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

在强相关电子系统的背景下，量子自旋液体（QSL）自1973年提出基本概念以来一直是一个积极研究的领域。QSL中可能出现的各种不同类型的非常规激发不仅在基础科学研究中产生了特别的兴趣，而且在可能的设备应用水平上也产生了特别的兴趣。例如，支持QSL态的材料被认为是拓扑量子计算的潜在平台。寻找能够实现QSL基态的系统需要几十年的材料探索。这些努力的结果一度令人失望，但最近已经报道了几种携带QSL物理特征的晶体化合物。在这个参照系中，我们希望利用先进的量子化学电子结构计算，阐明目前主要感兴趣的三角晶格化合物（d电子基和f电子基）中潜在的磁相互作用。在第一类中，我们的目标是三角形晶格体系KxIr(1-x/4)O2，以及Na2Pt0.5Ir0.5O3和K0.5RuCl3。这里的主要动机是最近的测量表明K0.85Ir0.79O2中存在QSL基态。这个项目要澄清的最有趣的方面是Kitaev和Heisenberg相互作用K/J之间的比率（大于6的比率，定义了所谓的Kitaev状态，在一些蜂窝iridates和RuCl3中发现），以及非对角线有效交换参数的强度（所谓的Γ’s）。这些信息对于理解三角形晶格KxIr(1-x/4)O2中磁性基态的性质是必不可少的。关于f电子的量子磁性，目前还没有关于固体中4f-4f超交换的详细的量子化学研究。通过这个研究项目，我们想改变这种现状。在这里起到催化剂作用的是目前对4f三角形晶格材料的兴奋，它们是实现QSL的有力候选者。有理由相信，最大的磁耦合出现在三角晶格KCeS2中。为了首次了解4f-4f超级交换和校准目的，因此我们将重点放在KCeS2的第一步。可能的扩展名为“NaYbSe2”、“NaYbS2”和“NaYbO2”。

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.