Can light cause superconductivity? New study reignites debate

Previous studies have shown that the reflectivity of cuprates, compounds containing copper and oxygen, changes temporarily when exposed to light. This change indicates that the resistance decreases for only a trillionth of a second, or picosecond. Critics have argued that this change could be caused by effects other than superconductivity.

New research strikes back. 1 cup emit a magnetic field When hit by light, physicist Andrea Cavalleri and colleagues reported on July 10. nature. They say that this expelling is a feature of superconductivity known as the Meissner effect.Serial Number: July 6, 2015).

“This observation is fundamentally a signature of superconductivity,” says Columbia University physicist Dmitry Basov, who was not involved in the study.

Not everyone is so convinced by the new work. “They see this change as ongoing. [about] “It’s picoseconds, and it’s not immediately clear whether this is something like the Meissner effect,” says Steve Dodge, a physicist at Simon Fraser University in Burnaby, Canada.

Superconductors are of intense interest to physicists, partly because of their technological potential. Superconductors operating at high temperatures could save enormous amounts of energy, for example, by allowing more efficient power transmission. And there are still mysteries surrounding this phenomenon. Cuprates are superconductors at higher temperatures than most, and the reasons for this are not yet fully understood.

Scientists knew that light could destroy superconductivity, but the idea that light could create it was unexpectedly controversial. And in previous studies, “things were a bit subjective, you ‘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 Cavallari and his colleagues turned to the Meissner effect. They studied a type of cuprate called yttrium barium copper oxide, or YBCO, a class of compounds that had previously shown signs of light-induced superconductivity.

But accurately measuring magnetic field changes on a picosecond scale is no easy task. “Existing technologies can’t do these measurements,” says Cavallari.

The team devised a way to measure magnetic fields using a gallium phosphide crystal placed next to YBCO. In experiments conducted in a conventional magnetic field, the researchers hit YBCO with a laser and then sent a second laser through the crystal. Traveling through the crystal changed the polarization of the laser (the direction of the electromagnetic wave) in a way determined by the magnetic fields inside the crystal. This effect allowed the team to determine how the magnetic field near YBCO changed when it was bombarded with light at temperatures generally above the superconducting limit of YBCO.

If YBCO were to become a superconductor, it would radiate magnetic fields from within due to the Meissner effect. This would then produce stronger magnetic fields at the edges of the YBCO, which is exactly what the team found. Basov says that the measurements had to be done very quickly to capture the short-lived Meissner effect. “It’s a great concept and a great execution.”

Nanlin Wang, a physicist at Peking University, is convinced that when laser pulses hit YBCO, magnetic fields are released. But it is unclear whether this suggests superconductivity as it is usually defined. It could be the result of amplification of existing small-scale superconducting currents, not typical large-scale superconductivity. “The underlying physics can be very complicated,” he says.

But Dodge argues that something other than superconductivity could be the cause. He points out that complex and unexpected phenomena can occur at high intensities of light. “I would like to see them carefully investigate other effects to make sure they don’t mistake the Meissner effect for something else.” Dodge says it’s not clear exactly what’s behind the magnetic field changes. He remains skeptical of the superconductivity claim, but says, “It’s a worthwhile experiment because it raises some questions that I don’t know the answers to.”