Neutron scattering studies of spin-phonon hybridization and superconducting spin gaps in the high-temperature superconductor La2−x(Sr,Ba)xCuO4

We present time-of-flight neutron scattering measurements on single crystals of La2−xBaxCuO4 (LBCO) with 0≤x≤0.095 and La2−xSrxCuO4 (LSCO) with x=0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high-temperature cuprate superconductivity, ranging from insulating, three-dimensional commensurate long-range antiferromagnetic order, for x≤0.02, to two-dimensional (2D) incommensurate antiferromagnetism coexisting with superconductivity for x≥0.05. Previous work on lightly doped LBCO with x=0.035 showed a clear enhancement of the inelastic scattering coincident with the low-energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore, we show that the low-temperature, low-energy magnetic spectral weight is substantially larger for samples with nonsuperconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low-energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO.