  Resistivity anomaly, a sharp peak of resistivity at finite temperatures, in
the transition-metal pentatellurides ZrTe5 and HfTe5 was observed four decades
ago, and more exotic and anomalous behaviors of electric and thermoelectric
transport were revealed recent years. Here we present a theory of Dirac
polarons, composed by massive Dirac electrons and holes in an encircling cloud
of lattice displacements or phonons at finite temperatures. The chemical
potential of Dirac polarons sweeps the band gap of the topological band
structure by increasing the temperature, leading to the resistivity anomaly.
Formation of a nearly neutral state of Dirac polarons accounts for the
anomalous behaviors of the electric and thermoelectric resistivity.
