Superconducting 3<i>R</i>-Ta_1+<i>x</i>Se₂ with Giant In-Plane Upper Critical Fields.

Abstract

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 3<i>R</i> phase with the self-intercalated Ta atoms, 3<i>R</i>-Ta_1+<i>x</i>Se₂, which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3<i>R</i>-Ta_1+<i>x</i>Se₂ film exhibits a superconducting (SC) critical temperature (<i>T</i>_c) of 3.0 K, which is the highest among all of the polymorphs in TaSe₂. Thickness-dependence measurements reveal that <i>T</i>_c decreases with decreasing thickness, accompanied by the development of the charge-density wave phase. The 3<i>R</i>-Ta_1+<i>x</i>Se₂ films exhibit large in-plane upper critical fields (<i>H</i>_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<i>R</i> stacking. Moreover, the temperature dependence of the in-plane <i>H</i>_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<i>R</i>-Ta_1+<i>x</i>Se₂ distinct from those of thermodynamically stable 2<i>H</i>-TaSe₂, demonstrating the essential importance of the MBE-based approach to the exploration of novel quantum phenomena in 2D materials research.