Quasiparticles in metals come in two familiar varieties. Gapped excitations require a minimum energy to create — insulators, superconductors, magnons above a spin gap. Gapless excitations exist at arbitrarily low energy — conduction electrons in metals, acoustic phonons, Goldstone modes. The gap, when it exists, is always in energy.
There is a third kind. Quasiparticles gapped in momentum.
In metals where translational symmetry is broken by impurities, certain excitations have a dispersion relation that becomes imaginary below a critical momentum. The quasiparticle energy is not merely gapped — it ceases to be real. Below the critical momentum, the excitation cannot exist as a propagating mode. It is not suppressed or broadened; it is structurally forbidden.
The authors identify Pines' demon — an acoustic plasmon recently observed in Sr₂RuO₄ — as exhibiting exactly this dispersion. The demon's observed energy-momentum relation deviated strongly from the expected massless linear behavior, and the explanation is the momentum gap: at long wavelengths, the mode drops into the imaginary regime and vanishes. The excitation exists only at short enough wavelengths, where the microscopic disorder cannot resolve it.
The gap opens in a dimension nobody was monitoring. Standard theory classifies excitations by their energy threshold. Momentum thresholds are invisible to energy-resolved spectroscopy — you need both energy and momentum resolution simultaneously to see the mode disappear as momentum decreases. The excitation looks gapless if you approach it from high momentum (energy smoothly approaches zero) but is absent if you look for it at low momentum.
Disorder does not merely blur the spectral function. It creates an entirely new classification: excitations that are forbidden from existing at long wavelengths. The gap is real, but it lives in the wrong variable.