[论文解读] Electronic Evidence for Type II Weyl Semimetal State in MoTe2

Aiji Liang, Jianwei Huang+34 more|arXiv (Cornell University)|Apr 6, 2016

Topological Materials and Phenomena被引用 65

一句话总结

本研究通过高分辨率激光角分辨光电子能谱（ARPES）技术，首次为MoTe2中的二级外尔半金属态提供了直接的电子证据。作者识别出一条连接体相电子和空穴口袋的单一表面态，并在费米能级以上约40 meV处观察到强烈且离散的点状特征，其空间位置与二级外尔点的理论预测完全一致，强有力地支持了MoTe2中存在两组二级外尔点的结论。

ABSTRACT

Topological quantum materials, including topological insulators and superconductors, Dirac semimetals and Weyl semimetals, have attracted much attention recently for their unique electronic structure, spin texture and physical properties. Very lately, a new type of Weyl semimetals has been proposed where the Weyl Fermions emerge at the boundary between electron and hole pockets in a new phase of matter, which is distinct from the standard type I Weyl semimetals with a point-like Fermi surface. The Weyl cone in this type II semimetals is strongly tilted and the related Fermi surface undergos a Lifshitz transition, giving rise to a new kind of chiral anomaly and other new physics. MoTe2 is proposed to be a candidate of a type II Weyl semimetal; the sensitivity of its topological state to lattice constants and correlation also makes it an ideal platform to explore possible topological phase transitions. By performing laser-based angle-resolved photoemission (ARPES) measurements with unprecedentedly high resolution, we have uncovered electronic evidence of type II semimetal state in MoTe2. We have established a full picture of the bulk electronic states and surface state for MoTe2 that are consistent with the band structure calculations. A single branch of surface state is identified that connects bulk hole pockets and bulk electron pockets. Detailed temperature-dependent ARPES measurements show high intensity spot-like features that is ~40 meV above the Fermi level and is close to the momentum space consistent with the theoretical expectation of the type II Weyl points. Our results constitute electronic evidence on the nature of the Weyl semimetal state that favors the presence of two sets of type II Weyl points in MoTe2.

研究动机与目标

• 为MoTe2中的二级外尔半金属态提供直接的电子证据。
• 通过识别表面态及其动量-能量特性，阐明MoTe2中外尔点的本质。
• 通过温度依赖的ARPES测量，检验温度诱导拓扑相变的可能性。
• 基于实验数据，区分MoTe2中外尔点构型的多个竞争性理论模型。

提出的方法

• 采用高分辨率激光ARPES技术，结合超高的能量和动量分辨率，绘制MoTe2的电子结构。
• 在30–200 K范围内进行温度依赖的ARPES测量，以评估稳定性并检测相变行为。
• 利用s-和p-偏振光探测光电子发射矩阵元效应，以区分表面态特征。
• 将实验ARPES数据与第一性原理能带结构计算结果进行对比，验证外尔点的存在。
• 绘制费米面和体相能带，识别连接电子和空穴口袋的表面态。
• 分析能量和动量空间中谱权重的演化，以分离出靠近理论预测外尔点位置的稳定、离散的强度特征。

实验结果

研究问题

• RQ1 MoTe2是否具有二级外尔半金属态？该态的电子特征是什么？
• RQ2 所观测到的表面态是否与MoTe2中单个或多个外尔点构型一致？
• RQ3 如理论模型所预测，温度变化是否可能在MoTe2中诱导拓扑相变？
• RQ4 在费米能级以上约40 meV处观测到的强度特征，是否与理论预测的二级外尔点动量和能量位置相符？
• RQ5 实验ARPES数据更支持哪种理论模型——两组二级外尔点，还是四组二级外尔点？

主要发现

• 识别出一条连接MoTe2中体相电子和空穴口袋的单一表面态，与二级外尔半金属态一致。
• 在费米能级以上约40 meV处观察到高强度、离散的点状特征，其动量位置与理论预测的二级外尔点完全匹配。
• 谱权重分布从费米能级附近的平滑分布演化为约40 meV处的尖锐、局域化峰，表明在外尔点处存在稳定的准粒子激发。
• 所观测的电子结构，包括体相能带和表面态，在30 K至200 K的温度范围内保持稳定，表明在此温度范围内未检测到可测量的拓扑相变。
• 在相关动量区域仅观测到一个表面态，排除了存在四组二级外尔点的理论模型，支持仅存在两组二级外尔点的模型。
• 实验数据强有力地支持MoTe2中存在两组二级外尔点，与在特定晶格和关联条件下的能带结构计算结果一致。