Quantum Critical Dynamics of S=1/2 Antiferromagnetic Heisenberg Chains Studied in CuPzN by ESR

We found the dramatic crossover of the quantum critical dynamics of the chain antiferromagnet CuPzN which manifests itself in a change of symmetry properties of the ESR lineshape at low temperatures. We argue that such a crossover reveals the subtle interplay of two different perturbations of the isotropic Heisenberg model: i) the exchange anisotropy and ii) the staggered magnetic field induced by alternating g-factors of nonequivalent Cu sites of the unit cell of this compound. We estimate the magnitude of these perturbations, which drive the system from the ideal Tomonaga-Luttinger-Liquid state to the state with the field-induced gapped spin excitation spectrum.