Can light spark superconductivity? A new study reignites debate

The previous research showed a temporary change in the reflectivity of cuprates, compounds containing copper and oxygen, when blasted with light. That change indicated a drop in resistance lasting mere trillionths of a second, or picoseconds. Critics argued that the change could be caused by effects other than superconductivity. 

The new study claps back. One cuprate expels magnetic fields when hit with light, physicist Andrea Cavalleri and colleagues report July 10 in Nature. That expulsion, they say, is a hallmark of superconductivity known as the Meissner effect (SN: 7/6/15).

The observation is “basically an unmistakable signature of superconductivity,” says physicist Dmitri Basov of Columbia University, who was not involved with the research.

Not everyone is so convinced by the new work. “They’re seeing this change that lasts for [about] a picosecond, and it’s not immediately obvious that it’s the same thing as the Meissner effect,” says physicist Steve Dodge of Simon Fraser University in Burnaby, Canada.

Superconductors attract intense interest from physicists, in part because of their technological potential. A superconductor that operates at high temperatures could allow for more efficient power transmission, for example, potentially saving vast amounts of energy. And mysteries still shroud the phenomenon. Cuprates are superconducting at higher temperatures than most, and it’s still not fully understood why.

Scientists knew that light could disrupt superconductivity, but the idea that light could also birth it was unexpected and controversial. And in previous studies, “things were a bit subjective, they kind of ‘smelled’ like a superconductor but … you couldn’t really be sure,” says Cavalleri, of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg.

So Cavalleri and colleagues set their sights on the Meissner effect. They studied a type of cuprate called yttrium barium copper oxide, or YBCO. That’s a class of compounds that had previously shown signs of light-induced superconductivity. 

But precisely measuring magnetic field changes over picoseconds is no easy feat. “No existing technique allows you to do this measurement,” Cavalleri says.

The team devised a scheme that used a crystal of gallium phosphide placed next to the YBCO to measure magnetic fields. In experiments performed within a preexisting magnetic field, the researchers hit the YBCO with the laser, and sent a second laser through the crystal. The trip through the crystal changed the laser’s polarization — the orientation of its electromagnetic waves — in a way dictated by the magnetic field within the crystal. That effect allowed the team to determine how the magnetic field changed near the YBCO as it was bombarded with light at temperature normally above the YBCO’s superconducting limit. 

If the YBCO became a superconductor, it would expel magnetic fields from within due to the Meissner effect. That would result in a stronger magnetic field at the YBCO’s edge, which is exactly what the team found. The measurements had to be made extremely quickly to capture the short-lived Meissner effect, Basov says. “This is a brilliant concept and brilliant execution.” 

Physicist Nan-Lin Wang of Peking University is convinced that magnetic fields are expelled when the laser pulse hits the YBCO. But whether that implies superconductivity as it is normally defined is unclear. It might be the result of preexisting, small-scale superconducting currents being amplified, rather than of typical large-scale superconductivity. “The underlying physics could be very complicated,” he says.

But Dodge contends that something other than superconductivity could be responsible. At high intensities of light, he notes, complex and unexpected phenomena can occur. “I would like to see … some careful scrutiny to ensure that they’re not mistaking some other effect for a Meissner effect.” What, exactly, is behind the change in the magnetic field is not clear, Dodge says. While he’s still skeptical of the superconductivity claim, he says “it’s a worthwhile experiment because it raises some questions that I certainly don’t know the answer to.”