Quantum breakthrough sheds light on perplexing high-temperature superconductors

Superfast levitating trains, long-range lossless power transmission, faster MRI machines—all these fantastical technological advances could be in our grasp if we could just make a material that transmits electricity without resistance—or "superconducts"—at around room temperature.

In a paper published in Science, researchers report a breakthrough in our understanding of the origins of superconductivity at relatively high (though still frigid) temperatures. The findings concern a class of superconductors that has puzzled scientists since 1986, called "cuprates."

"There was tremendous excitement when cuprate superconductors were discovered [in 1986], but no understanding of why they remain superconductive at such high temperatures," says Shiwei Zhang, a senior research scientist at the Flatiron Institute's Center for Computational Quantum Physics (CCQ). "I think it's surprising to everybody that almost 40 years later, we still don't quite understand why they do what they do."

In the new paper, Zhang and his colleagues successfully recreated features of cuprate superconductivity with a simple model called the two-dimensional Hubbard model, which treats the materials as if they were electrons moving around a quantum chessboard. The breakthrough comes only a few years after the same researchers demonstrated that the simplest version of this model couldn't perform such a feat. Such straightforward models can spark a deeper understanding of physics, says study co-author Ulrich Schollwöck, a professor at the University of Munich.

"The idea in physics is to keep the model as simple as possible because it's difficult enough on its own," Schollwöck says. "So in the beginning we studied the simplest version imaginable."

In the new study, the researchers added to the 2D Hubbard model the ability for electrons to make diagonal hops, like bishops in chess. With this tweak and thousands of weeks-long simulations on supercomputers, the researchers' model captured the superconductivity and several other key features of cuprates previously found in experiments. By showing that the humble Hubbard model can describe cuprate superconductivity, the authors prove its worth as a platform for understanding why and how superconductivity emerges.

For most of the last century, physicists thought they understood why some materials superconduct. They thought that superconductivity only existed at extremely low temperatures below about minus 243 degrees Celsius (around 30 degrees above absolute zero). Such low temperatures require expensive cooling systems that use liquid helium.

When cuprates were discovered in 1986, they shocked the science world by superconducting at much higher temperatures. By the mid-1990s, scientists had discovered cuprates that remained superconductive up to around minus 123 degrees Celsius (about 150 degrees above absolute zero). Such temperatures can be reached using relatively cheap liquid nitrogen.

You can imagine a cuprate as a lasagna of copper oxide layers alternating with layers of other ions. (The name "cuprate" comes from the Latin word for copper.) Superconductivity arises when electricity flows with no resistance through the copper oxide layers. The simplest version of the 2D Hubbard model uses only two terms to picture each layer as a chessboard where electrons can hop north, south, east and west.