[论文解读] Large and unsaturated negative magnetoresistance induced by the chiral anomaly in the Weyl semimetal TaP

Chandra Shekhar, F. Arnold+14 more|arXiv (Cornell University)|Jun 22, 2015

Topological Materials and Phenomena被引用 24

一句话总结

本研究在Weyl半金属TaP中展示了大范围、未饱和的负磁阻效应，即使由于费米能级远离Weyl节点而导致手征性不明确时亦如此。通过角分辨量子振荡光谱学与能带结构计算，研究识别出自旋极化的香蕉形费米口袋，并证实当电场与磁场共线时，手征磁效应依然存在，为在非理想Weyl体系中探测手征异常提供了稳健的框架。

ABSTRACT

Weyl semimetals (WSMs) are topological quantum states wherein the electronic bands linearly disperse around pairs of nodes, the Weyl points, of fixed (left or right) chirality. The recent discovery of WSM materials triggered an experimental search for the exotic quantum phenomenon known as the chiral anomaly. Via the chiral anomaly nonorthogonal electric and magnetic fields induce a chiral density imbalance that results in an unconventional negative longitudinal magnetoresistance, the chiral magnetic effect. Recent theoretical work suggests that this effect does not require well-defined Weyl nodes. Experimentally however, it remains an open question to what extent it survives when chirality is not well-defined, for example when the Fermi energy is far away from the Weyl points. Here, we establish the detailed Fermi surface topology of the recently identified WSM TaP via a combination of angle-resolved quantum oscillation spectra and band structure calculations. The Fermi surface forms spin-polarized banana-shaped electron and hole pockets attached to pairs of Weyl points. Although the chiral anomaly is therefore ill-defined, we observe a large negative magnetoresistance (NMR) appearing for collinear magnetic and electric fields as observed in other WSMs. In addition, we show experimental signatures indicating that such longitudinal magnetoresistance measurements can be affected by an inhomogeneous current distribution inside the sample in a magnetic field. Our results provide a clear framework how to detect the chiral magnetic effect.

研究动机与目标

• 研究当Weyl半金属中的手征性不明确时，手征异常及其相关的负磁阻效应是否依然存在。
• 利用量子振荡测量与第一性原理计算，绘制TaP的详细费米面拓扑结构。
• 确定在费米能级附近的Weyl节点不明确的材料中，手征磁效应可被实验观测的条件。
• 识别并表征可能影响纵向磁阻测量的实验伪影，如电流分布不均匀性。

提出的方法

• 采用角分辨de Haas–van Alphen量子振荡光谱学，探测TaP的费米面拓扑结构。
• 通过第一性原理能带结构计算，验证观测到的费米面几何形状及Weyl点位置。
• 在电场与磁场共线条件下测量纵向磁阻，以探测手征磁效应。
• 利用输运测量与理论建模，分析磁场中电流分布不均匀性的影响。
• 将观测到的磁阻行为与靠近Weyl点的自旋极化费米面口袋相关联。

实验结果

研究问题

• RQ1 当费米能级远离Weyl节点，导致手征性不明确时，手征磁效应是否依然存在？
• RQ2 TaP的费米面精确拓扑结构是什么？其与手征异常有何关联？
• RQ3 磁场中电流分布的不均匀性在多大程度上会扭曲纵向磁阻测量？
• RQ4 在具有复杂自旋极化费米面口袋的Weyl半金属中，能否可靠探测手征磁效应？

主要发现

• TaP的费米面由连接Weyl点对的自旋极化、香蕉形电子与空穴口袋构成，证实了复杂的拓扑结构。
• 尽管由于费米能级远离Weyl节点导致手征性不明确，但在电场与磁场共线条件下仍观测到大范围负磁阻。
• 负磁阻的幅度未饱和，表明手征异常响应具有强鲁棒性。
• 实验信号表明，磁场中电流分布的不均匀性可能显著影响纵向磁阻测量。
• 本研究建立了一个可靠的框架，即使在传统手征条件未严格满足的情况下，也能在Weyl半金属中探测手征磁效应。