Giant Bandgap Pulsation Driven by Hotspot Breathing Phonons in a Flat-Band Solid

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The electronic bandgap of solids is conventionally viewed as a static property at a given temperature, with only weak and stochastic thermal fluctuations under equilibrium conditions. Here, using ab initio molecular dynamics and first-principles electron-phonon calculations, we reveal a pronounced room-temperature bandgap pulsation at about 7.8 THz in a perovskite-like flat-band solid, with a maximum peak-to-peak variation approaching 0.95 eV. This behavior originates from a dual selection mechanism: the A1g-like breathing branch couples much more strongly to the flat conduction-band edge than other phonon branches, while real-space phase selectivity distinguishes its hotspot gamma-point and finite-q components. Although finite-q modes retain appreciable microscopic coupling, their intercell phase shifts produce smaller-amplitude shorter-recurrence-period responses, leaving the unit-cell-synchronous gamma-point A1g component to dominate the fundamental-period bandgap pulsation. The resulting band-edge dynamics further modulates the optical response on femtosecond timescales. These findings demonstrate that an unexpectedly ordered electronic response can emerge from intrinsically disordered thermal lattice fluctuations.