Extended Data Figure 7: Estimate of the intrinsic specific heat C_i by subtracting the specific heat due to magnetic defects.

Fitting the specific heat C in Extended Data Fig. 5 with the model for D(E) in Fig. 4c gives an estimate of the specific heat due to magnetic defects. The best-fitting function, represented dashed line in Extended Data Fig. 5, is subtracted from the total specific heat C(B, T) shown in Fig. 4a. The residual specific heat ΔC is almost independent of B and gives a measure of intrinsic specific heat C_i. ΔC/T appears to extrapolate almost to zero as the temperature approaches 0 K, within the uncertainty of the estimate of the contribution from magnetic defects. We do not attempt to separate the possible contribution from the bulk spin liquid to ΔC because of the difficulty in estimating the lattice contribution. The inset shows the plot of ΔC/T as a function of T².