Good (next-nearest) neighbors

While the study of high temperature superconductivity was once dominated by the cuprate family of compounds, the landscape has looked a little different since 2008 and the discovery of iron based superconducting materials. These new materials can be divided into two classes: the iron pnictides (parent compound CaFe₂As₂) and the iron chalcogenides (parent Fe_1+yTe). Despite the differences in composition, these materials have more in common than just their superconductivity, such as their band structures and magnetic excitations.

 

Now researchers in the US, China and the UK have found another link between these compounds which may reveal a common magnetic origin for the superconductivity [Lipscombe et al., Phys Rev Lett (2011) 106, 057004]. While the magnetic structures of the parent compounds are different, researchers have found commonalities that are impossible to ignore.

 

The team studied the non-superconducting parent Fe_1.05Te, using “time-of-flight inelastic neutron spectroscopy to determine the spin-wave excitations” [Lipscombe et al.], utilizing the Spallation Neutron Source and High Flux Isotope Reactor at Oak ridge National Laboratory and the ISIS spallation source in the UK. This technique allows spin wave excitations to be mapped out by monitoring the change in energy of an incident neutron beam as it interacts with the spin wave. As the energy of the incident beam is determined by the speed of the neutrons, the energy of the beam can be determined by measuring the “time of flight” between source and sample. Due to the weak interaction of neutrons with matter the team has to coalign 6 g of single crystal samples.

 

Magnetic ions interact with each other through so-called exchange interactions: electrostatic interactions that can only be understood through quantum mechanics. The researchers have found that while the magnetic structures and interactions between magnetic neighbors are different in the two compounds, the anti-ferromagnetic interactions between next-nearest neighbors (NNNs) are nearly identical. The isotropic nature of the exchange suggests that the interaction between magnetic NNNs is mediated through another ion, in a process known as superexchange. Their results contradict theoretical calculations which predict that the NNN interactions in the chalcogenide should be anisotropic. The authors suggest this difference may be due to the complicated orbital configuration in the material, or due to the effects of itinerant magnetism.