Atomic disorder in the heavy fermion superconductor CeCu_2+xSi₂

Superconducting, magnetic, and non-Fermi-liquid-like ground states evolve in CeCu_2+xSi₂ with small changes in the Ce/Cu ratio that leave the average crystal structure unchanged. We find, however, that the local atomic structure determined by neutron pair density function analysis for a superconducting sample with x=0.33 is fundamentally different from a nonsuperconducting sample with x=-0.08. The local lattice of the Cu-deficient sample exhibits long-range atomic order, while the Cu-rich sample is intrinsically disordered. Spin-lattice coupling in the disordered lattice produces an inhomogeneous magnetic state that allows superconductivity to develop and may contribute to observed non-Fermi-liquid effects.