Field-Induced Gap in a Quantum Spin-1/2 Chain in a Strong Magnetic Field

Due to recent progress in theory and the growing number of physical realizations, low-dimensional quantum magnets continue to receive a considerable amount of attention. They serve as model systems for investigating numerous fascinating phenomena in materials with cooperative ground states, in particular, induced by high magnetic fields. The way a magnetic field changes the ground-state properties and, correspondingly, the low-energy excitation spectrum of low-dimensional magnets is one of the fundamental aspects in quantum magnetism. Here, magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy in fields up to 63 T. In particular, we report on a minimum of the gap in the vicinity of the saturation field Hsat = 48.5 T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG investigation of the spin-1/2 Heisenberg chain with a staggered transverse field