14 April 2020

A Superconductor Full of Surprises

Physicist Nick Butch discovered a Lazarus-like form of superconductivity in uranium ditelluride. Physicist Nick Butch discovered a Lazarus-like form of superconductivity in uranium ditelluride. F. Webber/NIST

High magnetic fields usually kill superconductivity. But in this material, it brought it back to life.

Story by KRISTEN COYNE

Scientists have been chasing superconductivity for more than a century. It's the ideal form of electricity, a state in which electrons travel through a material with perfect efficiency. The more materials that researchers discover with this amazing property and the better they understand how they work, the closer we come to a long dreamed of superconducting revolution that could dramatically lower energy costs worldwide and liberate us from fossil fuels.

Nick ButchPhysicist Nick Butch

Physicist Nick Butch of the National Institute of Standards and Technology has been studying one such material: uranium ditelluride or UTe2. Last fall, in papers published in Science and Nature Physics, Butch and his colleagues revealed intriguing and promising properties of UTe2.

"This is a very recently discovered superconductor with a host of other unconventional behaviors," said Butch. "There's something different going on in there."

In Science they reported that UTe2's superconductivity involved an unusual electron configuration called spin triplets, in which pairs of electrons are aligned in the same direction. Eager to dig deeper, Butch brought UTe2 to the National High Magnetic Field Laboratory, where he pushed it even further.

Generally, magnetic fields are an enemy of superconductivity: When strong enough, they bust apart the electron pairs that are responsible for superconductivity. At the MagLab, the team tested UTe2 in gradually stronger fields to see at which point the magnet would crush superconductivity. They also adjusted the angle at which the UTe2 crystals were positioned vis-à-vis the direction of magnetic field.

In addition to the superconductivity they had previously observed at lower fields, the team observed that higher fields did indeed kill that superconductivity. But when they kept increasing the field and adjusting the sample orientation, they saw superconductivity come back to life in UTe2, a Lazarus-like effect called reentrant superconductivity. And the team observed this not just once, but twice.

Specifically, at about 16 teslas x A tesla is a unit of magnetic field strength. A typical fridge magnet has a field of about 0.01 tesla. , the material's superconducting state abruptly changed: While it died in most of the experiments, it persisted when the crystal was aligned at a very specific angle in relationship to the field. This behavior continued until about 35 teslas. At that point, not only did all superconductivity vanish, but the electrons shifted their alignment in the field, entering a new magnetic phase.

 
Magnetic field versus angle dependence of the superconducting phases in UTe<sub>2</sub>
 

Magnetic field versus angle dependence of the superconducting phases in UTe2. The green area indicates the unusual "Lazarus superconductivity" discovered by Butch and his team.
Credit: Nick Butch

 

But as the researchers ramped the field even higher while continuing to fiddle with angles, they found that a different orientation of the crystal to the field yielded yet another superconducting phase, one that persisted to at least 65 teslas, the maximum field tested. It was a record-busting performance for a superconductor and an unprecedented finding: the first time two field-induced superconducting phases have been found in the same compound. Instead of killing superconductivity, high fields appeared to stabilize it.

While it's not yet clear exactly what is happening at the atomic level, Butch said the evidence points to a phenomenon fundamentally different than anything scientists have seen to date. Said Butch, "It is sufficiently different, I think, to expect it will take a while to figure out what's going on."

Last modified on 16 April 2020