Unconventional Hall response in the quantum limit of HfTe5

Interacting electrons confined to their lowest Landau level in a high magnetic field can form a variety of correlated states, some of which manifest themselves in a Hall effect. Although such states have been predicted to occur in three-dimensional semimetals, a corresponding Hall response has not yet been experimentally observed. Here, we report the observation of an unconventional Hall response in the quantum limit of the bulk semimetal HfTe5, adjacent to the three-dimensional quantum Hall effect of a single electron band at low magnetic fields. The additional plateau-like feature in the Hall conductivity of the lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the longitudinal electrical resistivity and its magnitude relates as 3/5 to the height of the last plateau of the three-dimensional quantum Hall effect. Our findings are consistent with strong electron-electron interactions, stabilizing an unconventional variant of the Hall effect in a three-dimensional material in the quantum limit

Observation of a three-dimensional fractional Hall response in HfTe5

Interacting electrons in two dimensions can bind magnetic flux lines to form composite quasiparticles with fractional electric charge, manifesting themselves in the fractional quantum Hall effect (FQHE). Although the FQHE has also been predicted to occur in three dimensions, it has not yet been experimentally observed. Here, we report the observation of fractional plateaus in the Hall conductivity of the bulk semimetal HfTe5 at magnetic fields beyond the quantum limit. The plateaus are accompanied by Shubnikov-de Haas minima of the longitudinal electrical resistivity. The height of the Hall plateaus is given by twice the Fermi wave vector in the direction of the applied magnetic field and scales with integer and particular fractional multiples of the conductance quantum. Our findings are consistent with strong electron-electron interactions, stabilizing a fractionalized variant of the Hall effect in three dimensions.Comment: 35 pages with 17 figure

Propagation of longitudinal acoustic phonons in ZrTe₅ exposed to a quantizing magnetic field

The compound ZrTe5 has recently been connected to a charge-density-wave (CDW) state with intriguing transport properties. Here, we investigate quantum oscillations in ultrasound measurements that microscopically originate from electron-phonon coupling and analyze how these would be affected by the presence or absence of a CDW. We calculate the phonon self-energy due to electron-phonon coupling, and from there deduce the sound-velocity renormalization and sound attenuation. We find that the theoretical predictions for a metallic Dirac model resemble the experimental data on a quantitative level for magnetic fields up to the quantum-limit regime. © 2021 American Physical Societ