Abstract
We have measured the specific heat of the coupled spin-dimer antiferromagnet to in temperature and to in the magnetic field. The experiment extends to the midpoint of the field region of the magnetization plateau at of the saturation magnetization, and reveals the presence of three ordered phases in the field region between that of the magnetization plateau and the low-field spin-liquid region. The exponent of the phase boundary with the thermally disordered region is smaller than the theoretical value based on the Bose-Einstein condensation of spin triplets. At zero field and , the specific-heat data show gapped behaviors characteristic of spin liquids. The zero-field data indicate that the gapped triplet excitations form two levels whose energies differ by nearly a factor of 2. At least the lower level is well localized. The data at reveal that the low-lying excitations at the magnetization plateau are weakly delocalized.
- Received 7 September 2005
DOI:https://doi.org/10.1103/PhysRevB.72.214434
©2005 American Physical Society