Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal

Exposing a Hidden State Shining intense laser light on a material can temporarily alter its properties. The effect usually subsides after a few picoseconds, unless the system is trapped in a metastable state, in which case the transient period may last as long as microseconds. Stojchevska et al. (p. 177 ) observed that, following exposure to a 35-femtosecond laser pulse, the layered dichalcogenide 1T -TaS 2 entered a stable “hidden” state not present in the equilibrium phase diagram and stayed there indefinitely. The switch to the hidden state could be reversed by heat or a train of laser pulses. Because the switch alters the sample's conducting properties, the phenomenon might also lead to practical applications.