  In most materials, transport can be described by the motion of distinct
species of quasiparticles, such as electrons and phonons. Strong interactions
between quasiparticles, however, can lead to collective behaviour, including
the possibility of viscous hydrodynamic flow. In the case of electrons and
phonons, an electron-phonon fluid is expected to exhibit strong phonon-drag
transport signatures and an anomalously low thermal conductivity. The Dirac
semi-metal PtSn4 has a very low resistivity at low temperatures and shows a
pronounced phonon drag peak in the low temperature thermopower; it is therefore
an excellent candidate for hosting a hydrodynamic electron-phonon fluid. Here
we report measurements of the temperature and magnetic field dependence of the
longitudinal and Hall electrical resistivities, the thermopower and the thermal
conductivity of PtSn4. We confirm a phonon drag peak in the thermopower near 14
K and observe a concurrent breakdown of the Lorenz ratio below the Sommerfeld
value. Both of these facts are expected for an electron-phonon fluid with a
quasi-conserved total momentum. A hierarchy between momentum-conserving and
momentum-relaxing scattering timescales is corroborated through measurements of
the magnetic field dependence of the electrical and Hall resistivity and of the
thermal conductivity. These results show that PtSn4 exhibits key features of
hydrodynamic transport.
