Magnetic field versus temperature phase diagram of the spin-1/2 bond-alternating-chain antiferromagnet F$_{5}$PNN

The S = 1/2 Heisenberg bond-alternating-chain antiferromagnet pentafluorophenyl nitronyl nitroxide (F$_{5}$PNN) exhibits Tomonaga-Luttinger-liquid behavior in the temperature dependence of the specific heat above the field-induced magnetic ordering temperature [1]. We have determined the magnetic phase diagram of this compound from the specific heat. For a single crystal, the boundary of the ordered phase in the field-vs-temperature diagram is symmetric with respect to the central field of the gapless region H$_{c1} \quad \le $ H $\le $ H$_{c2}$, whereas a distorted phase boundary is observed for a powder sample, whose ordering temperature is reduced. Calculations based on the finite-temperature density-matrix renormalization group suggest the possibility of a novel incommensurate phase due to frustration in the powder, in a narrow field range near the central field. [1]Y. Yoshida \textit{et al.,} Phys. Rev. Lett.\textbf{94}, 037203 (2005).