Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets
  By means of nuclear spin-lattice relaxation rate 1/T1, we follow the spin dynamics as a function of the applied magnetic field in two gapped one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl2-4SC(NH2)2 and the spin-ladder system (C5H12N)2CuBr4. In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapples Tomonaga-Luttinger-liquid state. In between, 1/T1 exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for 1/T1, compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum critical behavior. 
S. Mukhopadhyay, M. Klanj\v{s}ek, M. S. Grbi\'c, R. Blinder, H.   Mayaffre, C. Berthier, M. Horvati\'c, M. A. Continentino, ...
cond-mat.str-el