Unified theory of second sound in two-dimensional materials

We develop a unified theory for second sound in two-dimensional materials. Previously studied drifting and driftless are two limiting cases of the theory, corresponding to drift diffusive part energy flux, respectively. find that due presence quadratic flexural phonons, does not exist thermodynamic limit, while mode is less affected. This understood as result infinite effective inertia their constant density states divergent Bose-Einstein distribution long wavelength limit. Consequently, group velocity smaller than mode. However, upon tensile strain, becomes larger. Both them increase with strain linearization phonon dispersion. Our results clarify several puzzles encountered previously pave way exploring wavelike heat transport beyond hydrodynamic regime.