Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics.

物质的拓扑相为在超快时间尺度上实现无损电荷和自旋信息传输提供了令人兴奋的可能性。然而，这需要详细了解它们的非平衡特性。在这里，我们采用时间、自旋和角度分辨的光电发射来研究 Sb(111) 自旋极化表面状态对飞秒激光激发的超快响应。表面态表现出巨大的质量增强，在费米能级以上的能量-动量色散中观察到扭结结构。扭结结构源自表面态与体连续体的直接耦合，其特征是群速度突然变化约 70%，与我们的G W 一致基于能带结构的计算。我们对瞬态占据带结构中的这种连通性的观察能够对表面状态的拓扑性质进行明确的实验验证。我们对电子动力学的测量进一步证实了体表面耦合的影响，这表明体态和表面态表现为单个热化电子群，具有来自低k电子-电子和高k电子-声子散射的不同贡献. 这些发现对于拓扑半金属的未来应用及其在超快自旋电子学中的激发具有重要意义。