What seemed to be flaws in the structure of a mystery metal may have given physicists a glimpse into as-yet-undiscovered laws of the universe.
The qualities of a high-temperature superconductor — a compound in which electrons obey the spooky laws of quantum physics, and flow in perfect synchrony, without friction — appear linked to the fractal arrangements of seemingly random oxygen atoms.
Those atoms weren’t thought to matter, especially not in relation to the behavior of individual electrons, which exist at a scale thousands of times smaller. The findings, published Aug. 12 in Nature, are a physics equivalent of discovering a link between two utterly separate dimensions.
“We don’t know the theory for this,” said physicist Antonio Bianconi of Rome’s Sapienza University. “We just make the experimental observation that the two worlds seem to interfere.”
Unlike semiconductors, the metals on which modern electronics rely, superconductors allow electrons to pass through without resistance. Rather than bouncing haphazardly, the electrons’ movements are perfectly synchronized. They flow like a fluid, but without viscosity.
For most of the 20th century, this was possible only in certain extremely pure metals at temperatures approaching absolute zero, cold enough to quench all motion but that of quantum particles, which interact with each other in ways that defy the classic laws of space and time.
Then, in the mid-1980s, physicists Karl Muller and Johannes Bednorz discovered a class of ceramic compounds in which superconductivity was possible at much higher temperatures. The temperatures were still hundreds of degrees Fahrenheit below zero, but it wasn’t even thought possible.
Muller and Bednorz soon won a Nobel Prize, but subsequent decades and thousands of researchers have not yielded a theory of high-temperature superconductivity. “High temperatures should destroy the quantum phenomenon,” said Bianconi, who decided to investigate another odd property of these materials: They’re not quite regular. Oxygen atoms roam inside, and assume random positions as they freeze.
“Everyone was looking at these materials as ordered and homogeneous,” said Bianconi. That is not the case — but neither, he found, was the position of oxygen atoms truly random. Instead, they assumed complex geometries, possessing a fractal form: A small part of the pattern resembles a larger part, which in turn resembles a larger part, and so on.
“Such fractals are ubiquitous elsewhere in nature,” wrote Leiden University theoretical physicist Jan Zaanen in an accompanying commentary, but “it comes as a complete surprise that crystal defects can accomplish this feat.”
If what Zaanen described as “surprisingly beautiful” patterns were all Bianconi found, the results would have been striking enough. But they appear to have a function.
In Bianconi’s samples, larger fractals correlated with higher superconductivity temperatures. When the fractal disappeared at a distance of 180 micrometers, superconductivity appeared at 32 degrees Kelvin. When it vanished at 400 micrometers, conductivity went quantum at 42 degrees Kelvin.
At -384 degrees Fahrenheit, that’s still plenty cold, but it’s heading towards the truly high-temperature superconductivity that Bianconi describes as “the dream” of his field, making possible miniature supercomputers that run at everyday temperatures.
However, while the arrangement of oxygen atoms appears to influence the quantum behaviors of electrons, neither Bianconi nor Zaanen have any idea how that could be. That fractal arrangements are seen in so many other systems — from leaf patterns to stock market fluctuations to the frequency of earthquakes — suggests some sort of common underlying laws, but these remain speculative.
“This fractal defect structure is astonishing, and there is nothing in the textbooks even hinting at an explanation.”
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