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 ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_0.4{\mathrm{Sr}}_x{\mathrm{CuO}}₄$ (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 $H\ensuremath{\parallel}c$ in these single crystals. In contrast to earlier studies at lower doping, we observe that $H\ensuremath{\parallel}c$ in excess of $\ensuremath{\sim}2.5$ 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 $\mathbf{Q}=\mathbf{0}$ 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 ${\mathrm{\ensuremath{\Delta}}}_{\text{spin}}=3\ifmmode\pm\else\textpm\fi{}0.5$ meV for Nd-LSCO with $x=0.24$. This is shown to be consistent with spin-gap measurements as a function of superconducting ${T}_C$ over five different families of cuprate superconductors, which follow the approximate linear relation ${\mathrm{\ensuremath{\Delta}}}_{\text{spin}}=3.5{k}_B{T}_C$.