Superconductors are metals whose electrical resistance vanishes below a certain ‘critical temperature’ Tc. This remarkable phenomenon was first observed in 1911, so 2011 was the 100th anniversary. Before 1986 the highest known Tc of any superconductor was only 18 degrees above absolute zero, or a chilly minus 255 Celsius. Despite such low operating temperatures, superconductors are widely used to provide high magnetic fields for magnetic resonance imaging and other applications.

The material we have studied, LiFeAs, is a member of a new family of iron-based superconductors discovered in 2008. Compounds in this family tend to develop antiferromagnetic order, and this property is believed to offer a possible route to higher Tc. In the case of LiFeAs, however, theoreticians had predicted an anomalous tendency for ferromagnetism. In our experiment we probed the atomic-scale magnetic dynamics in LiFeAs directly by neutron spectroscopy and found an antiferromagnetic tendency after all. We also found that the magnetic signal increased on cooling below Tc, providing clear evidence that antiferromagnetic fluctuations in LiFeAs are intimately linked to superconductivity.

AE Taylor, MJ Pitcher (University of Oxford), RA Ewings, TG Perring (ISIS), SJ Clarke, AT Boothroyd (University of Oxford)

Research date: August 2011

Further Information

Contact: Professor AT Boothroyd, a.boothroyd@physics.ox.ac.uk
Further reading: AE Taylor et al., Phys. Rev. B 83 (2011) 220514(R)​