The temperature dependence of the in-plane, ${\lambda }_{\Vert }\mathrm{}\left(T\right),$ and interplane, ${\lambda }_{\perp }\mathrm{}\left(T\right),$ London penetration depth was measured in the metal-free all-organic superconductor ${\beta }^{″}-(\mathrm{ET}{)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$ ${(T}_c\approx 5.2\hspace{0.5em}\mathrm{K}).\mathrm{}$ $\Delta {\lambda }_{\Vert }\mathrm{}\left(T\right)\mathrm{}\propto T^3$ up to 0.5 $T_c,$ a power law previously observed only in materials thought to be $p-\mathrm{wave}$ superconductors. ${\lambda }_{\perp }$ is larger than the sample dimensions down to the lowest temperatures (0.35 K), implying an anisotropy of ${\lambda }_{\perp }/{\lambda }_{\Vert }\approx 400-800.$

• Received 7 September 2000

DOI:https://doi.org/10.1103/PhysRevB.63.052506