Towards atomically-thin regime in bulk 4H-NbSe2 with interlayer disorder

Polytypism in transition metal dichalcogenides (TMDs) introduces an additional degree of freedom for tailoring the electronic properties of layered van der Waals materials. Polytypes with larger unit cells, spanning four or six layers, can be viewed as natural homostructures, since their atomic composition remains identical across the layers. The resultant crystalline environments can potentially give rise to exotic electronic states, earning these materials recent attention. In this study, we examine structural and charge transport properties of metallic and superconducting 4H_a-NbSe₂. We find that the compound has a highly disordered stacking of layers, which impedes interlayer coherence, as demonstrated by detailed out-of-plane resistivity measurements, and effectively tunes the bulk system towards an atomically thin limit. The disordered structure largely accounts for the enhanced resistivity anisotropy and superconducting upper critical field, when compared to 2H_a-NbSe₂. This phenomenon can be exploited to promote quasi-two-dimensional physics in bulk crystals, and our study also underscores the importance of thorough structural characterization when investigating large-unit-cell polytypes of TMDs.