EXOTIC AND CONVENTIONAL EXCITATIONS IN THE KITAEV MAGNET α-RuCl₃

In 2006, Kitaev introduced an exactly solvable model of spins on a honeycomb lattice, which predicts a quantum spin liquid ground state hosting a variety of exotic quasiparticles. These quasiparticles have been proposed as a pathway for creating a solid-state, topologically protected platform for quantum computing that is inherently fault tolerant. The Van der Waals honeycomb compound α-RuCl₃ has emerged as a prime candidate for realizing this model in a material. This thesis presents results from a series of experiments aimed at investigating this material through neutron scattering and spin transport measurements.

It was expected that both the crystal structure and the spin-phonon coupling would greatly influence the magnetic properties in this material. Initial benchmarking of these properties revealed that at low temperatures, single crystals of α-RuCl₃ conform closely to thespace group, as determined by neutron Laue diffraction. Additionally, phonons in α-RuCl₃, which propagate or vibrate along the out-of-plane direction, were observed to exhibit significantly reduced velocities. Furthermore, low-energy interlayer phonons revealed a stacking structure consistent with thespace group, as observed through neutron Laue measurements.

Given the lack of prior evidence for fractionalized excitations via phonons, the focus was redirected toward directly investigating the magnetic modes in α-RuCl₃. This material exhibits an antiferromagnetic zigzag ground state at zero field, which is suppressed at a critical field, revealing a quantum-disordered state. The nature of this state remains under debate. This thesis presents high-resolution neutron scattering data, collected up to 13.5 T, resolved in both energy and momentum, to clarify the shape of the modes and the spin gaps. The results show that the excitation spectrum above 8 T retains a low-energy gap for both field directions at the 2D Γ-point. Above the gap, the dispersions remain broad, with even the “sharper” features consistently broader than the elastic resolution across all fields up to 13.5 T. These features are particularly flat below 10 T for both field directions—perpendicular and parallel to the Ru-Ru bonds—with diminished out-of-plane dispersion, highlighting a reduced contribution of interplane interactions to the overall spin behavior. These findings challenge the conventional magnon picture and complicate a multimagnon interpretation. Instead, the results support the existence of a field-induced spin-liquid regime, characterized by the interplay of a continuum and bound states of fractionalized excitations, which are pushed to higher energies by the applied magnetic field.

Once the energy dependence of all modes as a function of T and H was determined, device geometries were designed to explore if similar information could be extracted in devices, as miniaturization is necessary, and neutrons are not ideal for miniaturization and devices. Moreover, for temperatures below the observed spin gap at the 2D Γ-point, topologically protected edge modes are expected to play a crucial role in both heat and spin transport. Spin Seebeck effect has been widely employed in the spintronics community to study various properties of magnetic insulators, including elementary excitations, magnetic order, and domains, and is increasingly being applied to quantum magnets, making it a powerful tool to probe spin transport phenomena and a promising approach for investigating the underlying magnetic excitations in α-RuCl₃. This dissertation explores the use of SSE as a novel method to investigate evidence of Majorana fermions in this material. A spin Seebeck device was fabricated using a thin flake of RuCl₃, and angle-dependent measurements revealed distinct voltage dips under applied magnetic fields. These features, absent in control measurements, suggested that they originated from intrinsic excitations within the material. While the nature of these excitations remains uncertain, the results provide compelling evidence of field-induced excitations. Overall, the results demonstrate the potential of using SSE to probe field-induced excitations, though further detailed studies are necessary to clarify their connection to quantum spin liquid behavior.