The good samarium

Replacing lanthanum with samarium boosts the critical temperature of the iron pnictide family of superconductors to 43 K

Credit: © 2008 NPG

The promise offered by the properties of superconductors is only matched by the frustration at how inaccessible they are. For some eighty years after their discovery, the highest temperature at which superconductivity (T_c) was observed was around 20 K. The discovery of copper oxide materials with T_c far exceeding that (up to 135 K) led to great excitement, but interest has dwindled after improvements reached a plateau. However, a new family of layered iron pnictide materials has been shown to superconduct at temperatures rivalling the early copper oxides.

The first material in the family to create interest, La[O_1-xF_x]FeAs, had¹ a T_c of 26 K, which could be pushed² to 43 K at high pressure (~4 GPa). Now, X. H. Chen and colleagues from the University of Science and Technology of China, Hefei, have made³ a samarium-containing analogue with a T_c of 43 K at atmospheric pressure. The material is SmFeAsO_1-xF_x and contains alternating layers of iron arsenide and samarium oxide, with the fluoride ions taking the place of around 15% of the oxide ions. The Fe₂As₂ layer, with the iron atoms tetrahedrally surrounded by arsenic, is thought to be the path through which current passes without resistance.

The standard model for describing superconductivity is the Bardeen–Cooper–Schreiffer theory, which predicts that the maximum T_c should be around 40 K. It is therefore clear that, like the cuprate family, the new iron pnictides do not fit the theory. Regardless of how they work, the race is on to find superconductors with ever higher T_c, with room temperature as the ultimate goal.