Strong coupling between Weyl fermions and phonons in TaAs - Dr. Bing Xu

APRIL 5, 2017

Scientists from HPSTAR and IOP-CAS , in collaboration with Dr. Rohit Prasankumar’s group at LANL, studied the temperature evolution of electronic and phonon properties of the Weyl semimetal TaAs using infrared spectroscopy. They found explicit evidence for strong coupling between an infrared-active A1 phonon and Weyl fermions in TaAs. The study is published in Nature Communications (DOI: 10.1038/ncomms14933).

Three-dimensional (3D) topological Weyl semimetals (WSMs) represent a novel state of quantum matter, which has an unusual band structure with two non-degenerate bands crossing at the Fermi level in 3D momentum space. At the band crossing points (Weyl points), the electronic dispersion is linear in all three directions, resembling a 3D version of graphene, and the low-energy excitations can be described by Weyl equations, producing a condensed-matter realization of Weyl fermions. Recently, such a Weyl semimetal state has been discovered in non-centrosymmetric transition-metal monoarsenides and monophosphides (TaAs, TaP, NbAs and NbP).

In these materials, since the Weyl points are located in close proximity to the Fermi level, interband electronic transitions near the Weyl points occur at a very low energy. This energy scale overlaps optical phonon frequencies. Consequently, strong coupling between electronic transitions near the Weyl points (Weyl fermions) and phonons may arise.

The coupling between discrete phonon and continuous electron–hole pair excitations can induce a pronounced asymmetry in the phonon line shape, known as the Fano resonance. Such interesting phenomenon has been observed in bilayer or few-layer graphene, topological insulators, stripe-phase nickelates and high-Tc superconductors etc. However, the coupling revealed in all these materials is associated with or has considerable contributions from coupling between phonons and conventional massive fermions.

In the present study, the scientists revealed an intrinsic, strong and temperature-tunable Fano resonance, which arises from quantum interference between phonons and massless Weyl fermions, in the recently discovered WSM TaAs. Using a model for strong electron-phonon coupling, they successfully elucidated the mechanism underlying the temperature-tunable phonon line shape.

“Our finding is the first observation of a temperature-tunable Fano resonance in TaAs.” Said Dr. Bing Xu, the co-first author of this work from HPSTAR, “More importantly, the Fano resonance is purely induced by the quantum interference between phonons and non-trivial massless Weyl fermions, which has great significance in studies of topologically non-trivial quantum phases.”

“These observations not only open a novel avenue for exploring exotic quantum phenomena in Weyl semimetals, but also set the stage for a variety of potential applications that take advantage of the ability to tune the Fano resonance using different parameters, for example, temperature, light or magnetic/electric fields.” added Dr. Yaomin Dai, the co-first author of this work from LANL.

Caption: (a) Fano resonance shown by the infrared phonon lineshape at different temperatures. The black solid lines through the data denote the Fano fitting results. (b) Temperature dependence of the Fano parameter 1/q2. The red solid lines through the data represent the modelling results.

最近，伴随着外尔半金属体系在TaAs家族被理论预言并首次被实验证实，外尔半金属材料独特的拓扑特性以及其新奇的量子效应受到了广泛的关注和研究。这类材料具有表面态费米弧，手性反常导致的负磁阻特性，反常的量子霍尔效应,以及极高的电子迁移率，在低能耗电子器件，量子计算等领域具有潜在的应用价值。来自北京高压科学研究中心,中科院物理所,以及美国洛斯阿拉莫斯国家实验室的研究团队通过光谱手段研究了外尔半金属TaAs的电子和声子性质,首次发现了外尔费米子和声子之间存在的很强的耦合作用，而且这种耦合作用可以被温度有效的调控。此研究不仅揭示了外尔费米子和声子之间的耦合机制，也为研究外尔半金属的新奇量子特性提供了新的有效途径。相关结果发表在近期的《自然-通讯》杂志上。