Heat-current-induced nonreciprocal phonon damping

Overview & Synopsis

Time-reversal symmetry enforces equal equilibrium linewidths for time-reversed phonon modes at opposite wave vectors. Here we show that a steady heat current can produce nonreciprocal anharmonic damping even in centrosymmetric nonmagnetic crystals. The effect arises from nonequilibrium occupations of scattering partners, which change the statistical weights of time-reversed three-phonon channels while their squared matrix elements remain equal. Within the single-mode relaxation-time approximation, we derive a linewidth correction that is linear in the temperature gradient and odd under wave-vector reversal. The response depends on scattering-partner velocities and equilibrium lifetimes and retains signed channel weights absent from the equilibrium linewidth. First-principles calculations for centrosymmetric MgO and diamond yield nonzero, mode-dependent linewidth corrections at selected finite wave vectors, demonstrating the mechanism in both ionic and covalent crystals. Reversing the heat current at fixed local temperature exchanges the broadening and narrowing of paired modes. This current-reversible linewidth contrast provides a spectral signature of occupation feedback, with potential implications for nonlinear phonon transport.