(a) Calculated geometric spin echo decay induced by bath spins decomposed into single (green) and dimer (blue) 13C nuclear spin contributions. Solid (dashed) lines shows the decay under a magnetic field of 0 mT (0.4 mT) measured along the NV axis. (b) Schematic spin bath configuration around the vacancy electron spin under a zero magnetic field. Single 13C nuclear spins are quantized along the electron spin hyperfine field (grey arrows). Dimer 13C nuclear spins are quantized along the dipolar magnetic field within each of the dimers.