Exploring R-Stacked Semiconductors: Spin-Valley Locking, Optical Properties, and Superconductivity

The unique role of rhombohedral (R)-stacking of 2D layers was first recognized in the transition metal dichalcogenide (TMD) MoS₂. The valley-contrasting spin splitting was theoretically predicted in 2012 [1] but not experimentally observed, simply because the exfoliated monolayer MoS₂ was too small for the spin- and angle resolved photoemission spectroscopy (ARPES) experiments. In sharp contrast to the centrosymmetric 2H-MoS₂, 3R polymorph keeps the broken inversion symmetry of monolayer and thus the electronic structure of monolayers even in bulk. The bulk 3R MoS2 enables the spin-resolved ARPES, and provides the first proof of valley dependent spin polarization [2]. The unique optical properties were subsequently revealed such as photoluminescence and second harmonic generation (SHG) [3,4]. This proves the 3R stacking is extremely useful for investigating the symmetry controlled properties of TMDs. Giant bulk photovoltaic effect in the plane was observed that can be quasi-continuously tuned by tensile strain [5]. Another interesting feature of 3R TMD is that in addition to the in-plane inversion symmetry breaking, the out-of-plane inversion symmetry is broken. This results in the observation of the out-of-plane photovoltaic effect with high efficiency [6]. These findings are significant not only for fundamental physics but also positions the family of rhombohedral TMDs as promising candidates for applications such as energy-efficient photodetection with high speed and programmable polarity. On the other hand, superconductor with 3R polymorph is very rare. An interesting and generic feature of the 3R polymorph is that the interlayer hopping is exactly zero, and thus the dimensionality becomes purely 2D with reducing the carrier density. This was demonstrated in the LixZrNCl superconductor, which exhibits a 2D BCS-BEC crossover [7]. Finally, we touch on the symmetry controlled superconducting transport properties on the recently discovered 3R type-Ta_1+xSe₂ [8].