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 Ba6Nb11S28 have received much interest. Here we report the crystal structure, Hall resistivity, and 2D superconductivity of its sister compound, Ba6Nb11S28. The atomic-resolution transmission electron microscopy images reveal an alternate stacking of 2H-NbSe2 with a 3 x 3 commensurate block layer, which is isostructural to Ba(6)Nb(11)S(28 )and with a larger NbX2 (X = S, Se) interlayer distance (12.8 angstrom). Transport property measurements demonstrate a twofold enhancement in superconducting temperature (T-c(0) = 2.2 K), meanwhile the anisotropy in resistivity and carrier mobility in Ba(6)Nb(11)S(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 Ba(6)Nb(11)S(28 )be Pauli-limit breaking (2 times the mu H-0(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.