Pursuit of Quantum Spin Liquids in Exfoliated Anti-Ferromagnetic Insulators

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Nontechnical Abstract: Conducting materials display a host of fascinating properties, but remarkable behavior can persist even when conduction ceases. For example, electrons can behave roughly like small bar magnets, and one may roughly imagine the material as composed of arrays of magnets, fixed in place but each rotating in response to all the others. Usually this results in alignement of the magnets with their neighbors. In extremely rare cases, quantum mechanics inhibits the alignment of these electron magnets so their motion remains fluid; such a "quantum spin liquid" is one of the most highly sought after phenomena in modern condensed matter research and may hold the key to understanding potentially groundbreaking technologies like room temperature superconductivity. Recently, a quantum spin liquid has been proposed in a bulk material made of layered sheets of ruthenium chloride where the ruthenium atoms - each surrounded by many chlorines - support the magnetic behavior. Here, isolated individual flakes of this layered material (down to only one layer thick) are used to study the quantum spin liquid using techniques from the field of 2D physics. To build intuition for the physics of magnetism, a table-top system of freely rotating bar magnets is used as a demonstration in introductory physics courses. Finally, this work supports outreach programs to African-American middle and high school aged girls in the St. Louis area through a collaboration between the principal investigator and faculty in the Brown School of Social Work. Technical Abstract: Quantum spin liquids, as an ensemble of spins frustrated from achieving an ordered state, with dynamics dominated by quantum fluctuations, are an intriguing and highly sought after phase of magnetic quantum matter. The two-dimensional Kitaev quantum spin liquid (QSL) has recently been observed in the layered antiferromagnet RuCl3. This work focuses on exfoliation of RuCl3 flakes to few and single-layer systems seeking to stabilize the QSL by removing interlayer interactions. Several important milestones along the way will lead to new methods of investigating quantum magnetism, focusing in particular on antiferromagnetic proximity effects in layered devices. In particular the usual techniques to explore magnetism are extremely difficult to apply to microscopic samples, so a new set of probes is under development in which flakes of RuCl3, along with other antiferromagnetic materials of interest, are placed on graphene, with the goal of inducing a proximity effect in graphene. The electronic properties of graphene are impacted by the presence of antiferromagnetic exchange and readily explored via electronic transport (quantum Hall effects and non-local transport), optical (infrared magnetospectroscopy), and thermodynamic probes (electronic compressibility and magnetization). An electronic method of measuring magnetization in microscopic flakes is employed, opening a new window on thermodynamic physics of microscopic systems. The link between structure and magnetism is explored through studying a variety of heterostructure devices where the type, thickness, and stacking order of the layers is varied to control interlayer magnetic couplings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.