Two-dimensional superconductivity in new niobium dichalcogenides-based bulk superlattices

Transition metal dichalcogenides exhibit many unexpected properties including two-dimensional (2D) superconductivity as the interlayer coupling being weakened upon either layer-number reduction or chemical intercalation. Here, we report the realization of 2D superconductivity in the newly synthesized niobium dichalcogenides-based bulk superlattices ${\mathrm{Ba}}_{0.75}{\mathrm{ClNbS}}_{2}$ and ${\mathrm{Ba}}_{0.75}{\mathrm{ClNbSe}}_{2}$, which consist of the alternating stacking of monolayer $H\text{\ensuremath{-}}{\mathrm{NbS}}_{2}$ (or $H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$) and monolayer inorganic insulator spacer ${\mathrm{Ba}}_{0.75}\mathrm{Cl}$. Magnetic susceptibility and resistivity measurements show that both superlattices belong to type-II superconductor with ${T}_{c}$ of 1 K and 1.25 K, respectively. Intrinsic 2D superconductivity is confirmed for both compounds below a Berezinskii-Kosterlitz-Thouless transition and a large anisotropy of the upper critical field. Furthermore, the upper critical field along $ab$ plane $({H}_{c2}^{\ensuremath{\parallel}ab})$ exceeds the Pauli limit $({\ensuremath{\mu}}_{0}{H}_{p})$ in ${\mathrm{Ba}}_{0.75}{\mathrm{ClNbSe}}_{2}$, highlighting the influence of spin-orbit interactions. Our results establish a generic method for realizing the 2D superconducting properties in bulk superlattice materials.