Mercury-containing oxides offer new perspective on mechanism of superconductivity

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The Dutch physicist Heike Kamerlingh Onnes discoveredsuperconductivityone hundred years ago, when he noticed that theelectrical resistanceof mercury dropped to zero suddenly at 4.2 K.Superconducting materialsare now used routinely inmagnetic resonance imagingscanners.

In classical superconductors such as mercury, superconductivity arises through the combined vibrations of the atoms in the crystal. This makes the crystal structure a key factor for the superconducting properties of a material. In the case of Hg_xReO₃, theatomic structureconsists of rhenium (Re) and oxygen (O) building blocks. In the empty spaces between them, the mercury atoms arrange in chains (Fig. 1). However, some of the available places along these chains lack mercury atoms, and the team’s work suggests that this leads to an arrangement of paired mercury atoms.

"These pairs move within the channel in an oscillatory motion known as rattling", explains team-member Ayako Yamamoto from the RIKEN Advanced Science Institute in Wako. The rattling vibrations provide a strong feedback for the electrons, and therefore reinforce superconductivity in the material. In comparison to a similar structure lacking mercury pairs, the superconducting temperature of Hg_0.44ReO₃at 7.7 K is almost twice as high."Despite remaining below the present record of 135 K for a superconductor, there is potential for improving operation temperatures", says Yamamoto.“The application of pressure increases the superconducting temperature to 11.1 K, and this could mean that for the right crystal structure further enhancement is possible.”

Yamamoto and her colleagues are now working to optimize the crystal structure further—for example, by replacing rhenium with other elements. A better understanding of the influence of the mercury atoms’ rattling motion may also provide better insight into the mechanism of superconductivity in such structures.“Mercury seems to be a magic element in superconductivity, not only for its role in Kamerlingh Onnes’ discovery, but also for the fact that mercury is part of the material with the highest known superconducting temperature, HgBa₂Ca₂Cu₃O_x,” Yamamoto explains."Once more,mercuryis playing a key role for new superconductors,"she says.

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