Frontiers Physics World  May 2018

Quasicrystal seen to superconduct

Cool stuff Diffraction pattern from the first superconducting quasicrystal alloy. (Keiichiro Imura, Noriaki K Sato and Tsutomu Ishimasa)

Superconductivity has been observed for the first time in a quasicrystal – a solid with atoms arranged in an ordered pattern, but without any translational symmetry. The discovery was made by Keisuke Kamiya and Noriaki Sato at Nagoya University in Japan and colleagues, who created the quasicrystal from an alloy of aluminium, zinc and magnesium. They found it conducts without resistance when cooled below 0.05 K.

The researchers began with their alloy of aluminium, zinc and magnesium in an “approximant crystal” phase, which is a bit like a quasicrystal, but has a lattice that repeats in space. They then reduced the aluminium content of the alloy while keeping the magnesium content almost constant. In doing so, the critical temperature marking the onset of superconductivity fell gradually from 0.8 K to about 0.2 K.

When the alloy had just 15% aluminium, it transformed into a quasicrystal. The specific heat of the material jumped dramatically when cooled below 0.05 K, while the magnetic flux inside the material was almost entirely blocked – both signs that a transition to a superconducting phase had occurred. The team says that this “extraordinarily low” critical temperature explains why it had previously been difficult to observe superconductivity in quasicrystals.

The conventional theory of superconductivity says it is due to correlated electrons forming “Cooper pairs” that flow without resistance. Closer inspection of the properties of the quasicrystal superconductor, however, suggests that the Cooper pairs arise from the weak coupling of electrons. Although relatively uncommon, weak coupling is seen in other materials including the approximant crystal phase of the alloy in the study. Sato says this similarity could mean that the observed superconductivity is not related to the quasicrystalline nature of the alloy – but is rather “dirty superconductivity” that occurs in imperfect crystals.

However, he adds, the theory of quasicrystals also predicts another form of superconductivity, based on fractal geometry in quasicrystals. “There is a strong possibility that fractal superconductivity makes at least some contribution, and we would be excited to finally measure it,” Sato says. The team is now examining the interplay between this fractal geometry and the weakly coupled electron pairs (Nature Comms 9 154).

Sam Jarman