Quasinormal Modes Criticality across Black Hole Extremality
We investigate the extremal solution branch to nonextremal branch transition in Einstein-Euler-Heisenberg anti-de Sitter black holes, where the nonlinear electromagnetic parameter $λ$ governs the existence of the extremal branch solution. At fixed pressure and charge, extremal black hole only exist for $λ\leqλ_c$. For $λ>λ_c$, the lowest Hawking temperature keeps nonzero. Near this threshold, the lowest temperature follows a linear critical scaling, $T_{\min}\propto(λ-λ_c)^{α_T}$. We investigate the corresponding dynamical behavior by computing the quasinormal modes of a massless scalar perturbation along the lowest-temperature black hole branch. As the critical threshold is approached, two complex-frequency quasinormal modes branches collide on the imaginary axis and reorganize into distinct purely imaginary branches. Then the imaginary parts of these modes, together with higher overtones, reach zero at the critical threshold, described by $Γ_n\propto(λ-λ_c)^{α_n}$, with $α_n\simeqα_T$. The resulting zero-damped modes become increasingly localized near the event horizon, and their relative damping rates closely follow the overtone spacing predicted by the near-horizon AdS$_2$ spectrum. Our results show that the thermodynamic and dynamical critical behavior remain closely connected even when extremal black hole solutions cease to exist, providing further evidence for the relevance of near-extremal spectral behavior in this distinct critical regime.