Atomic disorder in the heavy fermion superconductor CeCu2+xSi2

Superconducting, magnetic, and non-Fermi-liquid-like ground states evolve i n CeCu2+xSi2 with small changes in the Ce/Cu ratio that leave the average c rystal structure unchanged. We find, however, that the local atomic structu re determined by neutron pair density function analysis for a superconducti ng sample with x = 0.33 is fundamentally different from a nonsuperconductin g sample with x = - 0.08. The local lattice of the Cu-deficient sample exhi bits long-range atomic order, while the Cu-rich sample is intrinsically dis ordered. Spin-lattice coupling in the disordered Lattice produces an inhomo geneous magnetic state that allows superconductivity to develop and may con tribute to observed non-Fermi-liquid effects.