If you were asked to picture how electrons move, you could be forgiven for imagining a stream of particles sluicing down a wire like water rushing through a pipe. After all, we often describe electrons as “flowing” in an “electric current.”
In reality, water and electricity flow in completely different ways. Whereas water molecules move together to form a swirly, coherent substance, electrons tend to fly past one another. “Water is seeing nothing but other water,” said Cory Dean, a physicist at Columbia University, “but in an electronic system, in a wire, that’s manifestly not the case.” Water molecules unite to flow, but each electron acts on its own.
This every-particle-for-itself movement serves as the foundation for all of electronic theory. It explains why a warm wire resists more than a cold wire, and why a round wire conducts as well as a square wire.
But since the 1960s, theorists have suspected that electrons can be coaxed to act more like their watery counterparts, and to form an electron fluid.
In recent years, a string of experiments has confirmed that prediction. Last fall, in the most dramatic demonstration yet, Dean and his collaborators arranged for electrons to form a type of shock wave that occurs when a quickly flowing fluid crashes into a slowly flowing fluid. It was a surefire sign that electrons were flowing at extremely high speeds. “That’s really the frontier right now,” said Thomas Scaffidi, a physicist at the University of California, Irvine who was not involved in the experiment.
Making electrons behave like water might someday lead to the development of new kinds of electronic devices. And extending the familiar theory of water to electrons could spawn a new way of thinking about quantum materials.
Thudding vs. Flowing
Andrew Lucas, a theoretical physicist at the University of Colorado, Boulder, compares electrons traveling down a wire to pinballs traveling around a pinball machine. Once they enter the playing field, pinballs bounce around in every direction, flying off flippers and bumpers. They travel up the machine, down the machine, and all around it. Similarly, when electrons in a copper wire collide with vibrating copper atoms or with “impurities” in the metal — spots where some other atom has usurped an atom of copper — they ricochet in all directions.
On average, pinballs do tend to travel farther down than up; in this sense they “flow” downward. Analogously, the “flow” of electrons emerges only in an average sense; an electric field, perhaps generated by a battery, establishes an ever-so-slightly preferred direction in the wire.
Cory Dean, a physicist at Columbia University, leads the lab that created the most dramatic recent demonstration of an electron fluid. Courtesy of Cory Dean
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