Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices compatible with optical communication and information technology. Here, we use MeV Ultrafast Electron Diffraction (UED) to show how a transient three-dimensional (3D) Dirac semimetal state can be induced by a femtosecond laser pulse in a topological insulator ZrTe₅. We observe marked changes in Bragg diffraction, which are characteristic of bond distortions in the photoinduced state. Using the atomic positions refined from the UED, we perform density functional theory (DFT) analysis of the electronic band structure. Our results reveal that the equilibrium state of ZrTe₅ is a topological insulator with a small band gap of ~ 25 meV, consistent with angle-resolved photoemission (ARPES) experiments. However, the gap is closed in the presence of strong spin-orbit coupling (SOC) in the photoinduced transient state, where massless Dirac fermions emerge in the chiral band structure. The time scale of the relaxation dynamics to the transient Dirac semimetal state is remarkably long, τ ~ 160 ps, which is two orders of magnitude longer than the conventional phonon-driven structural relaxation. The long relaxation is consistent with the vanishing density of states in Dirac spectrum and slow spin-repolarization of the SOC-controlled band structure accompanying the emergence of Dirac fermions.

由量子和拓扑保护产生的具有独特性质的物质新阶段，是现代研究的重要方向。特别感兴趣的是使用超快速外部刺激（例如光激发）按需设计这些阶段，这为将其集成到与光通信和信息技术兼容的未来设备中提供了前景。在这里，我们使用MeV超快电子衍射（UED）来显示飞秒激光脉冲在拓扑绝缘体ZrTe _5中如何感应瞬态三维（3D）Dirac半金属态。我们观察到布拉格衍射的显着变化，这是光诱导状态下键畸变的特征。使用从UED精炼的原子位置，我们对电子能带结构进行密度泛函理论（DFT）分析。我们的结果表明，ZrTe ₅的平衡态是拓扑绝缘体，其带隙约为25 meV，与角度分辨光发射（ARPES）实验一致。但是，在光诱导的瞬态中存在强自旋轨道耦合（SOC）的情况下，该间隙是闭合的，其中无质量的狄拉克费米子出现在手性能带结构中。弛豫动力学到瞬态狄拉克半金属态的时间尺度非常长，τ 〜160 ps，比传统的声子驱动的结构弛豫长两个数量级。长时间的弛豫与Dirac光谱中消失的状态密度以及伴随Dirac费米子出现的SOC控制的能带结构的缓慢自旋复极化是一致的。