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Researchers used high-resolution scanning tunneling microscopy and spectroscopy to show that simply changing how the atomic layers stack in the material niobium diselenide (NbSe2) can reorganize both its charge order and its superconducting behavior. They compared two stable stackings — called 2H and 4Ha — that keep the same in-plane atomic lattice but differ in how layers sit on top of one another. The 4Ha form hosts a mixture of a unidirectional, lattice-locked (commensurate) stripe pattern and the usual threefold, non-locked (incommensurate) triangular pattern, while the 2H form shows only the triangular incommensurate pattern under the same conditions.
What the team did was image the surface and measure the local electronic spectra at very low temperature (0.34 K) with scanning tunneling microscopy and spectroscopy (STM/STS). These tools map both the atomic layout and the way electrons are arranged in space and energy. In spectra, both polytypes show multiband superconductivity (more than one pairing gap), but the 4Ha stacking reduces the superconducting energy scales and broadens the features. Fits to a two-band model give gap values for 4Ha of about 0.99 ± 0.08 meV and 0.27 ± 0.13 meV, with most of the tunneling weight coming from the larger gap. By contrast, 2H shows gaps near 1.08 meV and 0.72 meV and clearer features from both bands.
To study the charge order, the authors used large-scale STM images and a phase-resolved analysis that maps the local wavevector and phase of the charge density waves (CDWs). A commensurate state means the charge modulation locks to an integer multiple of the atomic lattice spacing; an incommensurate state means it does not. In 4Ha they saw intrinsic coexistence of a commensurate unidirectional CDW (called 1QC) and an incommensurate triangular CDW (3QI). The incommensurate wavevector differs from the commensurate one by a small momentum mismatch δ (about 0.021 times the Bragg vector). Importantly, the appearance of the unidirectional 1QC regions in 4Ha did not correlate with any measurable surface strain, unlike in 2H where similar stripe regions are linked to strained areas.
A striking result came from mapping the CDW phase gradients. The team found vortex-like defects bound to the interfaces between 1QC and 3QI regions. These vortices are places where the CDW phase jumps by 2π (a full cycle). The researchers interpret these as localized phase slips that absorb the momentum mismatch δ and so allow a commensurate and an incommensurate order to coexist at the nanoscale. They also adapted a Ginzburg–Landau free-energy model (a common theoretical framework for ordered phases) to include a commensurability term with period-seven modulation. That model reproduces a first-order transition between 3QI and 1QC states and supports phase coexistence near the transition.
Why this matters: the work shows that layer stacking alone — without changing the in-plane atoms — can qualitatively change which collective electronic orders appear and how strong superconductivity is. This highlights stacking as a tunable control knob for correlated states in layered materials. Important caveats are that the results come from surface-sensitive STM/STS measurements at cryogenic temperature on single crystals prepared by chemical vapor transport, and the excerpt is from a partial text. The theoretical interpretation uses an adapted phenomenological model and builds on prior ab initio work that suggests stacking can split bands and break inversion symmetry; those points support the explanation but are not direct experimental proof in this paper. The authors also note that within the sensitivity of their strain-detection method they do not see strain driving the 1QC regions in 4Ha, distinguishing that case from earlier observations in 2H.