Antiferromagnetic spin excitations in single crystals of nonsuperconducting Li_1-xFeAs

We use neutron scattering to determine spin excitations in single crystals of nonsuperconducting Li_1-xFeAs throughout the Brillouin zone. Although angle resolved photoemission experiments and local density approximation calculations suggest poor Fermi surface nesting conditions for antiferromagnetic (AF) order, spin excitations in Li_1-xFeAs occur at the AF wave vectors Q=(1,0) at low energies, but move to wave vectors Q=(±0.5,±0.5) near the zone boundary with a total magnetic bandwidth comparable to that of BaFe₂As₂. These results reveal that AF spin excitations still dominate the low-energy physics of these materials and suggest both itinerancy and strong electron-electron correlations are essential to understand the measured magnetic excitations.