The Core Finding: An Electron "Ketchup" Effect
Feynman once predicted, during the golden age of condensed matter physics, that in sufficiently clean materials electrons would behave like a fluid rather than independent particles. Four decades later, that prediction was confirmed in materials such as graphene. This paper goes a step further: electron fluids cannot only "flow"—they can also behave like ketchup, showing the exotic properties of a non-Newtonian fluid.
Non-Newtonian fluids include a class called "shear-thickening" fluids—stir them fast and they thicken and harden; stir slowly and they flow freely. When ketchup won't pour, you smack the bottom of the bottle—because it liquefies under a sudden shear stress.
The paper finds that in a two-dimensional electron fluid, when the current density exceeds a certain threshold, the electron viscosity changes with the current gradient (the "shear rate")—classic 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.
The 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 "low current, high resistance" to "high current, low resistance"—like a filament whose resistance drops once it glows. The key point: the material's properties do not change. Instead, the hydrodynamic interactions between electrons themselves generate the nonlinear collective behavior.
This opens possibilities for new electronic devices—purely electronic switches and memory elements that need no phase-change materials, relying only on the 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