The temperatures are rising—and now, so are the pressures. The recent discovery of superconductivity in layered compounds based on iron and nickel has lead to a flurry of work with these materials, including the discovery that substituting fluorine for oxygen in LaOFeAs leads to a superconducting transition temperature (Tc) of ~ 26 K. Now, scientists in Japan1 show that applying pressure increases the Tc of LaO1-xFxFeAs to 43 K.

Commenting on the importance of the work, lead author Hiroki Takahashi of Nihon University in Tokyo, says it provides evidence that, like their cuprate cousins, these materials may be considered high-temperature superconductors.

At the superconducting transition, the resistance of a material drops to zero and the material expels external magnetic fields. Takahashi and colleagues measured both of these signatures of superconductivity in polycrystalline samples of LaO1-xFxFeAs over a range of pressures from 0 to 30 GPa—about 300,000 times atmospheric pressure. They found that Tc increased with pressure up to 4 GPa—reaching a maximum of 43 K—and then decreased.

Fig. 1: The layered crystal structure of the superconductor LaO1-xFxFeAs (the F dopants substitute for oxygen in the LaO layers). Applying high pressure increases the superconducting transition temperature from 26 to 43 K, most likely because additional negative charge moves into the FeAs layers when the material is compressed.

Takahashi suggests that the effect of pressure is related to the crystal structure of LaO1-xFxFeAs, which consists of alternating layers of LaO and FeAs (Fig. 1). The LaO layers are believed to carry a slight positive charge, which is balanced by a negative charge on the FeAs layers. Substituting F (fluorine) for oxygen increases the number of mobile carriers, which are believed to reside in the FeAs layers, and increases Tc. Pressure may have a similar effect because it pushes adjacent layers closer together, which can also increase the number of mobile carriers in the FeAs layers and hence, Tc.

Research into these materials has been moving extremely rapidly in the past year. Theorists in particular are trying to understand the origin of ‘high-Tc’ superconductivity and compare it with cuprate or phonon-mediated superconductors. The results from this work, combined with other efforts on increasing Tc in these Fe and Ni based superconductors, are essential steps in this direction.