Abstract:

Spontaneous symmetry breaking process results in the formation of broken-symmetry states like superconductivity, ferromagnetism and charge-density wave. This broken-symmetry state is usually energetically preferred ground state of matter at low temperatures. With a femtosecond laser pulse excitation, a natural result is the excitation of electrons and subsequent lattice heating, which would suppress the ordered broken-symmetry states. For example, it's common to observe the ultrafast melting of a ferromagnetic order or charge density wave, but very rare to see the other way around --- light induced new symmetry broken states like superconductivity. In this talk, I'll present a case where we observed light induced CDW in CeTe3. Such a new state is a hidden ground state not allowed thermodynamically, but is driven to form after an interaction quench by an intense infrared laser pulse. With ultrafast electron diffraction, we capture the entire course of transformation from stripe CDW into a bi-directional checkerboard CDW state. This experiment demonstrates the new dimension of phase exploration in the non-equilibrium regime. I'll also discuss the photo-doping and photo-thermal effect based on experiments on another CDW material of 1T-TaS2.