In the golden age of condensed matter physics, Feynman predicted that in sufficiently pure materials, electrons would behave like a fluid rather than as independent particles. Four decades later, this prediction was confirmed in materials such as graphene. But a new paper goes a step further: electron fluids cannot only 'flow' — they can behave like ketchup, exhibiting the exotic properties of non-Newtonian fluids.
Core Finding: The Electron 'Ketchup' Effect
One class of non-Newtonian fluids is shear-thickening — stirring fast makes them thicker and harder, while slow stirring lets them flow freely. When ketchup won't pour, a tap on the bottle works — because it liquefies under sudden shear stress.
The paper reports that in a two-dimensional electron fluid, when current density exceeds a threshold, the electron viscosity changes with the current gradient (the 'shear rate') — a hallmark of non-Newtonian behavior. The microscopic mechanism is local electron heating: large currents make electrons hotter in certain regions, altering their scattering rate and viscosity.
Even more striking: within a certain parameter range, the system exhibits bistability — at the same voltage, the current can settle into either a high or a low stable state. This produces an S-shaped current-voltage curve, with resistance switching and hysteresis loops.
Physical Picture
Imagine a narrow wire. At low voltage, the current grows linearly (normal ohmic behavior). But once the voltage exceeds a critical value, the electron fluid abruptly switches from a 'low-current, high-resistance' state to a 'high-current, low-resistance' state — like a glowing filament whose resistance suddenly drops. Crucially, this is not due to a change in the material itself, but to the hydrodynamic electron-electron interactions generating nonlinear collective behavior on their own.
This opens possibilities for new electronic devices — purely electronic switches and memory elements that do not need phase-change materials, relying only on the intrinsic bistability of the electron fluid itself.
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*Paper*: Flow bistability in non-Newtonian electron fluid / Afanasiev & Alekseev / arXiv:2605.12150 / cond-mat.mes-hall