By Andrew Grant
A solid material that conducts electricity perfectly at certain temperatures may also qualify as an almost perfectly flowing liquid.
If the result is confirmed, the superconducting material would become the first nearly perfect fluid that isn’t among the hottest or coldest substances in the universe. The result also suggests a new way of deciphering a class of materials that could eventually whisk electricity around the power grid with no energy losses.
In 2002, researchers chilled a cloud of lithium atoms to less than a ten-millionth of a degree Celsius above absolute zero to create what’s called a Fermi gas. Three years later, a particle accelerator at Brookhaven National Laboratory in Upton, N.Y., slammed gold nuclei together to form a trillion-degree-C quark-gluon plasma, a substance thought to resemble matter in the universe just after the Big Bang.
The two substances couldn’t be any more different in terms of temperature — the Fermi gas is colder than outer space, the quark-gluon plasma hotter than the core of any star. But they share a surprising quality: Instead of resembling gases, both concoctions behave like liquids with exceptionally low viscosity, or resistance to flow. Scientists consider them nearly perfect fluids (SN: 4/25/09, p. 26).
Peter Johnson, a condensed matter physicist at Brookhaven, studies a seemingly unrelated class of materials called high-temperature superconductors. Such compounds conduct electricity with no resistance when chilled to temperatures that are well above absolute zero but still very cold. Yet Johnson says that Fermi gases, quark-gluon plasmas and high-temperature superconductors have an important trait in common: The interactions between their constituent particles are so strong that the particles behave collectively rather than individually. The collective behavior of electrons in high-temperature superconductors has baffled physicists, frustrating attempts to understand these materials and perhaps build wires that can shuttle electricity resistance-free at room temperature (SN: 10/18/14, p. 22).
Intrigued by a possible connection between collective behavior and a liquidlike state, Johnson measured the viscosity of a high-temperature superconductor, bismuth strontium calcium copper oxide. He and his colleagues injected light into the compound, which kicked out electrons. The researchers measured the properties of each exiting electron to determine how quickly it flowed through the material. Johnson’s team concludes October 13 in Physical Review B that the electron flow is speedy enough to call the superconducting compound a nearly perfect fluid at temperatures around –180˚ C.
But the experimental technique doesn’t sit well with Jan Zaanen, a theoretical condensed matter physicist at Leiden University in the Netherlands. “It’s a blunder,” he says. “It doesn’t make sense.” Measuring single electrons to estimate the flow of all the electrons is like measuring one water molecule to determine the flow of a river, he says.
Study coauthor Jonathan Rameau of Brookhaven defends the research. Each measurement provides a clue about what electrons encounter within the material, he says. “Even though we measure how fast this electron comes out, it actually gives us information about the medium from which the electron came.”
While also questioning the technique, Sean Hartnoll, a theoretical physicist at Stanford University, is more focused on the big picture. “This paper is a first, bold attempt to determine the viscosity of the electrons in a high-temperature superconductor,” he says. The work may encourage physicists to look at high-temperature superconductors as flowing liquids rather than simply electrical conduits, he says, which could help inspire ways to manipulate the materials for higher-temperature use.
