Determination of magnetic form factors for organic charge-transfer salts: A first-principles investigation

Organic charge-transfer salts show a variety of complex phases ranging from antiferromagnetic long-range order, spin liquid, bad metal, or even superconductivity. A powerful method to investigate magnetism is spin-polarized inelastic neutron scattering. However, such measurements have often been hindered in the past by the small size of available crystals as well as by the fact that the spin in these materials is distributed over molecular rather than atomic orbitals, and good estimates for the magnetic form factors are missing. By considering Wannier functions obtained from density-functional theory calculations, we derive magnetic form factors for a number of representative organic molecules. Compared to Cu2+, the form factors |F(q)|2 fall off more rapidly as function of q, reflecting the fact that the spin density is very extended in real space. Form factors |F(q)|2 for TMTTF, BEDT-TTF, and (BEDT-TTF)2 have anisotropic and nonmonotonic structures.