In Situ Cryo 4D STEM of CDW Phase Transitions in Layered Materials

Many quantum materials possess complex electronic phase diagrams where correlated electronic phases, such as superconductivity, magnetic ordering, and charge density waves (CDWs), exist near each other. The proximity of these phases suggests that phase transitions must be understood to establish the microscopic origin for these correlated phases and to use them for applications. These phase transitions have mostly been studied using ensemble-average techniques, lacking real space information of nucleation and growth of these often-competing electronic phases. Nevertheless, such real-space information is essential for applications of quantum materials, which will be at the nanoscale where reduced dimensionality, confinement, local heterogeneities, interfaces, and defects will greatly modify the phase transitions.<br/><br/>In this talk, I will discuss our group’s efforts on combining cryogenic scanning transmission electron microscopy (cryo STEM) and <i>in situ</i> cryo 4D STEM with <i>in situ</i> transport measurements to study the phase transition of TaS₂ in real-space with nanometer resolution as it undergoes a transition from the commensurate CDW (insulating phase) to the nearly commensurate CDW (metallic phase). We directly visualize the nucleation and growth of the NC-CDW phase out of the C-CDW phase and correlate this to the changing transport data. We establish that the phase transition starts at extended defects present in TaS₂. We also examine the pulse-induced CDW phase transition in TaS₂and reveal the role of defects and the transition mechanism. Our findings are extended to CDWs in rare-earth tri-tellurides (RTe₃) and other layered materials.