Anisotropic quantum magnetism in novel rare-earth materials

Background and rationale: Quantum magnetism is one of the central fields in the realm of quantum materials, the thriving realm that unifies complex magnetic states with other non-trivial many-body phenomena, such as unconventional superconductivity and topological quantum matter, graphene and Weyl semimetals, etc. The common thread is the manifestation of quantum mechanics on the macroscopic scale where the basic appeal arises from the fundamental intricacy of quantum nature. Recently, we have discovered a new type of emergent quantum phenomena, an Ising-like quantum spin liquid (QSL), that is a magnetic state that exhibits highly anisotropic spin correlations but remains quantum disordered at any temperature. This state was found in a triangular lattice antiferromagnet build of Nd3+ ions, coming from the rare-earth (RE) family, for which large magnetic anisotropy is a defining property. Still, the exact nature and origin of this extraordinary state remain unclear. Even more exciting is imagining other possible exotic magnetic states and excitations that could emerge when the Nd3+ magnetic ions are replaced with other RE ions or when the triangular spin lattice is replaced with a lattice that possesses different topology, e.g., the kagome lattice. Objectives and specific aims: The main objective of this proposal is to establish the link between strong magnetic anisotropy and QSL states on simplest and fundamentally the most important frustrated lattices in two dimensions, that is the triangular and the kagome lattice. To achieve that, we will investigate two novel families of RE-based antiferromagnets, as these materials have recently become state-of-the-art in the context of QSL candidates because of their minimal structural disorder and perturbations. The specific goals include i) unveiling further details about the Ising QSL and ultimately revealing its origin, ii) assessing the role of individual RE ions in selecting particular QSL states on the triangular lattice, and iii) determining the effect of the reduced spin lattice coordination on the ground state in cases of large magnetic anisotropy. Methods to be used: To reach the project goals, combining various complementary expertise in experimental and theoretical physics, as well as materials science is essential. Our team has an optimized balance of world-renown senior scientists in their corresponding fields, perspective younger scientists, and bright PhD students. We will employ a number of complementary experimental and numerical techniques that are perfectly suited for investigations of quantum magnetism, including local-probe techniques of muon spectroscopy (mSR), nuclear magnetic resonance (NMR) and electron spin resonance (ESR), neutron scattering techniques, as well as numerical techniques including ab-initio and finite temperature Lanczos calculations. These will be performed at the Jožef Stefan Institute (JSI), with the exception of more specific experiments, like mSR and neutron scattering experiments that require large-scale facilities, as well as NMR and ESR at high fields and very low temperatures, which will be performed in specialized partner laboratories. Expected results and impact for the field: Our systematic study will address some of the most pressing open fundamental problems in the vibrant field of QSLs, where effects of large magnetic anisotropy are almost completely unexplored. This is most likely due to very scarce experimental reports, providing little drive for the theory to develop in this direction. Having demonstrated that anisotropic spin systems can develop highly anisotropic QSL states and knowing that the magnetic anisotropy is highly RE-ion specific, we can confidently expect to observe a plethora of novel phases and phenomena to arise from the proposed study. Moreover, deeper understanding of the highly-anisotropic QSL states could prove very beneficial in exploiting the vast potential of QSLs in new-generation quantum technologies.