Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

Symmetry plays a key role in both conventional and topological phases of matter, making the ability to drive symmetry changes optically a critical step in developing future technologies that rely on such control. Here, we use femtosecond optical pulses to transiently lower symmetry in the prototypical type-I Weyl semimetal TaAs. Using second harmonic generation spectroscopy, we observe an ultrafast reduction of magnetic point group symmetry without any structural change, indicating the electronic origin of the transition. We argue that this effect is brought on by photocurrent generation, which breaks both spatial and time-reversal symmetry through introducing an asymmetry in the non-equilibrium distribution of charge carriers following photoexcitation. Our results demonstrate that optically driven photocurrents explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales.