Fig. 2. Coupling to only few MTSSL spin-labels.
(A) Coherent oscillation due to dipolar couplings between sensor and network spins, clearly visible by contrast inversion (blue shaded area) of the NV probe spin state, which suggests a low number of participating network spins. For the measurement, we choose a constant evolution time of 19.8 μs for the NV probe spin Hahn echo sequence, maximizing interaction time. Previous to the measurement, T2 = 14.0 μs of the NV center under investigation was measured. To access the coupling between sensor and network spins, we sweep the application point of the network π pulse within the free evolution of the sensor spin coherence. The measurement data are accompanied by simulations of coupling strength oscillations caused by n = 2, 6, and 10 randomly distributed spins above the diamond surface. We would like to stress that it is difficult to estimate the precise number of spins here, and the relative strength of the overshoot below the NV sensor mixed state (0.5) heavily depends on the couplings (positions) of the participating network spins. (B) Hahn echo measurement on the external network spins directly probing their T2 time via the readable signal from the probe spin (red). The blue curve was measured on a pulsed EPR spectrometer at 10 K in a frozen solution of water/glycerol (4:1) using the same peptides. A low concentration guarantees a proper separation between peptides, and the agreement of both coherence times points toward comparable SDSL peptide-peptide distances in both samples.