Specific heat study of an S=1/2 alternating heisenberg chain system:: F5PNN in a magnetic field -: art. no. 037203

We have measured the specific heat of an S=1/2 antiferromagnetic alternating Heisenberg chain pentafulorophenyl nitronyl nitroxide in magnetic fields up to H>H-C2. This compound has a 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. The 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.