A metal is a sea of electrons that move almost freely, colliding only occasionally. But in some materials, this sea suddenly becomes viscous and unpredictable: electrons stop behaving like independent particles. Physicists call this state 'non-Fermi liquid,' and it remains a puzzle.
In a new study, scientists found an unexpected culprit — crystal lattice vibrations that act on electric current rather than charge density. These vibrations are like waves in a crowd: if people sway in sync, they can pull their neighbors along. But there is a special regime where the wave decays too quickly, as if the crowd quenched its own motion through friction. Such 'overdamped' phonons (quanta of vibrations) can radically change electron behavior.
The authors calculated how electrons interact with such phonons in two-dimensional materials, such as twisted graphene. It turns out that depending on the sign of the orbital susceptibility (a quantity describing the electron response to a magnetic field), the behavior changes dramatically. If the susceptibility is positive, the usual Fermi liquid behavior persists only in a very narrow energy range, then gives way to unusual behavior. If it is negative, then even at the lowest energies electrons behave like a 'marginal Fermi liquid'—a state first proposed for high-temperature superconductors.
This finding is important because it does not require exotic conditions like proximity to a quantum critical point. Ordinary lattice distortions that create the needed phonons are sufficient. This means such behavior may occur in many materials, not just rare samples.
In twisted graphene, two layers of graphene are rotated relative to each other by a 'magic' angle of about 1.1 degrees. At this twist, electrons start moving very slowly, and their interaction with the lattice sharply increases, just what is needed for unusual phonons to appear.
In Greg Egan's novel 'Schild's Ladder', a universe is described where physical laws vary from place to place. Materials with non-Fermi liquid behavior could be a first step toward such 'islands of different physics' in our reality.