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

Quantum criticality in iron pnictides involves nematic and antiferromagnetic degrees of freedom in a concurrent way [1,2], but the relationship between the two types of fluctuations has yet to be clarified. 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 start from an effective Ginzburg-Landau theory for both the Ising-nematic and antiferromagnetic fluctuations in the presence of a small external uniaxial potential, which breaks the C4-symmetry in B1g channel, and establish an identity that connects the spin excitation anisotropy with the dynamical magnetic susceptibility and static nematic susceptibility. Using this identity, we show that the dynamical nematic susceptibility in the quantum critical regime can be determined, and we illustrate it by considering the case of the optimally Ni-doped BaFe2As2 [3].



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, X. Lu, D. L. Abernathy, D. W. Tam, J. L. Niedziela, W. Tian, H. Luo, Q. Si, and P. Dai, Phys. Rev. B 92, 180504 (2015).