Spin density distribution in a partially magnetized organic quantum magnet

Polarized neutron diffraction experiments on an organic magnetic material reveal a highly skewed distribution of spin density within the magnetic molecular unit. The very large magnitude of the observed effect is due to quantum spin fluctuations. The data are in quantitative agreement with direct diagonalization results for a model spin Hamiltonian, and provide insight on the actual microscopic origin of the relevant exchange interactions.Comment: 5 pages 4 figure

Specific Heat Study of an S=1/2 Alternating Heisenberg Chain System F_5PNN Under Magnetic Field

We have measured the specific heat of an S=1/2 antiferromagnetic alternating Heisenberg chain pentafulorophenyl nitronyl nitroxide under magnetic fields up to H>H_C2. This compound has the field-induced magnetic ordered (FIMO) phase between H_C1 and H_C2. Characteristic behaviors are observed depending on the magnetic field up to above H_C2 outside of the H-T boundary for the FIMO. Temperature and field dependence of the specific heat are qualitatively in good agreement with the theoretical calculation on an S=1/2 two-leg ladder. [Wang et al. Phys. Rev. Lett 84 5399 (2000)] This agreement suggests that the observed behaviors are related with the low-energy excitation in the Tomonaga-Luttinger liquid.Comment: 4pages, 4figures, replaced with revised version accepted to Physical Review Letter

Bose-Einstein condensation of semi-hard bosons in S=1 dimerized organic compound F2PNNNO

An analysis of the energy spectrum and the magnetization curve of two-dimensional organic antiferromagnet F2PNNNO with a spin-one dimerized structure shows that a behavior of the compound in an external magnetic field can be explained within a lattice boson model with an extended Pauli's exclusion principle, i.e. no more than two bosons per a dimer. The unusual magnetization curve observed experimentally in the compound reflects a sequence of phase transitions intrinsic for a lattice boson system with strong on-site and inter-site repulsions due to a tuning of magnon density by the applied magnetic field