Optical and Magnetic Field Manipulation of the Charge Density Wave in RbV3Sb5.
Broken time-reversal symmetry in the absence of spin order indicates the presence of unusual phases such as orbital magnetism and loop currents. The recently discovered family of kagome superconductors AV3Sb5 (A = K, Rb, or Cs), hosting an exotic charge-density wave (CDW) state, has emerged as a strong candidate for this phase. In this work we use laser-coupled scanning tunneling microscopy (STM) to study RbV3Sb5. We track the Fourier intensities of the CDW peaks with magnetic field and with laser illumination. Even in the absence of any external stimulation, we find that the three Fourier peak intensities of the CDW are different, implying that the CDW breaks rotational and mirror symmetries. First, we apply linearly polarized light along the CDW direction. We find that the relative intensities of the CDW peaks can be reversibly switched by the light, implying a substantial electro-striction response, indicative of strong non-linear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that, in turn, requires time-reversal symmetry-breaking. We show that the simplest CDW that satisfies these constraints and reconciles previous seemingly contradictory experimental data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. This laser-STM study opens the door to the possibility of dynamic optical control of complex quantum phenomenon in correlated materials.