Local Superconductor-to-Semiconductor Phase Transition in WS₂ Controlled by STM Tip

Transition metal dichalcogenides are a unique class of material that demonstrates two-dimensional (2D) nature and often have polymorphic crystal structures and each of them with distinct properties. There exists the topologically trivial semiconductor 2H phase, as well as the metastable topological superconducting 2M phase in WS2. The latter has been recently identified as a candidate of topological superconductor with a markedly high critical superconducting temperature of ~8.8 K with the observation of the Majorana Zero Modes (MZMs) near the center of the magnetic vortex. Thus, it is possible to create Josephson junction or channels for braiding MZMs from only WS2 through local precise phase control. In this study, we demonstrate the precise control of 2M to 2H phase transition in WS2 both on the sizes and locations using the STM tip "current pulsing" method. The resulting phase transition areas are notably sharply hexagonal following the 2H lattice orientation and can range from 20-350 nm in diameter. The effects of the setpoint bias and tunneling current during pulsing and their effects on the resulting transitions have also given further insight into the mechanism behind the observed structural phase transition.