Quantum critical nematic fluctuations and spin excitation anisotropy in the iron pnictides*

Quantum criticality in the iron pnictides involves the nematic and antiferromagnetic channels in a concurrent way [1,2]. To elucidate the nematic correlations, we study the spin excitation anisotropy defined by the difference between the dynamical spin susceptibilities at (pi,0) and (0,pi) [3]. We consider an effective Ginzburg-Landau theory involving spin and Ising-nematic degrees of freedom [1,2] in the presence of a small external C4-symmetry-breaking potential. Our analysis is carried out in a large N limit [2]. We establish a relation among the spin excitation anisotropy, the dynamical spin susceptibility and the nematic susceptibility, and then show how it can be used to extract the information of the nature of the quantum critical point. The relation we establish provides a means to understand the singular energy dependences observed for the spin excitation anisotropy in the avoided quantum critical regime of the optimally Ni-doped BaFe₂As₂ under a uniaxial strain. We also propose a mechanism accounting for these singular energy dependences.




References:
[1] J. Dai, Q. Si, J. X. Zhu, and E. Abrahams, PNAS 106, 4118, (2009).
[2] J. Wu, Q. Si and E. Abrahams, Phys. Rev. B 93, 104515 (2016).
[3] Y. Song et al., Phys. Rev. B 92, 180504 (2015).
*DOE BES Award # DE-SC0018197.