Non-Fermi-liquid metals — materials where the quasiparticle picture breaks down — typically require fine-tuning. Approach a quantum critical point, lower the temperature precisely, and the spectral function broadens beyond recognition. Away from the critical point, Fermi liquid theory recovers. The breakdown is fragile.
Gauge phonons make it generic.
Ordinary phonons couple to electronic density — they modulate the local potential that electrons feel. Gauge phonons couple to electronic currents — they modulate the effective vector potential, acting like a transverse gauge field. The distinction matters enormously. Density coupling preserves the quasiparticle: it shifts energy levels but doesn't destroy coherence. Current coupling does not preserve the quasiparticle: it dresses the electron with a fluctuating gauge field, and gauge fields famously destroy sharp excitations through infrared divergences.
The paper shows that overdamped gauge phonons — lattice vibrations with the wrong symmetry to couple to density, forced instead to couple to currents — produce non-Fermi-liquid behavior without any tuning. No quantum critical point required. The orbital susceptibility determines the character of the breakdown: its sign selects between marginal-Fermi-liquid behavior and stronger deviations. But both regimes are non-Fermi-liquid. Standard quasiparticle theory fails generically once the coupling channel is transverse.
The insight reframes why some metals are strange. The question is not “what special condition drives this metal to criticality?” but “what symmetry forces the phonon-electron coupling to be transverse?” The answer is crystallographic — it depends on the lattice symmetry and orbital content, not on proximity to any phase transition.
How symmetry-breaking couples to matter — density versus current, longitudinal versus transverse — determines whether the outcome is conventional or exotic. The mechanism of coupling, not its magnitude, is the decisive variable.