In a charge density wave (CDW), conduction electrons in a metal (typically a low-dimensional material) arrange themselves in a regular pattern, sometimes accompanied by lattice distortions. One material that undergoes a CDW transition is a compound of tantalum, selenium, and iodine [(TaSe4)2I]. Its quasi-one-dimensional structure consists of TaSe4 molecular chains interspersed with I– ionic chains.
The CDW transition in (TaSe4)2I is of particular interest because it’s potentially a mechanism that could lead to a spontaneous transformation into an “axion” state of matter. Axions are hypothetical particles that were proposed as a way to solve a well-known problem in particle physics. But the concept has crossed over to condensed matter physics, as a way to describe emergent properties in topological materials.
Prevailing theories predict that the CDW transition in (TaSe4)2I—a type of topological material known as a Weyl semimetal—should lead to a dispersion gap at points where linear Weyl bands intersect, but this has never been confirmed experimentally through angle-resolved photoemission spectroscopy (ARPES), because of difficulties arising from a very weak ARPES intensity near the Fermi level.
To address this, a team led by Meng-Kai Lin (National Central University, Taiwan) and Tai-Chang Chiang (University of Illinois at Urbana-Champaign) re-examined the electronic structure of (TaSe4)2I at Advanced Light Source Beamline 10.0.1 and other facilities, using high-statistics ARPES to bring out subtle features in the data.
“Despite the very low photoemission intensity near the Fermi level, the team was able to uncover the true band dispersions,” said Lin. “Upon CDW formation, no dispersion gap was observed. Instead, the Weyl bands recede away from the Fermi level to open a spectral gap wherein quasiparticles seem to simply disappear.”
These results are unexpected but can be explained by quasiparticle localization arising from the incommensurate modulation of the lattice by the CDW. The CDW lattice modulation increases as the temperature is lowered, and the spectral gap should widen accordingly, just as the team observed. Overall, the work tells us that the CDW in topological materials can be essentially different from that in other materials, a finding that should be carefully considered when designing quantum devices.
M.-K. Lin, J.A. Hlevyack, C.X. Zhao, P. Dudin, J. Avila, S.-K. Mo, C.-M. Cheng, P. Abbamonte, D.P. Shoemaker, and T.-C. Chiang, “Unconventional spectral gaps induced by charge density waves in the Weyl semimetal (TaSe4)2I,” Nano Lett. 24, 8778 (2024), doi:10.1021/acs.nanolett.4c02701.