Research: Phononic quantum oscillations indicate gapless state in ZrTe5

In recent years, there has been an intense discussion about the electronic structure and material properties of ZrTe5. One key debate is about whether or not this material forms a gapped charge-density-wave state at fields of a few Tesla. We now report on ultrasound measurements and their theoretical modelling that strongly suggests ZrTe₅ not to form a charge-density-wave. Our key observation are quantum oscillations of the ultra-sound waves: oscillatory changes of their properties in response to an applied magnetic field. Because phonons (which carry the ultrasound) are not charged, they do not couple directly to a magnetic field. Instead, the oscillations can only occur if the phonons couple to gapless electrons. The latter naturally feel the magnetic field, and via the electron-phon coupling hand down the field dependence to ultrasound.

T. Ehmcke, S. Galeski, D. Gorbunov, S. Zherlitsyn, J. Wosnitza, J. Gooth, T. Meng,
Propagation of longitudinal acoustic phonons in ZrTe₅ exposed to a quantizing magnetic field,
Phys. Rev. B 104, 245117 (2021) (arXiv)