New Investigations of Iron Pnictides Using Zero-Field Nuclear Magnetic Resonance

In Zero-Field Nuclear Magnetic Resonance ZFNMR, the hyperfine field HF and the electric field gradient EFG field dominate the nuclear Hamiltonian, as opposed to traditional NMR where they are simply perturbations. Iron pnictides, in the magnetically ordered state, have strong HF and EFG fields, yielding spectra that intricately depend on the relative size and orientation of the two fields. Previous studies of this effect have overlooked the polarization and Rabi, or nutation, frequency of the ZFNMR signal. It is known however, that signals from a pure EFG system are linearly polarized and that from a Zeeman system are circularly polarized. We present calculations of the signal’s polarization, as well as the Rabi frequency, when HF and EFG are comparable to reveal the additional information afforded by such studies. In conjunction the resonance frequencies, the chief metric used to date, are presented. Iron pnictides are particularly interesting candidates for this study due to the theoretical importance of the transitions, and fluctuations associated with the transitions, from magnetic to nematic to paramagnetic order. Furthermore, ab initio methods are used to calculate EFG and HF fields for iron pnictides.