Discovering and understanding quantum spin liquids

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Frustrated magnets are materials that have competing interactions between their electron spins, destabilizing conventional magnetic order and leading to various exotic phases that provide an ideal setting for studying emergent properties. When quantum fluctuations are large enough (typical of spin-1/2 moments), the spins can become highly quantum entangled and form what is known as a quantum spin liquid. This fascinating state of matter has a number of remarkable properties, including emergent excitations such as spinons, Majorana Fermions and magnetic monopoles.******The aim of this research is to discover new quantum spin liquid materials and to better understand the materials that we have already discovered. Four short-term objectives will be pursued: (1) We will explore the interplay of charge and spin degrees of freedom in frustrated lattices (kagome and triangular) to see how this can give rise to novel spin models and new types of quantum spin liquid. (2) We will apply hydrostatic pressure to materials that are understood to be close to a quantum critical point between antiferromagnetism and spin liquidity with the aim of tipping them past that critical point and into the quantum spin liquid phase. In particular, we hope to generate an example of the coveted Kitaev QSL which has potential quantum computing applications. (3) We will develop a new method of characterizing the spin excitations in quantum spin liquid candidate materials, including the kagome system Herbertsmithite, that relies on spinon-phonon coupling, as probed by acoustic measurements (sound velocity and attenuation). In particular, we hope to determine conclusively whether there is a spin gap in Herbersmithite or not. (4) We will aim to understand whether there is an important role for quantum fluctuations in certain "classical" spin liquids. While several materials with large, navely classical spins do not order, it remains unclear whether these systems are stuck in a metastable state or whether they would in fact be best described as quantum spin liquids.******This project will primarily be carried out by 7 graduate students and 5 undergraduate interns funded by this grant, as well as one postdoc funded from another source. A diverse set of research techniques will be employed in my laboratory at the Université de Sherbrooke and at user facilities around the world. In particular, we will employ nuclear magnetic resonance (NMR), muon spin rotation (µSR), thermodynamic measurements (specific heat and magnetization) and ultrasound velocity and attenuation. Many of these experiments will be carried out at very low temperatures (down to 20 mK), at high magnetic fields (up to 18 T) and under hydrostatic pressure (up to 2 GPa).