Superexchange, effective interactions, and magnetic ground states in frustrated triangular-lattice quantum magnets

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In the context of strongly-correlated electronic systems, quantum spin liquids (QSL's) represent an area of active investigation ever since the basic concept was proposed in 1973. The various types of less conventional excitations that may arise in QSL's generated particular interest not only in fundamental scientific research but also at the level of possible device applications. For example, materials supporting QSL states were indicated as potential platforms for topological quantum computation. Finding systems in which QSL ground states are realized necessitated a few decades of materials exploration. The results of these efforts were for a while disappointing but more recently several types of crystalline compounds carrying the signatures of QSL physics have been reported. In this frame of reference, we would like to throw light on the underlying magnetic interactions in triangular-lattice compounds of major present interest, d-electron- and also f-electron-based, using advanced quantum-chemical electronic-structure computations.In the first category, we target the triangular-lattice system KxIr(1-x/4)O2 but also Na2Pt0.5Ir0.5O3 and K0.5RuCl3. The main motivation here is provided by recent measurements suggesting a QSL ground state in K0.85Ir0.79O2. The most interesting aspect to be clarified with this project is the ratio between the Kitaev and Heisenberg interactions K/J (ratios larger than 6, defining the so-called Kitaev regime, are found in some honeycomb iridates and in RuCl3) but also the strength of the offdiagonal effective exchange parameters (so-called Γ's). Such information is essential for understanding the nature of the magnetic ground state in triangular-lattice KxIr(1-x/4)O2.In relation to f-electron quantum magnetism, no detailed quantum-chemical investigations are available to our knowledge on 4f-4f superexchange in solids. With this research project we would like to change this present state of affairs. What acted here as a catalyst is the current excitement around 4f triangular-lattice materials, strong candidates to the realization of QSL's. There are reasons to believe that the largest intersite magnetic couplings arise in triangular-lattice KCeS2. For first insights into 4f-4f superexchange and for calibration purposes, we will therefore focus in a first step on KCeS2. Possible extensions are then NaYbSe2, NaYbS2, and NaYbO2.