In most conductors, resistance increases with temperature. Hotter material means more atomic vibrations, more scattering, harder passage for electrons. This is first-year physics. It is also wrong, in one precise limit.
In ultra-pure double-layer graphene, electrons collide with each other far more often than with impurities or lattice defects. This sounds like it should increase resistance — more collisions, more scattering, more impediment to current flow. But electron-electron collisions don't dissipate momentum the way electron-impurity collisions do. When two electrons scatter off each other, their total momentum is conserved. The pair changes direction, but the collective flow doesn't slow down. Only collisions with fixed objects — impurities, phonons, boundaries — remove momentum from the electron fluid.
When inter-electron collisions dominate, the electron gas transitions from a collection of independent particles to a fluid. The fluid has viscosity, pressure, and flow patterns. It obeys hydrodynamic equations, not Boltzmann transport theory. And in this regime, resistance decreases with temperature, because higher temperature means more electron-electron collisions, which means more efficient momentum redistribution, which means the fluid flows more smoothly around obstacles.
Researchers built a de Laval nozzle in graphene — the same converging-diverging geometry that accelerates rocket exhaust to supersonic speeds. The electron fluid entered the narrow throat, accelerated, and exited supersonically (relative to the electron fluid's own sound speed). Shock waves formed. The electrons were behaving not like particles in a semiconductor but like a compressible gas in an aerospace engine.
The condition for this regime is severe: the material must be pure enough that the mean free path for electron-impurity scattering exceeds the mean free path for electron-electron scattering by a large margin. In dirty materials, impurity scattering dominates, momentum dissipates, and the electron system behaves as particles. In clean materials, the hierarchy inverts, and collective behavior emerges. The transition is not gradual — it is a qualitative change in the physics.
What makes this finding structurally interesting is the inversion of the usual relationship between interaction and disorder. In conventional materials, interactions between electrons are a complication — they scatter, they disrupt, they heat. In this regime, the same interactions that normally scatter instead organize. Electron-electron collisions enforce collective behavior by redistributing momentum faster than boundaries can drain it. The interactions that create noise in one limit create coherence in another.
The practical implication is that material purity unlocks a fundamentally different transport regime. You don't get hydrodynamic electrons by adding something to the material. You get them by removing everything except the electrons themselves. The cleaner the material, the more the electrons interact with each other instead of with defects, and the more the system behaves like a fluid instead of a gas. The endpoint of purification is not particle physics made simpler. It is fluid dynamics emerging from particle physics.