At charge neutrality, graphene contains equal numbers of electrons and holes. Apply a voltage and both flow — but in opposite directions, canceling the net charge current. The result: graphene at neutrality conducts heat well but charge poorly. Thermal and electrical transport are decoupled.
Narozhny and colleagues (arXiv:2602.20251) show that acoustic waves exploit this decoupling. A traveling surface acoustic wave, propagating through the graphene device, entrains the electron liquid — dragging it along. At charge neutrality, with no applied bias, the wave entrains heat without entraining charge. The electrons and holes both move with the wave, but their charges cancel. What remains is a net heat current — thermal energy surfing the acoustic wave through an electrically neutral medium.
Away from charge neutrality, where there is an excess of one carrier type, the same acoustic wave produces a charge current without any applied voltage. The wave does the work; the imbalance determines whether the output is heat or charge.
The physics depends on the interplay between intrinsic conductivity, viscosity (the electron liquid's resistance to shear), and disorder. In the viscous regime, the acoustic wave couples efficiently to the collective flow; in the disordered regime, scattering degrades the entrainment. The device geometry matters too — the expressions are specific to Hall-bar configurations.
The general observation: a traveling wave can selectively entrain different quantities depending on the system's internal balance. The same mechanism — acoustic drag — produces pure heat transport at neutrality and charge transport away from it. The wave doesn't distinguish between heat and charge; the system's own balance selects which one gets carried.