Superconducting 3 R -Ta 1+ x Se 2 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, TaSe ₂ , grown by MBE crystallizes into the pure 3 R phase with the self-intercalated Ta atoms, 3 R -Ta _{1+ x} Se ₂ , which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3 R -Ta _{1+ x} Se ₂ film exhibits a superconducting (SC) critical temperature ( T _c ) of 3.0 K, which is the highest among all of the polymorphs in TaSe ₂ . Thickness-dependence measurements reveal that T _c decreases with decreasing thickness, accompanied by the development of the charge-density wave phase. The 3 R -Ta _{1+ x} Se ₂ 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 3 R 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 3 R -Ta _{1+ x} Se ₂ distinct from those of thermodynamically stable 2 H -TaSe ₂ , demonstrating the essential importance of the MBE-based approach to the exploration of novel quantum phenomena in 2D materials research.