Superconducting 3 R-Ta_{1+ x}Se₂ with Giant In-Plane Upper Critical Fields

Molecular-beam epitaxy (MBE) enables the stabilization of a nonequilibrium material phase, providing a powerful approach to the exploration of emergent phenomena in condensed-matter research. Here we demonstrate that one of the metallic two-dimensional (2D) materials, TaSe₂, grown by MBE crystallizes into the pure 3R phase with the self-intercalated Ta atoms, 3R-Ta_1+xSe₂, which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3R-Ta_1+xSe₂ film exhibits a superconducting (SC) critical temperature (T_c) of 3.0 K, which is the highest among all of the polymorphs in TaSe₂. Thickness-dependence measurements reveal that T_c decreases with decreasing thickness, accompanied by the development of the charge-density wave phase. The 3R-Ta_1+xSe₂ films exhibit large in-plane upper critical fields (H_c2) in their SC states even in 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 H_c2 evolves from linear to square-root behavior with decreasing thickness, indicating crossover behavior from anisotropic three-dimensional superconductivity to 2D superconductivity. Our results unveil intriguing SC properties of metastable 3R-Ta_1+xSe₂ distinct from those of thermodynamically stable 2H-TaSe₂, demonstrating the essential importance of the MBE-based approach to the exploration of novel quantum phenomena in 2D materials research.