Spin Waves as New Probes for Graphene Quantum Hall Systems

Abstract

This thesis introduces two pioneering experimental techniques—magnon scanning gate microscopy (MSGM) and magnon transmission spectroscopy (MTS)—which utilize magnons in $\nu=1$ monolayer graphene quantum Hall ferromagnets (MLG QHFM) as probes to study spin orders in graphene-based systems. These methods leverage the principles of electrical generation and detection of spin waves (magnons) in graphene quantum Hall systems using two edge states with opposite spins. The first experiment, MSGM, provides spatially resolved insights into magnon transport within MLG QHFM. By employing a metallic scanning tip, we create a movable, localized scattering target that influences magnon transport in the system. Observations of tip-induced changes in two-terminal conductance and nonlocal voltage enable us to visualize the localized nature of magnon generation and absorption processes, as well as magnon propagation within the quantum Hall ferromagnet. Furthermore, this experiment uniquely highlights the importance of localized states in inducing spin flips across two chiral edge states, a process previously invisible to transport measurements alone. The second experiment, magnon transmission spectroscopy, leverages magnons from MLG QHFM to study spin order in a non-MLG system. Specifically, we launch magnons from $\nu=1$ MLG QHFM towards bilayer graphene (BLG) QHFM to probe the spin order of symmetry-broken states in BLG Landau levels. The sharp onset of nonlocal signals at Zeeman energy in BLG contacts indicates effective magnon transmission between the two systems, demonstrating the utility of magnons in revealing spin order within graphene multilayer devices. This method opens new avenues for assessing spin order in various exotic emergent phases in graphene heterostructures and other flat-band systems. Together, MSGM and MTS enhance our capability to probe and understand spin orders and collective excitations in graphene quantum Hall systems. The principles underlying these techniques are not limited to graphene systems but can be generalized to other Chern band systems, providing an exciting pathway for future exploration of quantum phases of matter.