London penetration depth of $\lambda$-(BETS)$_2$GaCl$_4$ superconductor

After many years of study, it is still unclear whether the superconductivity of organic charge transfer salts is conventional or not. In particular, symmetry of the superconducting gap is an important unresolved question. The gap anisotropy and nodal structure can be probed via precision measurements of London penetration depth, $\lambda(T)$, for which tunnel-diode resonator has been developed over past 20 years. Here we report $\lambda(T)$ in $\lambda$-(BETS)$_2$GaCl$_4$ crystals ($T_c \sim 5$~K ) measured down to 400 mK. We find that different samples exhibit different widths of superconducting transition, $\Delta T_c$. It is known for organic superconductors that cooling rate from room temperature is important for the properties probed at the low temperatures. We discuss observed variation of $\Delta T_c$ with the cooling rate as well as sample size and aspect ratio. Results are compared with thermal conductivity measurements [1]. \\[4pt] [1] M. A. Tanatar, T. Ishiguro, H. Tanaka, H. Kobayashi, Phys.Rev. B {\bf 66}, 134503 (2002).