Electricity Seen Flowing Like Liquid in Strange Metals

Flicking the switch on any kind of electrical device triggers a marching band of charged particles stepping to the beat of the circuit’s voltage.

But a new discovery in exotic materials known as strange metals has found electricity doesn’t always move in step, and can in fact sometimes bleed in a way that has physicists questioning what we know about the nature of particles.

The research was carried out on nanowires made of a precise balance of ytterbium, rhodium, and silicon, (YbRh₂Si₂).

By conducting a series of quantum measurement experiments on these nanowires, researchers from the US and Austria have uncovered evidence that could help settle a debate over the nature of electrical currents in metals that don’t behave in a conventional fashion.

Discovered late last century in a class of copper-based compounds known for having no resistance to currents at relatively warm temperatures, strange metals become more resistant to electricity as they heat up, just like any other metal.

Only they do so in a rather odd fashion, increasing in resistance by a set amount for every degree of temperature rise.

In normal metals, resistance varies depending on temperature, plateauing out once the material gets hot enough.

This contrast in the rules of resistance suggests currents in strange metals don’t operate in quite the same way. For some reason, the way charge-carrying particles in strange metals interact with the jostle of surrounding particles differs to the pinball slalom of electrons in your average strip of wire.

What we might picture as a current of negatively-charged spheres rolling through a tube of copper atoms is a little more complicated. Electricity is a quantum affair, after all, with the characteristics of a number of particles harmonizing to behave like single units known as quasiparticles.

Whether the same kinds of quasiparticles explain the unusual resistance behaviors of strange metals has been an open question, with some theories and experiments suggesting such quasiparticles may lose their integrity under the right circumstances.

To clarify whether there is a steady march of quasiparticles in the flow of electrons in strange metals, the researchers made use of a phenomenon called shot noise.

If you could slow time to a crawl, the photons of light emitted by even the most precise laser would pop and sputter with all of the predictability of sizzling bacon fat. This ‘noise’ is a feature of quantum probability, and can provide a measure of the granularity of charges as they flow through a conductor.

“The idea is that if I’m driving a current, it consists of a bunch of discrete charge carriers,” says senior author Doug Natelson, a physicist at Rice University in the US.

“Those arrive at an average rate, but sometimes they happen to be closer together in time, and sometimes they’re farther apart.”

The team found measures of shot noise in their super-thin sample of YbRh₂Si₂ was highly suppressed in ways that typical interactions between electrons and their environment couldn’t explain, suggesting quasiparticles probably weren’t at play.

Instead the charge was more liquid-like than currents in conventional metals, a finding that supports a model proposed more than 20 years ago by contributing author Qimiao Si, a condensed matter physicist from Rice University.

Si’s theory on materials approaching zero degree temperatures describes the way electrons within select locations no longer share characteristics that would allow them to form quasiparticles.

While conventional quasiparticle behavior can be tentatively ruled out, the team isn’t entirely certain of what form this ‘liquid’ current takes, or even if it might be found in other strange metal recipes.

“Maybe this is evidence that quasiparticles are not well-defined things or that they’re just not there, and charge moves in more complicated ways. We have to find the right vocabulary to talk about how charge can move collectively,” says Natelson.

This research was published in Science.