). The dot will be studied in an applied weak magnetic field in order to understand this transition and the properties of the spin doublet state with a degenerate ground state. The magnetic solution in a consistent theory must continuously match the non-magnetic one in the limit of the vanishing field. We further extend the static approximation from the reduced parquet equations to a dynamical one to make it applicable to low-dimensional lattice systems with long-range quantum coherence.

We study quantum coherence induced by electron correlations in microscopic models of hybrid nano-scale and low-dimensional bulk superconductors. We will use both analytic methods and numerical simulations to achieve a better understanding of impurity quantum phase transitions and the zero-temperature superconductivity in two-dimensional systems.