Glassy low-energy spin fluctuations and anisotropy gap in La_1.88Sr_0.12CuO₄

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductorLa_1.88Sr_0.12CuO₄ (T_c = 27 K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature T_N ≃ T_c inferred from elastic neutron scattering and the freezing temperature T_f ≃ 11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe-ordered ground state of La_1.88Sr_0.12CuO₄. The difference between T_N and T_f  implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly inelastic scattering-at least in the temperature range between T_f and T_c - which is not resolved in the present experiment. Combining the results obtained from our study of La_1.88Sr_0.12CuO₄ with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p = 0.12, where local probes indicate a sharp maximum in Tf(p), we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p = 0.12 and superconducting spin gaps dominating elsewhere.