Can light cause superconductivity? New research reignites debate

Previous work had shown that shining light on cuprates, compounds containing copper and oxygen, causes a temporary change in their reflectivity that reduces their resistance for just a trillionth of a second, or picoseconds. Critics argued that the change could have been caused by effects other than superconductivity.

New research refutes: One cuprate Eliminate magnetic fields Physicist Andrea Cavaleri and his colleagues discovered that when exposed to light, NatureThey say this emission is a hallmark of superconductivity known as the Meissner effect.SN: July 6, 2015).

The observation is “basically unequivocal evidence of superconductivity,” says Dmitri Basov, a physicist at Columbia University who was not involved in the work.

Not everyone is convinced by the new study. “They’re seeing this change last for a long time. [about] “It’s a phenomenon that occurs on the order of picoseconds, so it’s not immediately obvious that it’s the same as the Meissner effect,” says physicist Steve Dodge of Simon Fraser University in Burnaby, Canada.

Superconductors are of intense interest to physicists, in part because of their technological potential. Superconductors operating at high temperatures could, for example, enable more efficient power transmission and save enormous amounts of energy. And the phenomenon remains mysterious: cuprates superconduct at higher temperatures than most materials, but the reasons for this are not yet fully understood.

Scientists knew that light could inhibit superconductivity, but the idea that light could create it was unexpected and controversial, and in previous studies “things were a bit subjective; we could ‘smell’ something like superconductivity, but … we couldn’t really be sure,” says Cavaleri of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg.

So Cavaleri and his colleagues turned to the Meissner effect. They studied a type of copper oxide called yttrium barium copper oxide (YBCO), a class of compounds that previously showed signs of light-induced superconductivity.

But precisely measuring changes in magnetic fields on a picosecond time scale is no easy feat: “The technology that allows you to make these measurements doesn’t exist,” says Cavaleri.

The team devised a way to measure magnetic fields using a crystal of gallium phosphide placed next to YBCO. In experiments conducted in an existing magnetic field, the researchers shone a laser on the YBCO and passed a second laser through the crystal. As it passed through the crystal, the laser’s polarization (the direction of the electromagnetic waves) changed in a way that was determined by the magnetic field within the crystal. This effect allowed the team to determine how the magnetic field near the YBCO changes when it is illuminated with light at temperatures that are typically above YBCO’s superconducting limit.

When YBCO becomes a superconductor, the Meissner effect causes magnetic fields to radiate from within. As a result, the magnetic field should be stronger at the edges of the YBCO, which is exactly what the team found. The measurements had to be made very quickly to capture the short-lived Meissner effect, Basov says. “It’s a brilliant concept and brilliant execution.”

Nanlin Wang, a physicist at Peking University, is convinced that when a laser pulse hits YBCO, it emits a magnetic field. But it’s unclear whether that means superconductivity as it’s usually defined. This could be the result of an amplification of existing small-scale supercurrents, rather than typical large-scale superconductivity. “The underlying physics could be very complicated,” he says.

But Dodge argues that something other than superconductivity may be to blame. High light intensities can lead to complex and unexpected phenomena, he says. “I would encourage people to look carefully and make sure they’re not confusing the Meissner effect with other effects.” It’s unclear exactly what’s behind the magnetic field changes, Dodge says. He remains skeptical of the superconductivity claims, but says it’s “worth experimenting with because it raises some questions that we just don’t know the answers to.”