University of Connecticut physics researcher Ilya Sochnikov stands next to a dilution refrigerator in his lab. The findings by Sochnikov and his colleagues about how electrons behave in copper oxide superconductors may help scientists synthesize better high-temperature superconductors, with potential applications in transmission lines and magnetic trains. Photo: Sean Flynn/UConn.Physicists used to think that superconductivity – electricity flowing without resistance or loss – was an all-or-nothing phenomenon. But new evidence suggests that, at least in copper oxide superconductors, it's not so clear cut.
Superconductors have amazing properties, and in principle could be used to build loss-free transmission lines and magnetic trains that levitate above superconducting tracks. But most superconductors only work at temperatures close to absolute zero. This temperature, called the critical temperature, is often just a few degrees Kelvin and can only be reached using liquid helium, making these superconductors too expensive for most commercial uses.
A few superconductors, however, have a much warmer critical temperature, closer to the temperature of liquid nitrogen (77K), which is much more affordable. Many of these higher-temperature superconductors are based on a two-dimensional form of copper oxide known as cuprate.
"If we understood why copper oxide is a superconductor at such high temperatures, we might be able to synthesize a better one" that works at closer to room temperature (293K), says physicist Ilya Sochnikov at the University of Connecticut.
Together with colleagues at Rice University, the US Department of Energy's Brookhaven National Laboratory and Yale University, Sochnikov has now figured out part of that puzzle, as the team reports in a paper in Nature.
Their discovery concerns how electrons behave in copper oxide superconductors. Electrons are the particles that carry electric charge through our everyday electronics. When a bunch of electrons flow in the same direction, they generate an electric current. In a normal electric circuit, say the wiring in a house, electrons bump and jostle each other and the surrounding atoms as they flow. That wastes some energy, which leaves the circuit as heat. Over long distances, that wasted energy can really add up: long-distance transmission lines in the US lose on average of 5% of their electricity before reaching the consumer, according to the US Energy Information Administration.
In a superconductor below its critical temperature, however, electrons behave completely differently. Instead of bumping and jostling, they pair up and move in sync with the other electrons in a kind of wave. If electrons in a normal current are a rushing, uncoordinated mob, electrons in a superconductor are like dancing couples, gliding across the floor. It's this friction-free dance – coherent motion – of paired electrons, known as Cooper pairs, that makes a superconductor what it is.
The electrons are so happy in pairs in a superconductor that it takes a certain amount of energy to pull them apart. Physicists can measure this energy with an experiment that measures how big a voltage is required to tear an electron away from its partner; this is known as the 'gap energy'. The gap energy disappears when the temperature rises above the critical temperature and the superconductor changes into an ordinary material. Physicists assumed this is because the electron pairs have broken up. And in classic, low-temperature superconductors, it's pretty clear that is what happens.
But Sochnikov and his colleagues wanted to know whether this was also true for copper oxides, which behave a little differently. Even when the temperature rises well above the critical level, the energy gap in copper oxides persists for a while, diminishing gradually. It could be a clue as to what makes them different.
The researchers set up a version of the gap energy experiment to test this. They made a precise sandwich comprising two slices of copper oxide superconductor, each just a few nanometers thick, separated by a thin filling of electrical insulator. They then applied a voltage between them. This caused electrons to begin tunneling from one slice of copper oxide to the other, creating a current.
By measuring the noise in that current, the researchers found that a significant number of the electrons seemed to be tunneling in pairs instead of singly, even above the critical temperature. Only about half the electrons tunneled in pairs, and this number dropped as the temperature rose, but it tapered off only gradually.
"Somehow they survive," Sochnikov says, "they don't break fully." He and his colleagues are still not sure whether the paired states are the origin of high-temperature superconductivity, or whether it's a competing state that the superconductor has to win out over as the temperature falls. But either way, their discovery puts a constraint on how high temperature superconductors work.
"Our results have profound implications for basic condensed matter physics theory," says co-author Ivan Bozovic, group leader of the Oxide Molecular Beam Epitaxy Group in the Condensed Matter Physics and Materials Science Division at Brookhaven National Laboratory and professor of applied physics at Yale University.
Sochnikov agrees. "There's a thousand theories about copper oxide superconductors. This work allows us to narrow it down to a much smaller pool. Essentially, our results say that any theory has to pass a qualifying exam of explaining the existence of the observed electron pairs." He and his collaborators now plan to tackle the remaining open questions by designing even more precise materials and experiments.
This story is adapted from material from the University of Connecticut, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.