Magnetic field induced gap and staggered susceptibility in the S 1 2 chain [PMCu 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 H2O 2]n PM pyrimidine show that it behaves like a uniform S 1 2 antiferromagnetic Heisenberg chain, characterized by the exchange parameter. Specific heat measurements in 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 that strongly varies with 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 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 directio