Enhanced Superconductivity by Rare-earth Metal-doping in Phenanthrene

We successfully synthesized La- and Sm-doped phenanthrene powder samples and discovered superconductivity at $T_{\rm c}$ around 6 K in them. The $T_{\rm c}$s are 6.1 K for LaPhenanthrene and 6.0 K for SmPhenanthrene, which are enhanced by about 1 K and 0.5 K compared to those in $A_3$Phenanthrene ($A$=K and Rb) and in $Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors respectively. The superconductive shielding fractions for LaPhenanthrene and SmPhenanthrene are 46.1%$and 49.8$%$at 2 K, respectively. The little effect of the doping of the magnetic ion Sm$^{3+}$on$T_c$and the positive pressure dependence coefficient on$T_{\rm c}$strongly suggests unconventional superconductivity in the doped phenanthrene superconductors. The charge transfer to organic molecules from dopants of La and Sm induces a redshift of 7 cm$^{-1}$per electron for the mode at 1441 cm$^{-1}$in the Raman spectra, which is almost the same as those observed in$A_3$Phenanthrene ($A$=K and Rb) and$Ae_{1.5}$Phenanthrene ($Ae$ = Sr and Ba) superconductors.

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