Ising superconductivity and anomalous metallic states in a bulk crystal with artificial unidirectional stacking layers

Abstract

The two-dimensional (2D) limit in macroscopic bulk crystals provides a powerful platform for exploring exotic quantum phases. Here, we report the synthesis of a Sr0.75ClNbS2 superconductor that achieves unidirectional, parallel AA stacking-a configuration never before realized in a bulk crystal. Unlike conventional intercalation, which merely expands the interlayer spacing, our approach employs a planar Sr-Cl network to enforce a complete stacking reorganization, driving all NbS2 layers from the native antiparallel AB stacking into a unidirectional, parallel AA arrangement. This stacking switch globally breaks inversion symmetry, transforming centrosymmetric 2H-NbS2 into a noncentrosymmetric bulk crystal with D3h point group symmetry. Crucially, this structural design reproduces, in three dimensions, the electronic environment of an isolated monolayer, thereby preventing cancellation of the local Ising fields. As a result, strong Ising spin-orbit coupling and spin-split bands persist throughout the bulk. Transport measurements reveal extreme superconducting anisotropy ({\gamma} ~ 77), an in-plane upper critical field (~ 10.65 T) that far exceeds the Pauli paramagnetic limit, and clean-limit superconductivity indicative of high crystalline quality. Moreover, magnetotransport uncovers a novel magnetic-field-induced anomalous metallic state characterized by finite dissipation yet a vanishing Hall response. Direct band-structure measurements corroborate the layer-decoupled, quasi-2D electronic nature of the system. This work establishes stacking-geometry engineering as a powerful strategy to artificially enforce a globally noncentrosymmetric, quasi-2D superconducting state in bulk crystals, paving the way for designing quantum materials with tunable crystalline symmetry and electronic band topology.