Interaction-driven transport in a non-degenerate mixture of Dirac and massive fermions at charge neutrality point

Original Abstract

The interplay between distinct carrier species in systems with broken Galilean invariance gives rise to a rich landscape of interaction-driven transport phenomena. Here, we develop a comprehensive theory for the electrical conductivity of a non-degenerate two-dimensional mixture of massless Dirac and massive fermions, a system realized in HgTe quantum wells tuned to the charge neutrality point. In this regime, all carriers are thermally activated, enabling a self-consistent, temperature-dependent interplay between the two species. We demonstrate that the conductivity undergoes a distinct crossover as temperature increases: at low temperatures, transport is dominated by massless Dirac carriers, yielding a temperature-independent conductivity reminiscent of graphene's charge neutrality point. As the temperature rises, massive holes become thermally excited, and their mutual Coulomb scattering with Dirac carriers induces a negative, non-Drude correction to the conductivity. We show that this correction is governed by the dominant scattering mechanism: short-range interparticle interactions yield a stronger suppression than long-range Coulomb interactions, and it scales monotonically with temperature. Crucially, the charge neutrality condition ensures that the chemical potential is not externally pinned but is determined self-consistently, making the system's transport response an intrinsic probe of inter-species quantum friction. Our findings establish HgTe quantum wells at charge neutrality as a clean, highly tunable platform for isolating and quantitatively studying interaction-driven transport in the absence of Galilean invariance, offering a direct pathway to explore regimes where interparticle collisions dominate over disorder.