Bulk superconductivity and fully gapped superconducting state in Ba-doped phenanthrene

The superconducting state of Ba-intercalated phenanthrene, Ba${}_{1.5}$phenanthrene, has been investigated by specific-heat measurements down to 0.4 K. A clear anomaly associated with the superconducting transition at $T_c=4.7$ K indicates the bulk nature of the superconductivity. The temperature and the magnetic-field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The electron-phonon coupling constant estimated from the specific-heat jump is 0.55 to 1.06, suggesting that the superconductivity of Ba${}_{1.5}$phenanthrene can be explained within the BCS approach with intermediate coupling mediated by phonons of approximately 100 cm${}^{-1}$ in frequency, which is assigned as intercalant phonons or intermolecular phonon modes.

Figure 1

Main panel: x-ray diffraction patterns obtained with Mo source for pristine phenanthrene and Ba${}_{1.5}$phenanthrene. Inset: Temperature dependence of magnetic susceptibility ($\chi$) for Ba${}_{1.5}$phenanthrene in the zero field cooling (ZFC) measurements under a magnetic field of 10 Oe.

Figure 2

Main panel: Specific heat ($C_p$) divided by temperature ($T$), $C_p/T$, as a function of temperature at $H=0$ T and 3 T. The solid line (also in the inset) is the fit described in the text. Inset: $C_p/T$ at low temperatures as a function of $T^2$.

Figure 3

Main panel: Electronic part of specific heat $C_e/T$ obtained by subtracting lattice contributions and Schottky anomaly at low temperatures as a function of temperature. The solid line is a fit according to the model assuming an isotropically gapped state as described in the text. Left inset: Normalized electronic specific heat $C_e/{\gamma }_{s}T$, in which ${\gamma }_s$ corresponds to superconducting part of electronic specific coefficient, plotted on a logarithmic scale as a function of $T_c/T$. Right inset: Upper critical field $H_{c2}$ estimated from jump in $C_e/T$ in magnetic field. The solid and dashed lines show the Werthamer-Helfand-Hohenberg prediction.[23]

Figure 4

Main panel: Magnetic-field dependence of the normalized electronic specific heat $\gamma \left(H\right)/{\gamma }_n=C_e\left(H\right)/\left({\gamma }_{n}T\right)$. The dotted and dashed lines correspond to a linear fit for $\gamma \left(H\right)$ at $T\to 0$ K and the $\sqrt{H}$ dependence expected for $d$-wave symmetry, respectively. Inset: Low-temperature part of $C_p/T$ as a function of $T^2$ at various magnetic fields. The solid lines are linear fits to the data for $T<1.5$ K.