Probing topological states of matter using scanning tunneling microscopy and spectroscopy

Abstract

Bulk-boundary correspondence means a material’s bulk topology determines the presence of edge modes. This thesis uses low-temperature STM/STS to study edge states in a topological crystalline insulator and a topological superconductor. In Pb0.7Sn0.3Se, half-unit-cell step edges host 1D flat-bands. Surface-doping with 3d transition elements shifts the flat band to the Fermi level where it splits into multiple peaks due to enhanced interactions. Theory shows that these interactions drive time-reversal-symmetry breaking and stabilize flat-band Stoner ferromagnetism. In a CrCl3/NbSe2 heterostructure, STS reveals a fully gapped superconducting state which stays robust to strong magnetic fields. Furthermore the boundary of the CrCl3 islands hosts 1D edge modes. These features are consistent with helical p-wave superconductivity driven by interfacial Rashba SOC. Together, these results show how engineered materials enable control of exotic correlated and topological boundary states.