Ultrafast Nonsymmorphic Symmetry Breaking by Strong Light-Matter Interaction

In condensed matter physics, symmetry breaking lays the cornerstone of the formation of rich phases of matter, such as charge density waves and magnetism. Symmetry breaking can be on-demand controlled in equilibrium states by external fields, including electric/magnetic field and strain, leading to intriguing phase transitions and collective modes, such as tailoring topological Dirac nodal line, Mobius, and hourglass fermions by breaking nonsymmorphic symmetry. Beyond equilibrium states, strong light-matter interaction potentially provides unprecedented opportunities for breaking symmetry on an ultrafast timescale and even realizing exotic quantum states which have no counterpart in equilibrium states.<br/><br/>Here, by utilizing time- and angle-resolved photoemission spectroscopy (TrARPES), we demonstrate the realization of ultrafast symmetry breaking by strong light-matter interaction in a two-dimensional semiconductor. Driven by a strong mid-infrared light field, an emerged band crossing nodal ring is observed, which is gapped by the interactions and leaves nonsymmorphic symmetry-protected Dirac nodes. Interestingly, when the driving light field is tuned to polarize along the direction which breaks the nonsymmorphic symmetry, Dirac nodes are further gapped to form a fully gaped nodal ring, indicating a light-induced nonsymmorphic symmetry breaking. Moreover, the Dirac nodes are gapped only in the presence of the light field and recover gapless almost instantaneously (&lt;&lt;100 fs) when the light field is turned off, suggesting an ultrafast nonsymmorphic symmetry breaking. This work not only demonstrates light-matter interaction as an effective way to manipulate symmetry in quantum materials but also paves an important step for the long-sought Floquet topological insulator.