FPO - Sonia Zhang - "Topology in Quantum Magnets and Superconductors"

Fri, May 8, 2020, 1:00 pm
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A unifying theme in understanding the macroscopic properties of quantum materials is the concept of emergence, where novel effects and unusual phases arise from the interplay of spin-orbit effects, band topology and strong correlation. In this dissertation, using scanning tunnelling microscopy and spectroscopy (STM/S), we explore a series of unconventional spin-orbit materials including magnets and superconductors. First, we provide a brief introduction to the working principles of low temperature atomic resolution STM/S operating in conjunction with a vector magnetic field capability. Utilising this state-of-the-art capability we explore the effects of artificial quantum impurity and vortex defects on topological superconductor candidates LiFeAs and PbTaSe2. We find that a controlled deposition of a carefully chosen class of atomic scale magnetic impurities on their surfaces generates zero-bias peaks exhibiting signatures of Majorana zero modes, despite being absent in vortices in pristine samples. In a second line of research, we explore wavefunction topology in correlated kagome magnets. In Fe3Sn2, we discover a giant and anisotropic many-body spin-orbit tunability whose origin remains unclear in current theoretical models. In Co3Sn2S2 we find an unexpected negative magnetic response in the kagome flat band arising from the topology. Finally, we explore topological magnet Mn3Sn and show that the unique geometry of the kagome lattice leads to a remarkable manifestation of an apparent Kondo lattice-type effect, usually observed in strongly correlated heavy fermion materials. Our results taken collectively feature novel effects and phases arising from rich interplay among spin-orbit effects, band topology and many-body interactions in quantum magnets and exotic superconductors, that may potentially lead to new frontiers in condensed matter physics.