Center for High Pressure Science &Technology Advanced Research

New Weyl semimetal phase in TaAs – Dr. Wenge Yang

SEPTEMBER 30, 2016

A joint team co-led by HPSTAR scientist Wenge Yang studied the envolution of electronic and structural properties of Weyl semimetal TaAs using multiple methods. They found a pressure-induced new Weyl semimetallic phase with isoenergetic 12 Weyl nodes in TaAs. The study is published in the journal Physical Reviews Letters (DOI:https://doi.org/10.1103/PhysRevLett.117.146402).

Weyl fermions were first proposed by Hermann Weyl, a physicist and mathematician. It is a massless particle that also carried an electric charge, which was believed could greatly improve energy flow in electronics and thus will be ideal candidates for high-speed and energy-efficient electronic devices. Unfortuanatelly, it has never been found in nature while maybe realized in the form qusiparticles in crystals - called Weyl semi-metals, which behaves both like a metal, conducting electrons, and an insulator, blocking them.

Until 2015, Weyl fermions was first experimentally discovered in semimetallic TaAs, more than 8 decades later since it was first predicted. The Weyl semimetals possess novel transport properties due to the anomaly chirality in Weyl nodes, the band crossing points. Weyl nodes is connected by topological surface arcs always appear in pairs with opposite chirality.

Weyl semimetal TaAs has 24 Weyl nodes with two types at ambient conditions, locating at two different energy levels with some trivial state which makes the studies of topological surface states and novel properties more complicated and difficult.

In the present study, the scientists are aiming to minimize the trivial states interference by reducing some Weyl nodes in TaAs from high pressure technique, which has been widely used for tuning electronic structures in multiple materials (eg. ZrTe5).

A joint team of scientists compressed the TaAs crystals in diamond-anvil-cell. X-ray diffractions indicate that the tetragonal TaAs transitioned to a new crystal structure-called h-TaAs with hexagonal structure at 14 GPa. And the new phase can be quenched to ambient conditions. This new phase drives the team to further probe the band structure Fermi surface on this h-type sample.

What surprised the scientists is that the high-pressure phase, h-TaAs is also a Weyl semimetal but just shows 12 Weyl nodes, half of that of its tetragonal phase from their theoretical calculations. More interestingly, all 12 Weyl nodes belong to the same type with the same energy, coupling with each other.

This is the first discovery of isoenergetic Wely fermions induced by pressure, said Dr. Zhou, the lead author of the work. The released h-TaAs will allow us to study the interplay between surface states and other properties. And further experimental evidence like APRES (angle-resolved photoemission spectroscopy) is required to for supporting our theoretical predictions.

The Weyl nodes are very stable at ambient condition. This indicates that pressure is powerful in varying the topological properties in Wely semimetals. We are expecting more novel discoveries in this field using high-pressure techniques, added Wenge.

Caption:Caption: (Left): Representative Rietveld refinements tetragonal TaAs (up) and h-TaAs (bottom), respectively. (Right):TheWeyl nodes distribution in the first Brillouin zone.

伴随着拓扑绝缘体的发现,材料的拓扑特性以及新奇量子效应在过去的十年里受到了广泛的关注和研究,拓扑电子材料家族也从最初的拓扑绝缘体逐渐扩展到狄拉克半金属和外尔半金属等。外尔半金属具有表面态费米弧、手性反常导致的负磁阻等新奇物理性质,在低能耗电子器件、量子计算等领域具有潜在的应用价值。2015年,非中心对称的TaAs家族被理论预言并成为首个实验证实的外尔半金属体系,这类材料常压下为四方结构,其三维布里渊区内有24个外尔点,它们处在两个不同能级之上。压力作为一种洁净的维度,直接作用于晶格自由度,是调节量子相变、诱导新物态的一个直接途径。因而,压力下材料的拓扑特性如何演化不但有助于外尔半金属物理本质的研究,同时也将为设计新的拓扑电子材料提供新思路。通过理论分析并结合高压下的电输运测量和同步辐射X射线衍射等实验手段,本对TaAs单晶高压下的物理行为进行了详细研究(压力最高达到54GPa)。理论预测和实验测量结果一致表明,TaAs常压四方I41md结构中处在两个不同能级上的24个外尔点在低压区可以稳定存在。当压力超过14GPa后,新的高压六方相P-6m2出现。进一步的研究发现,该高压六方相属于一类新型拓扑半金属,它只拥有12个外尔点并位于相同能级之上,因而相比于常压相而言能带结构更为简单,另外这个新型拓扑半金属相可以稳定保留至常压,这为以后在常压下进一步研究这个新型外尔半金属相提供了可能 (http://www.hfcas.ac.cn/xwzx/jqyw/201610/t20161017_4678588.html).