A Triangular Affair


the spin density wave (SDW) gap — the typical splitting of the metallic bands in the magnetic state observed in the tunnelling spectrum at low temperatures. Based on this concurrence, they propose that the fluctuations must have spin character, and occur on a scale typical of magnetic energies. Although the results of Rosenthal et al. are definitely exciting, several questions remain. First, what is the basic mechanism for the creation of these highly anisotropic electronic states that form around simple defects in NaFeAs? It was recently shown that, theoretically10, similar magnetic states can form and grow surprisingly large in the SDW state, and tend to have a dimer-like structure similar to that observed in CaFe2As2 (ref. 8). It remains to be seen, however, whether such states can survive in the nematic phase and at higher temperatures. In particular, it would be very interesting to find out whether such dimerlike electronic structures could do a better job of reproducing the NaFeAs QPI data than the impurity potentials considered by Rosenthal and colleagues and rejected as an explanation of their data. Second, how airtight is the identification of spin degrees of freedom as the driving mechanism for nematicity? In the Lee–Rice–Anderson approach11 used by Rosenthal and colleagues, the low-temperature SDW gap does not appear explicitly, so the only connection with the energy of the nematic response to impurity atoms occurs through its coincidence with the typical energy scale of a spin fluctuation. Because orbital ordering occurs at very similar temperatures, could it be that the energy peak of the nematic signal lying within the SDW energy gap is fortuitous? A resolution of this question requires a thorough theoretical calculation of the QPI signal in the fluctuating regime, including both spin and orbital degrees of freedom. Finally, what ultimately drives the large nematic susceptibility responsible for the various remarkable phenomena (magnetism, orthorhombic-to-tetragonal structural transition and enhanced nematic fluctuations) observed for NaFeAs, but absent in LiFeAs, a structurally similar superconductor? As is now commonplace in the field of iron-based superconductors, the answer seems to depend on details, but an understanding of these details could prove crucial for uncovering the origin of superconductivity and finding a recipe to increase the critical temperature. ❐

Cite this paper

@inproceedings{Engel2014ATA, title={A Triangular Affair}, author={Michael Engel and Sharon C. Glotzer}, year={2014} }