Spin Dynamics in Quantum Spin Chains

Quantum fluctuations, significantly enhanced in spin systems with reduced dimensionality, give rise to a variety of strongly correlated spin states, making low-dimensional magnets an ideal ground for testing various theoretical concepts. The way a magnetic field changes the ground-state properties and, correspondingly, the low-energy excitation spectrum of such systems is one of the fundamental aspects in quantum magnetism. Here, we report on experimental studies of the spin dynamics in copper pyrimidine dinitrate (hereafter Cu-PM), a spin-1/2 antiferromagnetic chain with alternating g-tensor and Dzyaloshinskii-Moriya interactions, by means of high-field electron spin resonance (ESR) spectroscopy in magnetic fields up to 64 T. Due to the presence of the field-induced staggered moment, this material exhibits a field-induced gap. We show that the spin dynamics of Cu-PM can be effectively described using the quantum-field-theory sine-Gordon formalism (with soliton and breather modes forming the excitation spectrum). Furthermore, we show that in the fully spin-polarized state (H_sat = 48.5 T) the spectrum is formed by ordinary magnons. The soliton-magnon crossover is characterized by a minimum of the spin gap, predicted using DMRG calculations and clearly observed by us. The temperature and field evolution of the ESR parameters approaching the sine-Gordon regime is studied as well. Excellent agreement with the theory in all cases is found.