Probing and Controlling Density Waves at the Atomic Scale using Ultrafast STM

Measuring the femtosecond dynamics of complex order parameters using a real-space probe has been a long-coveted goal. To tackle this challenge, over the past decade different variations of pump-probe techniques that integrate pulsed-laser light with a scanning tunneling microscope have been developed. So far, these approaches have mainly been used to investigate dynamics of molecules or decay times of hot carriers, and often face limitations in energy resolution or stability. In this talk I introduce a novel multimodal STM-based pump-probe instrument that allows for the measurement of both phonon and carrier dynamics through a combination of optical pump-probe reflectance (OPPR), tunneling, and point-contact measurements. We use this technique to explore the amplitude and phase excitations of the unconventional CDW insulator (TaSe₄)₂I. Our time-domain current measurements reveal oscillations at a frequency of 0.22THz, exhibiting the characteristic temperature dependence of a longitudinal phason that acquires mass through the Higgs mechanism. Interestingly, we also detect a second mode at 0.11 THz with equal intensity, which, through comparison with concurrently measured OPPR, can be identified as a parametrically amplified, massless transverse phason which competes with and suppresses the amplitudon observed in OPPR at the same frequency. This study confirms the presence of massive phasons and their intricate interactions with other modes but paves the way for future investigations of dynamic phenomena.