QUANTUM MATTER SEMINAR

A new frontier in condensed matter physics involves developing techniques to measure non-equilibrium properties with spatial resolution. In this talk I will discuss two types of experiments on charge density wave (CDW) systems that were conducted using our newly developed multimodal pulsed laser STM.

In the first part of the talk, I will discuss our investigations of the CDW in the Kagome compound RbV3Sb5which is a strong candidate for orbital magnetism or a loop current phase that spontaneously breaks time-reversal symmetry. Our STM topography reveal that even without external stimulation, the intensities of the three CDW Fourier peaks differ, indicating a breakdown of rotational and mirror symmetries. By applying linearly polarized light along one of the CDW directions, we demonstrate that we can switch the relative intensities of these peaks, suggesting a significant electrostrictive response, indicative of strong non-linear electron-phonon coupling. We also observe similar intensity switching of the CDW in perpendicular magnetic fields, indicating an unusual piezo-magnetic response that requires time-reversal symmetry-breaking. We propose that the simplest CDW satisfying these criteria is an out-of-phase combination of bond charge order and loop current order, which we refer to as the congruent CDW flux phase. In the second part of the talk, I will present our experiments on the unconventional CDW insulator (TaSe4)2I where we use a combination of reflectance and current measurements in the time-domain with femtosecond time-resolution, to gain insights into the presence of a massive phase mode and its dynamic interactions with other modes in the system.