DYNAMICAL SIGNATURES OF EXOTIC PHASES IN FRUSTRATED HONEYCOMB MAGNETS

In magnetic systems, adding an ingredient known as frustration gives rise to a variety of intriguing phases. One of these phases is the quantum spin liquid (QSL), which hosts exotic quasiparticle excitations and exhibits long-range entanglement without conventional magnetic order. Recent experiments on a frustrated honeycomb material α-RuCl3 have observed a half-quantized thermal conductivity in the presence of an external magnetic field, potentially signalling the presence of a QSL known as a chiral Kitaev spin liquid (KSL). However, the extent of this half-quantization remains experimentally contentious, necessitating a deeper exploration of other mechanisms that may influence the thermal Hall effect. In this thesis, we investigate three types of dynamical probes for frustrated honeycomb magnets using semiclassical and classical simulations. First, we delve into the thermal Hall conductivity from magnetically ordered states, and we demonstrate various aspects of the experiment that can be explained using topological magnons. Indeed, we show that the true test for a chiral KSL will be a half-quantized plateau at low temperatures where the magnon contributions are suppressed. Next, we investigate inelastic neutron scattering (INS), one of the primary probes for investigating excitations of spin systems. In experiments, the thermal Hall conductivity is measured at finite temperature and field, where there is a crossover between Majorana fermions and magnons. We offer insights into this crossover region by computing the dynamical spin structure factor (DSSF) for various classical models with large Kitaev interactions. Our results for a minimal model for α-RuCl3 exhibit continuum-like behaviour in the DSSF for various classical orders, echoing the characteristics of the quantum model. Finally, given the limitations of INS in distinguishing between different excitation continua, we examine the nonlinear response using two-dimensional coherent spectroscopy (2DCS). We find distinguishing features between the KSL, classical spin liquids, and large unit cell magnetic orders, despite all producing a broad continuum in INS. Overall, this work demonstrates the potential of 2DCS in elucidating the complex physics underlying ambiguous spin excitation continua, thereby advancing our comprehension of the dynamics in these frustrated systems.

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