Magnetic field tuning of parallel spin stripe order and fluctuations near the pseudogap quantum critical point in La1.36Nd0.4Sr0.24CuO4

A quantum critical point in the single-layer, hole-doped cuprate system La1.6−xNd0.4SrxCuO4 (Nd-LSCO) near x=0.23 has been proposed as an organizing principle for understanding high-temperature superconductivity. Our earlier neutron diffraction work on Nd-LSCO at optimal and high doping revealed static parallel spin stripes to exist out to the quantum critical point and slightly beyond, at x=0.24 and 0.26. We examine more closely the parallel spin stripe order parameter in Nd-LSCO in both zero magnetic field and fields up to 8 T for H∥c in these single crystals. In contrast to earlier studies at lower doping, we observe that H∥c in excess of ∼2.5 T eliminates the incommensurate quasi-Bragg peaks associated with parallel spin stripes. But this elastic scattering is not destroyed by the field; rather it is transferred to commensurate Q=0 Bragg positions, implying that the spins participating in the spin stripes have been polarized. Inelastic neutron scattering measurements at high fields show an increase in the low-energy, parallel spin stripe fluctuations and evidence for a spin gap Δspin=3±0.5 meV for Nd-LSCO with x=0.24. This is shown to be consistent with spin-gap measurements as a function of superconducting TC over five different families of cuprate superconductors, which follow the approximate linear relation Δspin=3.5kBTC.