Iron-arsenide work points to novel superconductors

Physicists at the US Department of Energy’s Ames Laboratory have experimentally demonstrated that the superconductivity mechanism in the recently-discovered iron-arsenide superconductors is unique compared to all other known classes of superconductors.

The scientists claimed the finding – combined with iron-arsenide’s potential good ability to carry current due to their low anisotropy – may open a door to exciting possible applications in zero-resistance power transmission.

The research, led by Ames Laboratory physicist Ruslan Prozorov, has shown that electron pairing in iron-arsenides is likely to be very different to that of other types of known superconductors. In superconducting materials, electrons form pairs, called Cooper pairs, below a critical temperature and these electron pairs behave identically.

However, superconductors also have another inherent characteristic that distinguishes them from a perfect metal. Unlike perfect metals, superconductors expel a weak magnetic field from their interiors no matter whether they are cooled in a magnetic field or whether the magnetic field is applied after cooling. In either case, a weak magnetic field penetrates only a narrow region at a superconductor’s surface. The depth of this region is known as the London penetration depth.

“The change of the London penetration depth with temperature is directly related to the structure of the so-called superconducting gap, which in turn depends on the microscopic mechanism of how electron pairs are formed,” said Prozorov. “London penetration depth is one of the primary experimentally measurable quantities in superconductor studies.”

The variation of the London penetration depth with temperature depends on the superconducting gap structure and is already generally agreed upon in most other known classes of superconductors. In conventional superconductors – the class made up of periodic table elements, including lead and niobium – this dependence is exponential at low temperatures. In the high-temperature cuprate superconductors, the relationship is linear, and in magnesium-diboride superconductors the dependence is exponential, but requires two distinct superconducting gaps to explain the data in a full temperature range.

In contrast, the Ames Laboratory research group, found that iron-arsenide superconductors exhibit a power-law – almost quadratic – temperature variation of penetration depth. The team’s results were published in recent issues of Physical Review Letters and Physical Review B: Rapid Communications.

The iron-arsenide superconductors’ power-law variation of London penetration depth was observed across several FeAs-based systems. The Ames Lab group studied large single crystals of barium-iron-arsenic in which cobalt was substituted for part of the iron. They also studied neodymium-iron-arsenic-oxide and lanthanum-iron-arsenic-oxide samples.

As London penetration depth is tied to electron-pairing behaviour, the Ames Lab group’s findings suggest that the iron-arsenides also exhibit electron pairing different from any other known superconductor.

“The iron-arsenides are probably among most complex superconductors we – the superconductor research community – have encountered so far,” said Prozorov. “Altogether, analysis of the data collected on many samples shows that the iron-arsenides do not adhere to the previous superconductivity theories and that something else is happening. Of course, some theoretical models do exist, and we collaborate with leading theorists, including Ames Laboratory’s Jörg Schmalian, who has provided important insight into our observations. The unique qualities of the iron-arsenides cause me to believe that materials where transition temperature is closer to room temperature are possible.”

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