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.