  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.
