Using Nuclear Quadrupolar Resonance, Combined with First Principles Calculations, to Investigate Spin Fluctuation Dynamics in Iron Pnictides

Magnetic and superconducting properties of Fe-based superconductors are linked to spin fluctuations. Long-range slow Mermin-Wagner fluctuations (MWF), due to continuous degeneracy of the Heisenberg Hamiltonian, are believed to play a key role in the “nematic” phase, but also in the paramagnetic one close to the transition, and in the reduction of the ordered moments compared to fluctuating local ones. It is crucial to understand how low in frequency MWF extend. A finite asymmetry of the electric field gradient was reported in Ba(Fe_1-xCo_x)₂As₂ even in the tetragonal phase [1]. We explain this odd result in terms of the MWF tail being slower than anticipated, so even at the typical nuclear quadrupole resonance (NQR) frequency ν = 2-3 MHz there remains a residual stripe order. This tail can be characterized by an effective moment M emerging from averaging spin fluctuations faster than ν. We calculate M as a function of T and Co x in Ba122 by comparing the experimental NQR data with modified DFT calculations with tunable Stoner interaction, both in the antiferro- and paramagnetic phase. We also compare the extracted M with those from neutron studies (i.e., averaged over all spin fluctuations).

[1] Toyoda et al, Phys. Rev. B 97, 174507 (2018).