Universal spreading dynamics in quasiperiodic non-Hermitian systems

Non-Hermitian systems exhibit a distinctive type of wave propagation, due to the intricate interplay of non-Hermiticity and disorder. Here, we investigate the spreading dynamics in the archetypal non-Hermitian Aubry-Andr\'e model with quasiperiodic disorder. We uncover universal transport behaviors: subdiffusion with a spreading exponent $\ensuremath{\delta}=1/3$ in the localized regime and diffusion with $\ensuremath{\delta}=1/2$ in the delocalized regime, in stark contrast to their Hermitian counterparts (halted versus ballistic). We then establish a unified framework from a random-variable perspective to determine the universal scaling relations in both regimes for generic disordered non-Hermitian systems. An efficient method is presented to extract the spreading exponents from Lyapunov exponents. The observed subdiffusive or diffusive transport in our model stems from Van Hove singularities at the tail of imaginary density of states, as corroborated by a Lyapunov-exponent analysis.