Local nematic susceptibility in stressed BaFe2As2 from NMR electric field gradient measurements

The electric field gradient (EFG) tensor at the $⁷⁵\mathrm{As}$ site couples to the orbital occupations of the As $p$ orbitals and is a sensitive probe of local nematicity in ${\mathrm{BaFe}}₂{\mathrm{As}}₂$. We use nuclear magnetic resonance to measure the nuclear quadrupolar splittings and find that the EFG asymmetry responds linearly to the presence of a strain field in the paramagnetic phase. We extract the nematic susceptibility from the slope of this linear response as a function of temperature and find that it diverges near the structural transition, in agreement with other measures of the bulk nematic susceptibility. Our work establishes an alternative method to extract the nematic susceptibility which, in contrast to transport methods, can be extended inside the superconducting state.