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

Abstract

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 Ba${}_{0.75}$ClNbS${}_2$ and Ba${}_{0.75}$ClNbSe${}_2$, which consists of the alternating stacking of monolayer $H$-NbS${}_2$ (or $H$-NbSe${}_2$) and monolayer inorganic insulator spacer Ba${}_{0.75}$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 ($H_{c2}^{\parallel ab}$) exceeds the Pauli limit ($\hat{I}{¼}_0{H}_p$) in Ba${}_{0.75}$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.