Electron correlation and spin-orbit coupling in 4d and 5d transition metal oxides

In recent years, there has been an upsurge of interest in 4d and 5d transition metal oxides (TMOs) in which exotic states may emerge resulting from the subtle interplay between electronic correlation and relativistic spin-orbit interactions. It is commonly expected that 4d and 5d oxides are more metallic and less magnetic than their 3d counterparts because of the extended nature of the 4d and 5d orbitals. In contrast with these expectations, anomalous insulating behaviour was recently reported for 4d and 5d TMOs such as Ca2RuO4 and Sr2IrO4. Being at its infancy, this frontier research field poses several fundamental questions on the complicated balance between Hubbard`s U, Hund`s J, the bandwidth (W), the spin orbit interaction (SOI), and the splitting of the crystal field (CF). In order to improve our still poor understanding of these phenomena it is crucial to determine the size of the dominant effective interactions. Furthermore, for the 3d-4d-5d series of TMOs, the characteristics and the competing properties of these phenomena must be clarified. This project proposes to investigate this unexplored territory that is often challenging for the conventional understanding of TMOs but also rich in novel physical phenomena. This should be done by combining experimental, computational and theoretical methods complementarily. To this end, we gather a network of groups that are leading in their fields and equipped with the most advanced experimental facilities and theoretical methodologies. More specifically, the Indian group has a wide range of experimental spectroscopy techniques combined with an analysis based on a many-body model Hamiltonian. Additionally, the Austrian group has a vast competence in development and application of state-of- the-art first principles schemes. This includes, among others, the know-how of Density Functional Theory methods, Hartree-Fock methods, Green`s function based techniques, as well as wavefunction-based advanced quantum chemical methods. The research goal can then be subdivided into two stromgly interconnected areas of investigation: (i) A detailed qualitative and quantitative description of the evolution of the effective interactions (U, SOI, J, W, CF) for the 3d to 5d series of TMOs. This determines the boundary between the localized and itinerant regime, and the intersection between Mott-, Slater-, and Hund-type behaviours. (ii) A hand-in-hand experimental and theoretical work to clarify the phenomenology of 4d and 5d TMOs. Of particular interest are, e.g., phase transitions (i.e. insulator/metal-, order/disorder- and magnetic transition) and doping effects.

Materials with controllable quantum mechanical properties are of great importance for the electronics and quantum computers of the future. However, finding or designing realistic materials that actually have these effects is a big challenge. Understanding the physical origin and the nature of these interactions requires accurate mathematical models, advanced computational methods and fast computers. In this project we have employed quantum mechanical calculations based on the solution of the Schrodinger equation to study a new class of materials, called Quantum Materials, in which the macroscopic properties (e.g. electron mobility, magnetism) emerges from the collective behavior of a large number of different microscopic (quantum) constituents. In particular, we have focused our research on materials, primary oxides, formed by transition metals with quite diffuse (or delocalized) electrons which can interact with the crystal structure and exhibit a weak, but crucial, tendency to form spins (magnetism). A characteristic aspect of these materials is the relatively strong interaction between the spin and the orbit of the electron, a (quantum) effect called spin-orbit coupling. The research was directed towards two main lines: (A) optimization and assessment of the methodological apparatus and (B) applications to spin-orbit oxides and metal-organic solar- cells. Part (A) was conducted independently by the Vienna team, whereas the second part was developed in close collaboration with the Indian partners, who provided novel experimental measurements for the metal-organic solar cells and supported the theoretical analysis of the results obtained for the spin-orbit oxides. The most important aspect of our work was to recognize and explain the way in which the different microscopic constituents (electrons, spins, crystal structure) interact among each other and how to perturb this interactions using external forces (for instance an electric field or pressure) in order to modify the equilibrium and induce electronic or magnetic transitions. The control over quantum interactions in real materials represent not only a fundamental step in understanding the quantum nature of materials, but it can also expand the spectrum of potential applications in electronics, energy-devices and spintronics. The outcome of this project was reported in more than 15 scientific articles published in highly ranked scientific journals, including Nature Communications, Nature Quantum Materials and Physical Review, and was made possible also by the high-performance computing facilities provided by the Vienna Scientific Custer.