Vidya Madhavan

Dynamic manipulation of charge density waves

Abstract: A new frontier in condensed matter physics involves developing techniques to measure nonequilibrium 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 timedomain 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. Bio: Professor Madhavan received her bachelor's degree in metallurgical engineering in 1991 from the Indian Institute of Technology, Chennai, and a master of technology degree in solid state materials in 1993 from the Indian Institute of Technology, New Delhi. She obtained her phD from Boston University in 2000. She held a postdoctoral appointment at the University of California, Berkeley from 1999 to 2002, before joining the physics faculty at Boston College in 2002. She joined the faculty at Illinois in 2014 as a full professor.