NEUTRON SCATTERING AND TRANSPORT STUDIES OF QUANTUM MATERIALS

Quantum material is an multi-disciplinary research topic that continues to thrive in recent years. The term \textit{Quantum material} covers all systems which demonstrate physical phenomena beyond the scope of single-particle, semi-classical/quantum theory. Among many sub-fields of quantum materials, topological systems and strongly correlated systems are two topics which have receive growing attention from the scientific community. We begin with a discussion on a van der Waals magnet VI$_3$ in chapter three. VI$_3$ hosts ferromagnetism on a honeycomb lattice, which was one of the proposed models for topological magnon bands. There have been ample theoretical studies on ferromagnetic honeycomb lattice. However, there has not been any physical realization of such model. In our study, we show that the is a strong anomalous thermal Hall effect in VI$_3$, the underlying mechanism of which is the non-trivial topological nature of the magnon bands.In chapter four, we discuss our transport studies on some magnetic topological metals. The non-zero Berry curvature in the reciprocal space of topological metals can lead to anomalous transverse conductivities ($\kappa^A, \sigma^A, \alpha^A$) in the system. We found large anomalous transverse conductivities in TbMn$_6$Sn$_6$ and verified its intrinsic nature through first-principle calculations. Furthermore, we have found large exchange-bias behavior in TbMn$_6$Sn$_6$, which renders it as a promising system for anomalous Nernst effect based thermoelectric device. We will also discuss the topological Nernst effect observed in Fe$_3$Sn$_2$, which is potentially due to the Skyrmion bubble phase revealed by the Lorentz transmission electron microscopic studies.In chapter five, we discuss our inelastic neutron scattering study on a unique quantum spin chain system in Cu$_2$(OH)$_3$Br. The system hosts alternating ferromagnetic and anti-ferromagnetic spin chains with finite inter-chain couplings. This allows for the coexistence and interactions between magnons and spinons.