A magnet levitating above a cuprate high temperature superconductor. Photo: Robert Hill/University of Waterloo.
A magnet levitating above a cuprate high temperature superconductor. Photo: Robert Hill/University of Waterloo.

New findings from an international collaboration led by Canadian scientists may eventually lead to a theory of how superconductivity initiates at the atomic level. This is a key step in understanding how to harness the potential of materials that could provide lossless energy storage, levitating trains and ultra-fast supercomputers.

David Hawthorn, Michel Gingras, Andrew Achkar and Zhihao Hao from University of Waterloo's Department of Physics and Astronomy have experimentally shown that electron clouds in superconducting materials can snap into an aligned and directional order called nematicity.

"It has become apparent in the past few years that the electrons involved in superconductivity can form patterns, stripes or checkerboards, and exhibit different symmetries – aligning preferentially along one direction," said Hawthorn. "These patterns and symmetries have important consequences for superconductivity – they can compete, coexist or possibly even enhance superconductivity."

Their results, published in Science, present the most direct experimental evidence to date of electronic nematicity as a universal feature in cuprate high-temperature superconductors. "In this study, we identify some unexpected alignment of the electrons - a finding that is likely generic to the high temperature superconductors and in time may turn out be a key ingredient of the problem," said Hawthorn.

Superconductivity, the ability of a material to conduct an electric current with zero resistance, is best described as an exotic state in high temperature superconductors – challenging to predict, let alone explain. The scientists used a novel technique called soft x-ray scattering at the Canadian Light Source synchrotron in Saskatoon to probe electron scattering in specific layers of the cuprate crystalline structure. Specifically, they probed the individual cuprate (CuO2) planes, where electronic nematicity takes place, versus the crystalline distortions in between the CuO2 planes.

Electronic nematicity occurs when the electron orbitals align themselves like a series of rods. The term ‘nematicity’ more commonly refers to the alignment of liquid crystals under an electric field in liquid crystal displays. In this case, however, it is the electronic orbitals that enter the nematic state as the temperature drops below a critical point.

Recent breakthroughs in high-temperature superconductivity have revealed a complex competition between the superconductive state and charge density wave order fluctuations. These periodic fluctuations in the distribution of the electrical charges create areas where electrons bunch up in high- versus low-density clouds, a phenomenon that is now recognized as being generic to the underdoped cuprates.

Results from this study show that electronic nematicity also likely occurs in underdoped cuprates. Understanding the relation between nematicity and charge density wave order, superconductivity and an individual material's crystalline structure could prove important for identifying the origins of the superconducting and so-called pseudogap phases.

The authors also found that the choice of doping material impacts the transition to the nematic state. Dopants such as strontium, lanthanum and even europium create distortions in the structure of the cuprate lattice that can either strengthen or weaken nematicity and charge density wave order in the CuO2 layer.

Although scientists do not yet understand why electronic nematicity occurs, it may ultimately present another knob to tune in the quest to achieve the ultimate goal of a room temperature superconductor. "Future work will tackle how electronic nematicity can be tuned, possibly to advantage, by modifying the crystalline structure," says Hawthorn.

This story is adapted from material from the University of Waterloo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.