Vibronic Coupling, Jahn–Teller Instability, and the Emergence of Spontaneously Broken Symmetry

Spontaneous symmetry breaking (SSB) in molecular and condensed‑matter systems is shown to originate fundamentally from electronic degeneracy and vibronic coupling. Because nuclear configurations are governed by electron–vibrational interactions, any structural transformation necessarily involves more than one electronic state, making vibronic coupling the essential mechanism that links electronic structure to nuclear geometry. Within this framework, the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects provide the microscopic origin of instabilities in high‑symmetry configurations.

Although the adiabatic potential energy surface (APES) is widely used to rationalize symmetry‑lowering distortions, it becomes physically meaningful only when vibronic coupling is sufficiently strong to localize nuclear motion away from degeneracy points. The observability of symmetry breaking depends on the competition between tunneling dynamics and the probe's temporal resolution. A fast measurement can “catch the system” in a particular well, revealing a distorted configuration, whereas a slow probe averages over many tunneling events and reports an apparently high‑symmetry structure.

An important part of this presentation is the emergence of SSB in crystals, where local vibronic instabilities become cooperative. In perovskites such as BaTiO₃, the collective interaction of many PJT‑active centers deepens the effective wells on a macroscopic scale, suppresses tunneling, and produces a genuine phase transition characterized by long‑lived broken symmetry. This cooperative behavior links local vibronic physics to macroscopic-order parameters.

Finally, conceptual parallels are drawn between vibronic SSB and the Higgs mechanism in particle physics, emphasizing that degeneracy (in a broad sense, exact or approximate) is the universal prerequisite for symmetry breaking across atomic, crystalline, and field‑theoretic systems.