Defect-Mediated Structural Dynamics in 1T-TaS2 Using Ultrafast Electron Microscopy

Abstract

Recent advances in characterization techniques with high spatiotemporal resolution have opened a new landscape of ultrafast phase transitions and structural dynamics to explore. Strongly correlated materials experience electron-phonon coupling and structural deformation at the nanometer-picosecond regime. Ultrafast electron microscopy (UEM) has been developed as an excellent tool to elucidate these dynamics at the nanoscale. Considerable maintenance, operation and advancement was put into the current capabilities of the instrument which will be discussed throughout. Additionally, thorough characterization of the spatiotemporal capabilities for each component of the technique is included. Specifically, I examine the role of defects on charge-density wave (CDW) formation in 1T-TaS2 using diffraction and bright field imaging. During photoexcitation, ultrafast heating causes destruction of the CDW superlattice with an exponential decay away from defects. Additionally, phonon generation during photoexcitation was examined due to the competition which may occur during the induced phase transition. I discuss localized acoustic phonons which are visible in distinct specimen regions. Specifically, phonon propagation appears separated by local morphological defects along the examined viewing direction. These modes are induced by an initial c-axis longitudinal mode which causes in-plane modulations. Within each layer, independent phonons travel with properties representative of longitudinal and transverse motion. The phonons propagate with velocities agreeing with the longitudinal speed of sound for 1T-TaS2 in a direction parallel to the motion of an oscillation in the crystallographic planes. In diffraction, in-plane breathing modes oscillate with frequencies matching phonon propagation. However, there is also an out-of-plane structural distortion which bends the specimen in the shape of a transverse wave. As a result, I present the mechanism for phonon propagation in a single layer as a combination of simultaneous structural distortions. Each of the topics herein focus on the role of defects on photoexcited responses in 1T-TaS2.