Pseudogap theory puts physicists closer to high temperature superconductors

The theory explains the transition phase to superconductivity, or “pseudogap” phase, which is one of the last obstacles to developing the next generation of superconductors and one of the major unsolved problems of theoretical condensed matter physics.

Their work was published in this week’s issue of the prestigious journal Science.

Superconductivity is the phenomenon where electricity flows with no resistance and no energy loss. Most materials need to be cooled to ultra-low temperatures with liquid helium in order to achieve a superconductive state.

The team includes Professor Roger Melko, Professor David Hawthorn and doctoral student Lauren Hayward from Waterloo’s Physics and Astronomy Department, and Harvard Physics Professor Subir Sachev. Roger Melko also holds a Canada Research Chair in Computational Quantum Many-Body Physics.

“This amazing scientific collaboration actually came about by chance over lunch at the Perimeter Institute between Subir and myself,” said Hawthorn.

Hawthorn showed Sachdev his latest experimental data on a superconducting material made of Copper and the elements Yttrium and Barium. The material, YBa₂Cu₃O_6+x, had an unexplained temperature dependence. Sachdev had a theory but needed expert help with the complex set of calculations to prove it. That’s where Melko and Hayward stepped in and developed the computer code to solve Sachdev’s equations.

Melko and Sachdev already knew each other through Perimeter Institute, where Melko is an associate faculty member and Sachdev is a Distinguished Research Visiting Chair.

“The results all came together in a matter of weeks,” said Melko. “It really speaks to the synergy we have between Waterloo and Perimeter Institute.”

To understand why room-temperature superconductivity has remained so elusive, physicists have turned their sights to the phase that occurs just before superconductivity takes over: the mysterious “pseudogap” phase.

“Understanding the pseudogap is as important as understanding superconductivity itself,” said Melko.

The cuprate, YBa₂Cu₃O_6+x, is one of the few materials known to be superconductive at higher temperatures, but scientists are so far unable to achieve superconductivity in this material above -179°C. This new study found that YBa₂Cu₃O_6+x oscillates between two quantum states during the pseudogap, one of which involves charge-density wave fluctuations. These periodic fluctuations in the distribution of the electrical charges are what destabilize the superconducting state above the critical temperature.

Once the material is cooled below the critical temperature, the strength of these fluctuations falls and the superconductivity state takes over.

Superconducting magnets are currently used in MRI machines and complex particle accelerators, but the cost of cooling materials using Helium makes them very expensive. Materials that achieve superconductivity at a higher temperature could unlock the technology for new smart power grids and advanced power storage units.

The group plans to extend their work both theoretically and experimentally to understand more about the fundamental nature of cuprates.

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Nick Manning
University of Waterloo
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