Experiment reveals quantum tug-of-war limiting higher-temperature superconductivity

Simons Foundation

New work helps show why the dream of high-temperature superconductivity has been so hard to realize: a quantum brawl between superconductivity and a competing state of matter.

The finding resulted from a collaboration that included theorists with the Simons Foundation’s Flatiron Institute in New York City and experimentalists at Quebec’s Institut national de la recherche scientifique (INRS).

The researchers investigated a copper alloy called a ‘cuprate’ as it transitioned from the superconducting state to an enigmatic neighboring state of matter called the ‘pseudogap.’ Using laser pulses and computer simulations, they were able to understand the behavior of electrons inside the cuprate.

In a paper published October 20 in Nature Physics, the team shows how the two states — superconductivity and the pseudogap — wrestle for dominance. That finding challenges the idea that superconductivity in cuprates merely builds upon a pre-existing pseudogap phase.

“We’re really showing that these states are competing,” says study co-author Olivier Gingras, an associate research scientist at the Flatiron Institute’s Center for Computational Quantum Physics (CCQ). If scientists could figure out a way to suppress the formation of the pseudogap but retain superconductivity, he says, “there’s a chance of raising the superconductivity temperature.”

Pushing the Limits

In the superconductive state, electrons — which normally repel one another — bind together, creating a smooth, continuous highway for electrical current. This means electrical current can travel with no energy loss, like a sled sliding over a frictionless sheet of ice. That’s immensely useful for transporting electricity from place to place. It also means superconductors can generate strong magnetic fields, which have their own applications, such as MRI machines.

“Superconductivity is often seen as a potential technological revolution,” says Gingras, who serves as one of the new paper’s corresponding authors.

But the superconducting temperature range is a major barrier. Superconductivity only occurs below a critical temperature. Even the cuprates with the highest critical temperatures need to be kept below 151 Kelvin, or about minus 188 degrees Fahrenheit.

Above that critical temperature, some types of cuprates enter the pseudogap state. Here, electrical current can move freely in some directions but not in others. Understanding how the pseudogap develops might illuminate ways to prevent it, thereby creating the potential for higher-temperature superconductivity.

“There’s a lot of research in order to push that critical temperature to higher temperatures,” Gingras says. “The higher it is, the more useful it is.”

As part of this effort, Gingras and his co-authors wanted to better understand the phase transitions that happen in cuprates near the critical temperature. “You have a lot of exotic phases that are very interesting,” Gingras says. “It’s not so clear what is the relationship between them.”

The team focused on the transition from superconductivity into the pseudogap. Researchers have a few theories for how cuprates manage this transition.

One theory holds that superconductivity and the pseudogap cannot exist simultaneously. Under this competing states theory, researchers can try to stave off the formation of the pseudogap. If they succeeded, the cuprate would remain a superconductor by default.

But it’s also possible that the states are symbiotic — that in the pseudogap state, electrons are bonding, but not strongly enough to form a superconductor. “It would be kind of a preamble to the superconducting state,” Gingras says. In that case, researchers might look for ways to coax coherence, and thus a superconductor, out of the pseudogap.

“Superconductivity is really the thing that we want to use for applications,” Gingras says. “The pseudogap is a hint that something’s happening that we can leverage.”

Probing the States

To find out, collaborators at the INRS’s Advanced Laser Light Source used an experimental technique called angle-resolved photoemission spectroscopy (ARPES). ARPES involves shining light on the cuprate, causing it to emit electrons. The experimenters can then measure the electrons’ energy and momentum. Those measurements reveal which states of matter are in play.

The researchers then probed the transitions between phases by using light to excite a cuprate superconductor into the pseudogap state. They then observed the material as it transitioned between the two phases. They also took separate measurements of the material at equilibrium, both in the superconducting state and the pseudogap state.

“We’re able to melt the superconducting phase and see how things recombine into the superconducting phase,” Gingras says. “That’s where we can really see the dynamics.”

Dario Armanno, a Ph.D. student working under the leadership of the INRS’s Fabio Boschini, led the work acquiring the experimental data. With the data in hand, Gingras and other CCQ researchers took over to figure out which theory best explained the experimentalists’ observations. The only model that fit was the “competing states” theory. At equilibrium, only one state could be present.

Putting this into practice is still far in the future, Gingras says. “This work introduces a framework to study these dynamics without having to rely on very expensive numerics, but there is still this question of having high-precision, first-principles calculations,” he says. Outside of equilibrium, those calculations are prohibitively difficult. “It’s very slow and painful work.”