Field-induced gap in a quantum spin-1/2 chain in strong magnetic fields

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 high-frequency electron spin resonance spectroscopy in pulsed magnetic fields up to 64 T. In particular, we report on a minimum of the gap in the vicinity of the saturation field H_sat = 48.5 T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations in the magnetic excitation spectrum) to a spin-polarized state with magnon excitations (Fig. 1). 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. In addition, the development of the high-field Electron Spin Resonance facilities at the Dresden High Magnetic Field Laboratory is reported.
This work was partly supported by DFG and EuroMagNET II.