Canadian physicists discover new properties of superconductivity

Physicists worldwide are on a quest to understand the secrets of superconductivity because of the exciting technological possibilities that could be realized if it could be achieved closer to room temperatures. In conventional superconductivity, materials that are cooled to nearly absolute zero (−273.15 Celsius) exhibit the fantastic property of electrons pairing up and being able to conduct electricity with practically zero resistance. If superconductivity worked at higher temperatures, it could have implications for creating technologies such as ultra-efficient power grids, supercomputers and magnetically levitating vehicles.

New superconductivity findings published in journal Science

The new findings from an international collaboration, led by Waterloo physicists David Hawthorn and Dr. Andrew Achkar, along with CLS Scientists Feizhou He and Ronny Sutarto, present direct experimental evidence of what is known as electronic nematicity – when electron clouds snap into an aligned and directional order – in a particular type of high-temperature superconductor. The results, published in the prestigious journal Science, may eventually lead to a theory explaining why superconductivity occurs at higher temperatures in certain materials.

“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, a professor in Waterloo’s Department of Physics and Astronomy. 
The findings show evidence that electronic nematicity is a universal feature in cuprate high-temperature superconductors. Cuprates are copper-oxide ceramics composed of two-dimensional layers or planes of copper and oxygen atoms separated by other atoms. They are known as the best of the high-temperature superconductors. In the 1980s, materials that exhibit superconductivity under somewhat warmer conditions (but still -135 Celsius, so far from room temperature) were discovered. But how superconductivity initiates in these high-temperature superconductors has been challenging to predict, let alone explain.

“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,” says Hawthorn. “These patterns and symmetries have important consequences for superconductivity – they can compete, coexist or possibly even enhance superconductivity.”

Scientists use resonant soft X-ray scattering in superconductivity research

The scientists used a novel technique called resonant soft x-ray scattering to probe electron scattering in specific layers in the cuprate crystalline structure. Specifically, they looked at the individual cuprate (CuO2) planes where electronic nematicity takes place, versus the crystalline distortions in between the CuO2 planes.

Electronic nematicity happens when the electron orbitals align themselves like a series of rods. The term nematicity commonly refers to when liquid crystals spontaneously align under an electric current in liquid crystal displays. In this case, the electron orbitals enter the nematic state as the temperature drops below a critical point.

Cuprates can made to be superconducting by adding elements that will remove electrons from the material, a process known as “doping.”  A material can be optimally doped to achieve superconductivity at the highest and most accessible temperature, but in studying how superconductivity happens, physicists often work with material that is “underdoped,” which means the level of doping is less than the level that maximizes the superconducting temperature.

Results from this study show electronic nematicity likely occur in all underdoped cuprates.

Physicists also want to understand the relation of nematicity to a phenomenon known as charge density wave order. Normally, the electrons are in a nice, uniform distribution, but charge-ordering can cause the electrons to bunch up, like ripples on a pond. This sets up a competition, whereby the material is fluctuating between the superconducting and non-superconducting states until the temperature cools enough for the superconductivity to win.

Future work will tackle how electrons can be tuned for superconductivity

Although there is not yet an agreed upon explanation for 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 our advantage, by modifying the crystalline structure,” says Hawthorn.

Prof. Hawthorn is a Fellow of the Canadian Institute For Advanced Research.

Feizhou He and Ronny Sutarto are staff scientists on the REIXS beamline at the CLS synchrotron in Saskatoon, University of Saskatchewan campus.

A. J. Achkar, M. Zwiebler, Christopher McMahon, F. He, R. Sutarto, Isaiah Djianto, Zhihao Hao, Michel J. P. Gingras, M. Hücker, G. D. Gu, A. Revcolevschi, H. Zhang, Y.-J. Kim, J. Geck, D. G. Hawthorn. "Nematicity in stripe-ordered cuprates probed via resonant x-ray scattering”. Science. 5 Feb 2016 : 576-578. DOI:10.1126/science.aad1824

 

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