Quantum critical dynamics in the one-dimensional spin chain compound copper pyrazine dinitrate probed by NMR spectroscopy

The metalorganic compound copper pyrazine dinitrate is known to be one of the best realizations of the antiferromagnetic $S = 1/2$ Heisenberg chain model with a comparatively small nearest neighbor exchange constant $J/k_B$ = 10.7 K. The zero temperature saturation field $B_c$ = 14.6 T corresponds to a quantum critical point (QCP), where the system is driven from a Luttinger liquid state to ferromagnetic polarization. With an emphasis on the vicinity of the QCP, a comprehensive comparison of our experimental findings from $^{13}$C NMR spectroscopy with both numerical (quantum Monte Carlo) and analytical (conformal field theory) approaches is presented. In particular, this comparison reveals a well-defined maximum of $1/T_1$ $(B,T)$ \textit{below} $B_c$ as the signature of essential one-dimensional spin-spin interactions in the Luttinger liquid regime.