Three-dimensional superconductivity induced by an extremely small amount of Li in Li x SnSe 2

Unconventional superconductivity occurs often in materials with low dimensionality. Here, we report superconductivity observed in layered ${\mathrm{Li}}_x{\mathrm{SnSe}}₂$ with the superconducting transition temperature ${T}_c\ensuremath{\sim}6$ K. Through $\mathrm{L}{\mathrm{i}}⁺$ intercalation in semiconducting ${\mathrm{SnSe}}₂$ via electrochemical process, ${\mathrm{Li}}_x{\mathrm{SnSe}}₂$ is formed with an extremely small $x$ value as estimated from the $c$-axis lattice parameter, carrier concentration, and first-principles calculations. Electrical resistivity and magnetic susceptibility measurements allow the construction of both lower $({H}_c1^ab)$ and upper critical field (${H}_c2^ab\phantom{\rule{4pt}{0ex}}\mathrm{and}\phantom{\rule{4pt}{0ex}}{H}_c2^c$) phase diagrams. While ${H}_c2^c$ obtained from 10% and 50% resistivity drops exhibits linear temperature dependence, ${H}_c2^c\phantom{\rule{4pt}{0ex}}(90%\phantom{\rule{4pt}{0ex}}\mathrm{resistivity}\phantom{\rule{4pt}{0ex}}\mathrm{drop})$, ${H}_c1^ab$, and ${H}_c2^ab$ can be described by the empirical formula ${H}_ci(T)={H}_ci(0)[1\ensuremath{-}{(\frac{T}{{T}_c0})}²]$ ($i=1$, 2), giving ${H}_c2^c(0)=754$ Oe, ${H}_c1^ab$(0) = 27 Oe, and ${H}_{\mathrm{c}2}^ab$(0) = 1652 Oe. Using the Ginzburg-Landau formula, we further estimate that the $c$-axis penetration depth ${\ensuremath{\lambda}}_c$(0) = 396 nm and the coherence length anisotropy ${\ensuremath{\xi}}_ab$(0) = 66.1 nm and ${\ensuremath{\xi}}_c$(0) = 30.3 nm. The fact that ${\ensuremath{\xi}}_c$(0) is much longer than the interlayer separation implies three-dimensional superconductivity with superconducting anisotropy ${\ensuremath{\xi}}_ab(0)/{\ensuremath{\xi}}_c$(0) \ensuremath{\sim} 2.2.