Stacking-Selective Self-Intercalation and Suppressed Superconductivity in Epitaxial Nb_1+xSe₂ Films

​Hongguang Wang, Jiawei Zhang, Dennis Huang, Hidenori Takagi, Peter A. van Aken

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

Layered van der Waals materials offer a versatile platform for tailoring emergent electronic phenomena through controlled stacking configurations and intercalation chemistry [1, 2]. In NbSe₂​, a prototypical layered superconductor, both the stacking polytype and the incorporation of excess Nb are known to significantly influence the low-energy electronic structure and superconducting properties. However, their coupled effects remain insufficiently understood, largely because previous investigations have relied on spatially averaged techniques that lack direct access to the atomic-scale distribution and chemical state of intercalant.

In this work, we examine epitaxial Nb_1+x​Se₂​ thin films grown on c-cut sapphire substrates by hybrid pulsed laser deposition, with global x ~ 0.3. Electrical transport measurements reveal metallic conductivity in the normal state but no superconducting transition down to 0.4 K. Instead, the films exhibit a logarithmic upturn in resistivity below approximately 20 K, accompanied by a low-field negative magnetoresistance, indicative of weak localization effects.

Atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy uncovers a nanoscale coexistence of 0°- and 180°-stacked NbSe₂​ layers. Notably, pronounced Nb self-intercalation is observed exclusively in the 180°-stacked regions, where interstitial Nb atoms occupy octahedral sites within the van der Waals gaps. In contrast, adjacent 0°-stacked layers remain largely free of intercalants. This spatial selectivity establishes a direct correlation between local stacking configuration and intercalation propensity.

To elucidate the electronic consequences of this structural heterogeneity, we employ angle-resolved photoemission spectroscopy (ARPES) in conjunction with density functional theory (DFT) calculations. Nb self-intercalation is found to induce substantial electron doping, resulting in a downward shift of the electronic bands by up to 0.8 eV in the fully intercalated limit. The experimentally observed band structure lies intermediate between pristine and fully intercalated NbSe₂, consistent with a mixed intercalation regime. This charge transfer reduces the size of Fermi surface hole pockets and diminishes the density of states at the Fermi level, thereby suppressing the superconducting transition temperature. Additional contributions from disorder and intercalant-induced pair-breaking scattering are likely to further inhibit superconductivity. Furthermore, DFT calculations indicate that the relative stability of stacking configurations depends sensitively on the intercalant concentration, placing the system in proximity to a stacking phase boundary at elevated Nb content.

These findings demonstrate that stacking order, self-intercalation, and electronic structure are strongly interdependent in Nb_1+xSe₂​ thin films, providing new insight into the microscopic mechanisms governing superconductivity in intercalated van der Waals materials. [3]

References

1. P. Ajayan et al. Phys. Today 69 (2016), 39-44.

2. K. S. Novoselov et al. Science 353 (2016), aac9439.

3. H. Wang et al. Nat. Commun. 15 (2024), 2541.