Resistively detected NMR in a triple-gate quantum point contact: Magnetic field dependence

We report a resistively detected nuclear magnetic resonance (RDNMR) measurement in a GaAs-based quantum point contact in the quantum Hall breakdown regime. We focus on the detection of the $^{75}\mathrm{As}$ RDNMR signal at different perpendicular magnetic fields in the presence of electrons occupying the lowest Landau level. We confirm successful RDNMR detection down to $B=1.25\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ ($B=3.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$) using a high-mobility (low-mobility) device. All the RDNMR signals exhibit a threefold spectra attributed to electric quadrupole interaction. We find that the separation between the central transition and its satellite, ${\mathrm{\ensuremath{\Delta}}}_{f}\ensuremath{\approx}20\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$, remains unaffected by magnetic-field variation. Interestingly, the central NMR linewidths vary with the field strength. Above $B=3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, the linewidth tends to saturate around 13 kHz. Below $B=3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, the linewidth decreases in proportion with field and approaches a lower limit of 1 kHz corresponding to the GaAs nuclear dipole interaction as the field decreases to $B=1.25\phantom{\rule{0.16em}{0ex}}\mathrm{T}$.