Stacking-tuned superconductivity and competing charge-density-wave states in NbSe${}_2$

Abstract

Layer stacking provides a powerful yet underexplored route for reshaping collective quantum order in van der Waals materials. Here we use high-resolution scanning tunneling microscopy and spectroscopy to show that the stacking sequence alone can qualitatively transform the charge density and superconducting orders in NbSe${}_2$, while preserving the same in-plane atomic structure. Comparing the 4Ha and 2H polytypes, we find that, unlike the ubiquitous triangular incommensurate $3Q^{\mathrm{I}}$ order of 2H-NbSe${}_2$, 4Ha-NbSe${}_2$ hosts two competing CDW states with no measurable correlation with local strain: a unidirectional commensurate $1Q^{\mathrm{C}}$ phase and a triangular incommensurate $3Q^{\mathrm{I}}$ phase, with $Q^{\mathrm{I}}=Q^{\mathrm{C}}+\hat{I}´$. We introduce a phase-resolved analysis that directly maps the gradient of the CDW phases and reveals vortices bound to the $1Q^{\mathrm{C}}$ - $3Q^{\mathrm{I}}$ phase boundaries. These vortices accommodate the momentum mismatch $\hat{I}´$ through abrupt $2\ddot{I}€$ phase slips, providing a mechanism by which distinct charge orders coexist. Superconductivity is also reshaped by stacking, while both polytypes exhibit multiband pairing.

5 pages, 4 figures, includes SI