Mobile charge carriers are supposed to help superconductivity. The BCS mechanism requires electrons to move — pairing happens in momentum space, mediated by lattice vibrations. More mobility means better sampling of the Fermi surface, more phase space for pairing, stronger condensation.
In the doped Kitaev-Heisenberg ladder (arXiv:2603.12198), more mobility destroys pairing. The threshold is sharp: t/K < 0.65. Below it, doped holes develop dominant superconducting correlations in rung-singlet regions. Above it, the kinetic energy of the holes actively prevents pairing formation.
The mechanism is a competition between itinerant and localized physics. Kitaev spin liquids generate pairing through frustrated magnetic interactions — bonds that cannot simultaneously satisfy all their constraints, producing entangled ground states with topological order. Doped holes inherit this entanglement and pair. But only if they move slowly enough that the spin-liquid correlations survive their passage. Fast holes shred the magnetic texture they need for pairing.
This is not the usual “disorder kills superconductivity” story. The system is clean. The obstruction is kinetic: the very mobility that enables pairing in conventional superconductors prevents it here. The standard technique of increasing carrier mobility fails because it assumes pairing and transport share the same energy scale. In Kitaev systems, they compete.
Charge-density-wave order appears at weak doping near phase boundaries. Spin-density wave patterns emerge in other parameter regions. The phase diagram is rich precisely because the kinetic obstruction creates competition rather than simple suppression. Fast holes don't produce featureless metal. They produce a different kind of order — one incompatible with the pairing the system would otherwise prefer.