Soliton-magnon crossover in spin-1/2 Heisenberg chains

Copper pyrimidine dinitrate ([PM-Cu(NO₃)₂(H₂O)₂]_n, PM = pyrimidine) is a spin-1/2 antiferromagnetic chain material with alternating g-tensor and Dzyaloshinskii-Moriya interactions, whose spin dynamics can be described using the effective sine-Gordon model. Due to the presense of alternating staggered magnetization, this material exhibits a field-induced spin gap, formed by first breather. Here, we report on electron spin resonance (ESR) studies of this material in magnetic fields up to 64 T. In particular, a minimum of the gap in the vicinity of the saturation field Hsat = 48.5 T associated with a transition from the quantum disordered sine-Gordon (with soliton-breather elementary excitations in the magnetic excitation spectrum) to a spin-polarized state (with excitation spectrum formed by magnons) has been observed. This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG calculations for spin-1/2 Heisenberg chain with a staggered transverse field [1]. Such a behavior appears to be a general feature of the high-field excitation spectrum of quantum spin-1/2 chain systems with alternating g-tensor and/or Dzyaloshonskii-Moriya interactions