A team of Canadian physicists has found that the phenomenon of charge ordering, which interferes with high-temperature superconductivity, is actually a series of stripes rather than checkered, ending a long-running debate over its shape. They also showed that charge ordering competes with superconductivity much more strongly in one direction than the other, increasing our knowledge of factors that drive or hinder superconductivity, and potentially leading to new approaches to engineering room-temperature superconductors.

To comprehend the effect of charge ordering, it is key to identify its shape, so the study, published in Science [Comin et al. Science (2015) DOI: 10.1126/science.1258399], explored whether the pattern of charge ordering using resonant x-ray scattering of very cold yttrium barium copper oxide to measure the two-dimensional structure factor in the superconductor in reciprocal space. This showed the pattern is striped, that the electrons self-organize only in one direction rather than two, providing a unifying picture of how charge order looks like in these materials.

They also found that the well-established competition between charge order and superconductivity is stronger for charge correlations across the stripes than along them, offering further evidence for the intrinsic unidirectional nature of the charge order. As lead author Riccardo Comin asks, “Is charge ordering just an anomaly, or is it there in all these systems because there is an underlying interaction which isn't completely removed from superconductivity? The two phenomena are competing but in a sense they're also interconnected.”

Charge ordering brings instability in various metals at temperatures greater than around –100oC, with the result that some electrons can reorganize into new periodical static patterns that compete with superconductivity. However, this could also be responsible for propelling electrons into the tight pairs that allow them to travel with no resistance. On the temperature cooling down sufficiently, the charge ordering tapers off allowing superconductivity to take over so that the electrons are able to move around freely with no resistance, and are no longer limited to a single dimension.

The knowledge that charge ordering has a striped pattern could have major implications for theories on superconductivity, helping to further refine research into the mystery of unconventional superconductivity. The team now intends to carry out more experiments in a bid to fully characterize how charge order behaves in other materials in their quest to understand ever-greater detail about the behavior of superconducting materials down to the nanoscale.