LAUSR

Molecular-beam epitaxy (MBE) enables stabilization of a non-equilibrium material phase, providing a powerful approach for exploration of emergent phenomena in condensed-matter research. Here we demonstrate that one of metallic two-dimensional (2D) materials TaSe2 grown by MBE crystalizes into pure 3R phase with the self-intercalated Ta atoms, 3R-Ta1+xSe2, which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3R-Ta1+xSe2 film exhibits the superconducting (SC) critical temperature (Tc) of 3.0 K, which is the highest among all the polymorphs in TaSe2. Thickness-dependence measurements reveal that Tc decreases with reducing thickness accompanied by the development of the charge-density wave phase. Those 3R-Ta1+xSe2 films exhibit large in-plane upper critical fields (Hc2) in their SC states even at the thick-enough regime, most likely due to the suppression of the interlayer hopping associated with the unique 3R stacking. Moreover, the temperature dependence of the in-plane Hc2 evolves from linear to square-root behavior with reducing thickness, indicating crossover behavior from anisotropic three-dimensional superconductivity to 2D superconductivity. Our results unveil intriguing SC properties of metastable 3R-Ta1+xSe2 distinct from those of thermodynamically-stable 2H-TaSe2, demonstrating essential importance of MBE-based approach for exploration of novel quantum phenomena in 2D materials research.