For all measurements presented in Fig. , each NMR signal was fitted with a complex Lorentzian. In the case of (1/T 1 ) measurements, the fitted values of intensity as a function of the relaxation delay τ were in turn fitted with a monoexponential function of the form k 1 e -t/T1 + k 2 , where k 1 and k 2 are constants. In the case of data taken at 100 µT, the signal should be more adequately fitted by a bi-exponential decay function , arising from the different relaxation rates of 1 H and 13 C spins. However, since protons contribute more greatly to the initial thermal polarization, mono-exponential fitting sufficed to fit the data with R 2 > 0.99. The T 2 time constant was inferred from the fitted full width at half maximum (FWHM) Γ using the relation Γ = 1/ (πT 2 ).