Field-Induced Gap in the Spin-1/2 Heisenberg Chain Compound Cu-Pyrimidine Dinitrate: ESR Studies in Magnetic Fields up to 63 T

Results of electron spin resonance studies of copper pyrimidine dinitrate (Cu-PM, [PM-Cu(NO₃)₂(H₂O)₂]_n, PM= pyrimidine), a spin-1/2 antiferromagnetic chain material with alternating g-tensor and Dzyaloshinskii-Moriya interactions, in pulsed magnetic fields up to 63 T are reported. The field-induced gap shows a nonmonotonic behavior with a minimum in the vicinity of the saturation field H_sat = 48.5 T. This is associated with a transition from the sine-Gordon region to a spinpolarized state with magnon excitations. By comparing the entire set of experimental data with results of density matrix renormalization group calculations for Cu-PM, the validity of the used theoretical approach is proven.