Magnetic-field induced gap and staggered susceptibility in the S=1/2 chain [PM•Cu(NO3)2•(H2O)2]n (PM = pyrimidine)

Single-crystal magnetic susceptibility and specific heat studies of the one-dimensional copper complex [PM . Cu(NO3)(2)bullet>(*) over bar * (H2O)(2)](n) (PM = pyrimidine) show that it behaves Like a uniform S = 1/2 antiferromagnetic Heisenberg chain, characterized by the exchange parameter J/k(B) = 36 K. Specific heat measurements in the applied magnetic field, however, reveal the formation of a field-induced spin excitation gap, whose magnitude depends on the magnitude and direction of the field. This behaviour is inconsistent with the ideal S = 1/2 Heisenberg chain. In the low-temperature region, a contribution to the susceptibility, approximately proportional to 1/T, is observed which varies strongly with the varying direction of the magnetic field. The field-induced gap and the 1/T contribution are largest for the same field direction. Previous observations of a field-induced gap in the related compounds copper benzoate and Yb4As3 have been explained by the alternating g tensor and alternating Dzyaloshinkii-Moriya interaction, producing an effective staggered magnetic field at the Cu and Yb ions. We apply this model to [PM . Cu(NO3)(2)bullet>(*) over bar * (H2O)(2)](n) and obtain a consistent quantitative explanation of the low-temperature susceptibility, the field-induced gap and their dependence on the magnetic-field direction.