Specific Heat Study of anS=1/2Alternating Heisenberg Chain System:F5PNNin a Magnetic Field

We have measured the specific heat of an $S=1/2$ antiferromagnetic alternating Heisenberg chain pentafulorophenyl nitronyl nitroxide in magnetic fields up to $H>H_{\mathrm{C}2}$. This compound has a field-induced magnetic ordered (FIMO) phase between $H_{\mathrm{C}1}$ and $H_{\mathrm{C}2}$. Characteristic behaviors are observed depending on the magnetic field up to above $H_{\mathrm{C}2}$ outside of the $H\mathrm{\text{−}}T$ 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.