Optical Manipulation of the Charge Density Wave state in RbV₃Sb₅ - Part 1

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. While initial experiments suggested that the CDW phase breaks time-reversal symmetry, this idea is being intensely debated due to conflicting experimental data. In this work we use laser coupled scanning tunneling microscopy (STM) to study RbV₃Sb₅. STM data shows that the Fourier intensities of all three CDW peaks are different, implying that the CDW breaks rotational and mirror symmetries. By applying linearly polarized light along high symmetry directions, we show that the relative intensities of the CDW peaks can be reversibly switched, implying a substantial electro-striction response, indicative of strong nonlinear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that 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. Our laser STM data opens up the possibility of dynamic optical control of complex quantum phenomena in correlated materials.