Physicists closer to high temperature superconductors

[rhoenix]

Physicists are one step closer to developing the world's first room-temperature superconductor thanks to a new theory from the University of Waterloo, Harvard and Perimeter Institute.

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, YBa2Cu3O6+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, YBa2Cu3O6+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 YBa2Cu3O6+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.

This is a small advance to be sure, but considering the field, it appears to be an important step along the path to achieving room-temperature superconductors.

[rhoenix]

Another article, with results from a different group of scientists:

From super-lubricants, to solar cells, to the fledgling technology of valleytronics, there is much to be excited about with the discovery of a unique new two-dimensional semiconductor, rhenium disulfide, by researchers at Berkeley Lab's Molecular Foundry. Rhenium disulfide, unlike molybdenum disulfide and other dichalcogenides, behaves electronically as if it were a 2D monolayer even as a 3D bulk material. This not only opens the door to 2D electronic applications with a 3D material, it also makes it possible to study 2D physics with easy-to-make 3D crystals.

"Rhenium disulfide remains a direct-bandgap semiconductor, its photoluminescence intensity increases while its Raman spectrum remains unchanged, even with the addition of increasing numbers of layers," says Junqiao Wu, a physicist with Berkeley Lab's Materials Sciences Division who led this discovery. "This makes bulk crystals of rhenium disulfide an ideal platform for probing 2D excitonic and lattice physics, circumventing the challenge of preparing large-area, single-crystal monolayers."

Wu, who is also a professor with the University of California-Berkeley's Department of Materials Science and Engineering, headed a large international team of collaborators who used the facilities at the Molecular Foundry, a U.S Department of Energy (DOE) national nanoscience center, to prepare and characterize individual monolayers of rhenium disulfide. Through a variety of spectroscopy techniques, they studied these monolayers both as stacked multilayers and as bulk materials. Their study revealed that the uniqueness of rhenium disulfide stems from a disruption in its crystal lattice symmetry called a Peierls distortion.

"Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces," says Sefaattin Tongay, lead author of a paper describing this research in Nature Communications for which Wu was the corresponding author. The paper was titled "Monolayer behaviour in bulk ReS2 due to electronic and vibrational decoupling."

"Typically the monolayers in a semiconducting transition metal dichalcogenides, such as molybdenum disulfide, are relatively strongly coupled, but isolated monolayers show large changes in electronic structure and lattice vibration energies," Tongay says. "The result is that in bulk these materials are indirect gap semiconductors and in the monolayer they are direct gap."

What Tongay, Wu and their collaborators found in their characterization studies was that rhenium disulfide contains seven valence electrons as opposed to the six valence electrons of molybdenum disulfide and other transition metal dichalcogenides. This extra valence electron prevents strong interlayer coupling between multiple monolayers of rhenium disulfide.

"The extra electron is eventually shared between two rhenium atoms, which causes the atoms to move closer to one another other, forming quasi-one-dimensional chains within each layer and creating the Peierls distortion in the lattice," Tongay says. "Once the Peierls distortion takes place, interlayer registry is largely lost, resulting in weak interlayer coupling and monolayer behavior in the bulk."

Rhenium disulfide's weak interlayer coupling should make this material highly useful in tribology and other low-friction applications. Since rhenium disulfide also exhibits strong interactions between light and matter that are typical of monolayer semiconductors, and since the bulk rhenium disulfide behaves as if it were a monolayer, the new material should also be valuable for solar cell applications. It might also be a less expensive alternative to diamond for valleytronics.

In valleytronics, the wave quantum number of the electron in a crystalline material is used to encode information. This number is derived from the spin and momentum of an electron moving through a crystal lattice as a wave with energy peaks and valleys. Encoding information when the electrons reside in these minimum energy valleys offers a highly promising potential new route to quantum computing and ultrafast data-processing.
"Rhenium atoms have a relatively large atomic weight, which means electron spin-orbit interactions are significant," Tongay says. "This could make rhenium disulfide an ideal material for valleytronics applications."

The collaboration is now looking at ways to tune the properties of rhenium disulfide in both monolayer and bulk crystals through engineered defects in the lattice and selective doping. They are also looking to alloy rhenium disulfide with other members of the dichalcogenide family.

A different approach, but one also showing fruit.