Seeking Higher Critical Temperatures for Superconductors

We are reliant on electricity to perform most of our daily tasks. As electricity moves through the wires and cables that transmit the electrical current, some of the energy is lost as heat due to resistance in the materials. While some materials conduct electricity with less resistance, superconductors do so without any energy loss at all. One caveat is superconductors require very low temperatures, below what is called the critical temperature, to function and this quality makes them impractical for wider use.

The record critical temperature under normal pressure is currently about minus 140 degrees C, a long way off from room temperature. However, in a paper recently published in Physical Review Letters, scientists, including UConn Department of Physics researcher and assistant professor Pavel Volkov, and Zhaoyu Han and Steven A. Kivelson from Stanford University, detail a mechanism that may help scientists develop superconductors that can perform at higher temperatures.

Volkov explains that superconductivity was discovered in 1913, but the first theory of the phenomenon was published in 1957 by Bardeen, Cooper, and Schrieffer who later received the Nobel Prize in physics in 1972. They attributed superconductivity to electrons pairing up, allowing these pairs to move through the material without dissipation. In the theory of Bardeen, Cooper, and Schrieffer (BCS), the electron pairs are held together via the vibrations and distortions of the material. Most metals consist of positively charged ions arranged in a lattice while negatively charged electrons can move through it, conducting electricity. As they move, they perturb the lattice because the ions are attracted to the electron.

“The deformations created by one electron are felt by the others, and like two bowling balls on a trampoline, electrons prefer to get closer to one another, eventually pairing up,” Volkov says. “This is one of the important preconditions for superconductivity to occur, and BCS have shown that this happens because of this interaction between electrons and the surrounding lattice.”

Scientists have been studying superconductivity and its theoretical consequences ever since, says Volkov, and the next question is how high can we make the critical temperature?

“What matters is the interaction between electrons and the lattice. If we make it stronger and stronger and stronger, can we [at least theoretically] get as high-temperature superconductivity as we wish? The answer is, unfortunately no, because when this interaction becomes too strong, the electrons have to carry with them very large deformations of the lattice, becoming very sluggish and ineffective at carrying current, which suppresses the critical temperature,” Volkov says.

One possible way out is to turn to other mechanisms of pairing electrons up. In this research, Volkov explains they have shown another way that electrons interact with the lattice that results in much faster electron pairs at extremely strong interaction strength, overcoming the difficulty.

“The conventional way to think about the interaction between the electron and lattice is through the deformations created by electrons,” Volkov says. “However, ions in a crystal are also quantum objects – they exist in multiple locations simultaneously.  We considered the impact of a passing electron on this quantum uncertainty of the ion’s positions. This quantum effect does lead to electron pairing and because there is no real lattice distortion occurring, the electrons do not get as heavy.”

Volkov says it can help to imagine the ion position as “smeared” due to quantum fluctuations. When an electron passes by, suddenly, the ion should become more localized, which is felt by other electrons, creating the same “balls on a trampoline” effect.

“These electrons want to stick together, and the average positions of the ions remain the same, it’s just this uncertainty of the position that changes, so there is much less of a problem for the electrons to drag this perturbation around. The lattice is not deforming strongly, it’s just the change of this quantum mechanical wave functions of ions, therefore, the pairs of electrons do not become as heavy. What we hope for is that this will allow us to get a much higher transition temperature,” Volkov says.

Researchers showed that with this mechanism, the coupling can be optimized by nanoscale engineering of the lattice.

“We have not only made model prediction, but also found out that using some techniques that have recently become of prominence due to the discovery of the twisted bilayer of graphene and analogous materials, that if you create superlattices, you can actually make that coupling quite a bit stronger,” Volkov says.

They show that depending on the period of the superlattice, the interactions of electrons with the ions can become stronger.

“There’s a sweet spot where the interaction strength is maximized,” Volkov says. “This is not so for the conventional electron lattice interaction that would go down as you increase the period of the superlattice. The same holds for the repulsive Coulomb interaction between electrons that are bad for superconductivity, they will also go down as you increase the superlattice period. That’s good because we don’t want this repulsion to hinder us.”

They believe a promising material to make superlattices is strontium titanate, but no superlattices of this material have been made yet to test the theory. This material has a fascinating history of its own, when it comes to superconductivity, which was first observed in strontium titanate 1964 but a theoretical consensus on its origin has not yet been reached. Two groups have recently shown that the interaction mechanism explored in the present work appears to be a reasonable candidate to explain it, Volkov says.

“We have yet to prove that it causes superconductivity in strontium titanate, but there is experimental evidence that this interaction is there and is fairly strong,” Volkov says. “Our current work is partially motivated by strontium titanate because now we know that that material has that kind of coupling.”

However, critical temperature of strontium titanate is extremely low, about minus 273 degrees C. If a superlattice on the surface of strontium titanate is created, Volkov says, they predict the transition temperature can be tuned to a much higher value.

“While it’s still probably not going to be higher than the current high-temperature superconductor record holder right now, demonstrating the potential of this coupling mechanism in a concrete material may motivate the search for materials where this coupling can be even stronger, and maybe we can get a transition temperature that is even higher,” Volkov says. “According to our results, it should be easier to realize truly high-temperature superconductivity with this mechanism than with the conventional one. But for now, we’re in a stage of finding just one material where we would know that this works.”