"Ultrafast measurements of massive phase oscillations of a condensate" with Professor Donald Biggar Willett Professor in Engineering Vidya Madhavan of University of Illinois Urbana-Champaign
Abstract: The Anderson-Higgs mechanism manifests in quantum materials, with the Meissner effect in superconductors serving as a notable example. Here, photons gain mass through interactions with condensed Cooper pairs, leading to the expulsion of magnetic fields from the superconductor's interior. A similar process has been predicted to occur in a charge density wave (CDW) condensate, where the collective phase oscillations (phasons), which are typically massless, could acquire mass via a Anderson-Higgs-like mechanism. Although the concept of massive phasons was proposed decades ago, it is only recently that THz emission from the unconventional CDW insulator (TaSe4)2I has been detected and linked to this mode. To directly confirm the presence of these massive phasons, local charge oscillations must be measured with femtosecond temporal resolution, which is a challenging task.
In this talk, I will present an ultrafast scanning tunneling microscope that enables local measurement of local charge dynamics. Using this tool, we observe charge oscillations at 0.22 THz, displaying the temperature dependence expected of an excitation acquiring mass through the Higgs mechanism. Remarkably, we also detect a second excitation with equal intensity. I will present evidence that this mode results from the splitting of the massive phason into two massless modes, akin to the decay of a neutral pion into two photons. This work not only confirms the existence of Higgs-ed massive phasons in (TaSe4)2I but also uncovers their complex interactions with other modes, opening new avenues for exploring dynamic phenomena such as light-induced superconductivity. Finally, as time permits, I will also show how the ultrafast STM can be used to obtain spatially resolved information on the dynamics of step edges hosting a 1D edge mode in the topological crystalline insulator PbSnSe.
