In familiar metals like copper, electrons move almost independently, like cars on an open highway. Resistance arises from rare collisions and can be described by simple formulas. But there is a whole class of materials, from cuprate ceramics to twisted graphene, where above the superconducting transition temperature, everything goes haywire. There, electrons seem to hit rush hour: everyone is bumping into everyone else, and motion turns into chaotic jostling.
This state is called a strange metal. Its resistance grows linearly with temperature, and the time between electron collisions becomes so short that it violates the so-called Mott-Ioffe-Regel limit — the boundary beyond which conventional metal theory breaks down. The authors of the new study built a model where the key role is played not by the electrons themselves, but by their interaction with quantum fluctuations — the trembling of charge or spin 'ripples.' These fluctuations are randomly distributed in space, like magnets scattered across a table.
The calculations showed that in such a system, resistance and electron lifetime are no longer tightly linked. The Boltzmann formula, which works in ordinary metals, breaks down here, as if the time spent in traffic jam no longer depended on the number of cars. Moreover, the model predicts the violation of another universal relation — between the viscosity of the electron fluid and its entropy (a measure of disorder). In ordinary fluids, this ratio has a lower bound, but here it vanishes.
This work is not just abstract theory. It provides a microscopic recipe for describing quantum-critical incoherent metals, from which superconductivity emerges upon cooling. Understanding the strange metal phase could be the key to creating superconductors that work at room temperature — materials that would revolutionize energy transmission and transportation.
as written by the authors arXiv:2605.03013
Numerous unconventional superconductors such as cuprates, heavy-fermions, and twisted-bilayer graphene exhibit incoherent metallic transport above the superconducting critical temperature. This phenomenon cannot be described with Fermi-liquid theory and has presented a significant theoretical challenge to overcome. We utilize the two-dimensional Yukawa-SYK model of fermions with spatially random coupling to quantum-critical bosons to study transport in a manner which is non-perturbative in the coupling strength. Our work provides a microscopic model of quantum-critical incoherent metals and their concomitant properties, including a non-Boltzmann transport formula between resistivity and quasi-particle lifetime, violation of the Mott-Ioffe-Regel resistivity bound, and violation of the Kovtun-Son-Starinets shear viscosity to entropy density bound.
The Mott-Ioffe-Regel limit can be imagined as a situation where the electron's mean free path becomes shorter than the distance between atoms. At that point, the electron seems to forget where it is, and the classical picture of motion falls apart.
In Peter Watts's novel Blindsight, alien life is built on unusual conducting states where the familiar laws of solid-state physics don't apply — somewhat reminiscent of strange metals.