  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.
