Two-dimensional superconductivity in a bulk superlattice van der Waals material Ba6Nb11Se28

The observation of clean 2D superconductivity and bosonic Landau levels below 0.85 K in superlattice van der Waals material ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ have received much interest. Here we report the crystal structure, Hall resistivity, and 2D superconductivity of its sister compound, ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$. The atomic-resolution transmission electron microscopy images reveal an alternate stacking of $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ with a 3 \ifmmode\times\else\texttimes\fi{} 3 commensurate block layer, which is isostructural to ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ and with a larger $\mathrm{Nb}{X}_{2}$ ($X=\mathrm{S}$, Se) interlayer distance (12.8 \AA{}). Transport property measurements demonstrate a twofold enhancement in superconducting temperature $({{T}_{\mathrm{c}}}^{0}=2.2\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, meanwhile the anisotropy in resistivity and carrier mobility in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ decrease drastically compared to its sulfur counterpart, driving the superconductor towards the dirty limit. Further measurements of angular-dependent upper critical field suggest the superconductivity in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ be Pauli-limit breaking (2 times the ${\ensuremath{\mu}}_{0}{H}_{P}$), and can be well described by the 2D Tinkham form. Our findings offer a valuable platform to investigate 2D superconductivity in bulk superlattice materials based on the transition-metal dichalcogenides.