Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg   chain studied by 13C-NMR spectroscopy
  We present a 13C-NMR study of the magnetic field driven transition to complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local magnetization as well as the low-frequency spin dynamics, probed via the nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the high field limit and at temperatures from the quantum regime (k_B T << J) up to the classical regime (k_B T >> J). The experimental data show very good agreement with quantum Monte Carlo calculations over the complete range of parameters investigated. Close to the critical field, as derived from static experiments, a pronounced maximum in 1/T_1 is found which we interpret as the finite-temperature manifestation of a diverging density of zero-energy magnetic excitations at the field-driven quantum critical point. 
H. Kuehne, H.-H. Klauss, S. Grossjohann, W. Brenig, F.J. Litterst,   A.P. Reyes, P.L. Kuhns, M.M. Turnbull, C.P. Landee