  Oscillations of conductance observed in strong magnetic fields are a striking
manifestation of the quantum dynamics of charge carriers in solids. The large
charge carrier density in typical metals sets the scale of oscillations in both
electrical and thermal conductivity, which characterize the Fermi surface. In
semimetals, thermal transport at low-charge carrier density is expected to be
phonon dominated, yet several experiments observe giant quantum oscillations in
thermal transport. This raises the question of whether there is an overarching
mechanism leading to sizable oscillations that survives in phonon-dominated
semimetals. In this work, we show that such a mechanism exists. It relies on
the peculiar phase-space allowed for phonon scattering by electrons when only a
few Landau levels are filled. Our measurements on the Dirac semimetal ZrTe5
support this counter-intuitive mechanism through observation of pronounced
thermal quantum oscillations, since they occur in similar magnitude and phase
in directions parallel and transverse to the magnetic field. Our phase-space
argument applies to all low-density semimetals, topological or not, including
graphene and bismuth. Our work illustrates that phonon absorption can be
leveraged to reveal degrees of freedom through their imprint on longitudinal
thermal transport.
