Synopsis: How to make CuO sit up straight

CuO in thin-film form could be a prototype material for exploring magnetism that is similar to what is found in high-temperature superconductors.
Synopsis figure
Illustration: W. Siemons et al., Phys. Rev. B (2009)

The parent compounds of cuprate high-temperature superconductors are typically antiferromagnets where the magnetic interaction between the spins on the copper sites is unusually large (100meV or >1000K). Since they may play a role in the superconducting mechanism, researchers have explored similarly large magnetic interactions in other copper-oxide compounds.

Moving from left to right on the periodic table, CuO is the last member of the transition metal rock-salt series that includes MnO, FeO, CoO, and NiO. Except for CuO, each of these oxides has a cubic structure, like salt, where the transition metal ion is surrounded by six oxygen ions. From MnO to NiO, the antiferromagnetic (Néel) transition temperature, which scales with the magnetic interaction between the spins on the transition metal sites, increases from 100 to 500K. Following this trend, CuO should have a Néel temperature as high as 900K, but in bulk form, CuO has a low-symmetry, distorted rock-salt structure and a transition temperature of only 200K.

Wolter Siemons and colleagues at the University of Twente in The Netherlands and collaborators at Stanford University in the US report in Physical Review B that they have succeeded in using pulsed laser deposition to grow thin films of CuO with a structure that is an elongated (tetragonal) version of its rock-salt cousins.

While Siemons et al. have determined the structure with extensive crystallography, magnetic measurements will be necessary to determine if the magnetic interactions in this tetragonal form of CuO compare with those of the high-temperature superconducting oxides. – Jessica Thomas


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