Topological semimetals with massless Dirac and Weyl fermions represent the forefront of quantum materials research. In two dimensions, a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction was predicted 16 years ago. These highly exotic quasiparticles—the semi-Dirac fermions—ignited intense theoretical and experimental interest but remain undetected. Using magneto-optical spectroscopy, we demonstrate the defining feature of semi-Dirac fermions—B2/3 scaling of Landau levels—in a prototypical nodal-line metal ZrSiS. In topological metals, including ZrSiS, nodal lines extend the band degeneracies from isolated points to lines, loops, or even chains in the momentum space. With calculations and theoretical modeling, we pinpoint the observed semi-Dirac spectrum to the crossing points of nodal lines in ZrSiS. Crossing nodal lines exhibit a continuum absorption spectrum but with singularities that scale as B2/3 at the crossing. Our work sheds light on the hidden quasiparticles emerging from the intricate topology of crossing nodal lines and highlights the potential to explore quantum geometry with linear optical responses. Published by the American Physical Society 2024

中文翻译：

---

拓扑金属中的半狄拉克费米子


具有无质量 Dirac 和 Weyl 费米子的拓扑半金属代表了量子材料研究的前沿。在二维空间中，一类奇特的费米子在一个方向上是无质量的，而在垂直方向上是有质量的。这些高度奇特的准粒子——半狄拉克费米子——激发了强烈的理论和实验兴趣，但仍然没有被发现。使用磁光光谱法，我们展示了原型节点线金属 ZrSiS 中半狄拉克费米子的决定性特征——朗道能级的 B2/3 缩放。在包括 ZrSiS 在内的拓扑金属中，节点线将能带简并性从孤立的点延伸到动量空间中的线、环甚至链。通过计算和理论建模，我们将观察到的半狄拉克谱精确定位到 ZrSiS 中节点线的交叉点。交叉节点线表现出连续吸收光谱，但在交叉处具有 B2/3 的奇点。我们的工作阐明了从交叉节点线的复杂拓扑结构中出现的隐藏准粒子，并强调了利用线性光学响应探索量子几何的潜力。 美国物理学会 2024 年出版