Probing the symmetry of superconductivity at the atomic level

Friday, February 26, 2016

Dave Hawthorn and the resonant soft x-ray scattering equipmentElectron orbitals can exhibit surprising and unexpected patterns in superconducting cuprates, according to the latest finding by Waterloo physicists Professor David Hawthorn and doctoral student Andrew Achkar.

Their results could have implications for understanding how competing states arise, behave and dissipate in superconducting materials, eventually helping scientists to create more practical superconductors that can operate at ambient temperatures.

The electron orbitals were observed as part of a Charge Density Wave order, a quantum state that appears to suppress superconductivity, but also weakens when superconductivity finally emerges. Their results were published this month in the journal Nature Materials.

“Until three years ago, we never considered a scenario like this. Charge density wave orders were just oscillations in the charge. But now we see there’s much more texture,” says Hawthorn.

The work appears on the heels of another paper on the properties of superconducting cuprates that was published by the same group last month in the prestigious journal Science. In both papers, Hawthorn’s group used a novel technique called resonant soft x-ray scattering, located at the Canadian Light Source synchrotron in Saskatoon.

Superconductivity and Charge Density Wave Order

The strange and spectacular phenomenon known as superconductivity occurs when certain materials cooled below a critical temperature are able to conduct electricity with zero resistance.

With conventional superconductors, this critical temperature ranges between 11 and 39 K (−262° and −234° Celsius). If superconductivity worked at more ambient temperatures (0°C or higher) it would open the way for technologies such as lossless energy storage, levitating trains and ultra-fast supercomputers.

That’s why physicists worldwide are focusing their attention on superconducting cuprates. These copper oxide ceramics doped with additional elements can achieve a superconductive state at relatively high temperatures (−140° Celsius) – a discovery that instantly earned Dr. Johannes Bednorz and Prof. Karl Müller the Nobel Prize in Physics in 1987.

Yet our understanding of how these unconventional superconductors work at the atomic level remains challenging to predict, let alone explain.

For example, cuprates transition through a series of competing quantum states prior to entering the superconducting state.

One competing state Hawthorn and his group have been particularly interested in is the Charge Density Wave order, which appears during the mysterious pseudogap phase and dissipates just above the critical superconducting temperature. Under certain conditions, Charge Density Wave order has been shown to supress superconductivity.

Hawthorn and Achkar’s recent Science paper has also shown cuprates can exhibit nematicity, a property where electron orbitals line up with a directional order separate from the symmetry of the crystalline lattice.

“How all these phenomenon are intertwined and relate to one another is not quite clear,” says Hawthorn. “Time will tell if this is the key or an added complication.”

Soft x-ray scattering – the key to probing electron orbital states

Developed in consultation with Hawthorn and Prof. George Sawatzky of the University of British Columbia, Canadian Light Source’s resonant soft x-ray scattering instrument is known as one of the world’s best for probing electron orbital states within ordered electron “superstructures.”

“It’s one of the jewels of Canadian Light Source,” says Hawthorn. “Amongst other high impact publications, it’s produced five Science papers and three Nature Materials papers since 2012, a superb achievement.”

Similar to how x-ray diffraction is used to determine the locations of atoms in crystalline structures, resonant soft x-ray diffraction (RSXD) can be used to study the location, orbital, spin and charge of electrons in complex materials like cuprates under superconducting and near-superconducting states.

RSXD works by using x-ray energies at special resonant frequencies to excite electrons from a core state into an unoccupied orbital. The absorption lines are individual to both the element and the orbital direction, making it several orders of magnitude more sensitive than conventional x-ray diffraction.

So sensitive that it can probe the electron states of the individual two-dimensional layers of copper and oxygen atoms that make up the cuprate structure, as well as other elements (such as Yttrium and Barium) doped into the lattice.

This research was supported by the Canadian Foundation for Innovation, the Canadian Institute for Advanced Research and the Natural Sciences and Engineering Research Council of Canada.

Andrew Achkar graduated with his doctoral degree from the University of Waterloo’s Department of Physics and Astronomy last December and currently works for Miovision Technologies in Kitchener, Ontario.

The authors also include collaborators from Canadian Light Source, the Leibniz Institute for Solid State and Materials Research, Brookhaven National Laboratory, the University of British Columbia, the Canadian Institute for Advanced Research, and the University of Salzburg.

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