| We present a comprehensive NMR study of the magnetic field-driven quantum phase transitions in the S=1/2 spin chain systems Cu(C4H4N2)(NO3)2 and (phzH)2CuCl4H2O. The static and dynamic experimental NMR properties are compared with both quantum Monte Carlo calculations and Luttinger liquid theory. The first compound, Cu(C4H4N2)(NO3)2, is known to be one of the best realizations of the antiferromagnetic S=1/2 Heisenberg chain (AFHC) model with a low coupling constant J. The zero temperature saturation field Bc = 14.6 T corresponds to a quantum critical point, where the system is driven from a Luttinger liquid state to ferromagnetic polarization. In the vicinity of this point in the corresponding B- and T- parameter space a divergent behavior of the nuclear 13C-spin-lattice relaxation rate is observed and in good agreement with theory [1,2]. In addition, we present a detailed 14N-study of the angular dependent hyperfine fields at the nitrogen sites in the nitrate groups. This allows to determine the EFG-Tensor, the distribution of nearby local spin moments and fortifies the comparibility between theory and experiment of the 13C-results. The second compound, (phzH)2CuCl4H2O, is a recently synthesized [3] spin chain system. 1H- and 35Cl-NMR experiments consistently yield a field- and temperature dependent behavior of 1/T1 similar to that of the first compound. But, in contrast, a pronounced second maximum is observed at about 3/4 of the saturation field Bc = 12.2 T. This effect is not found in the local or macroscopic magnetization, suggesting a more complicated magnetic interaction scheme. |
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