Thickness-driven phase selection for epitaxial helical tellurium on a van der Waals superconductor.

The unique chiral structure and strong spin-orbit coupling of helical Te make it a compelling platform for realizing novel topological quantum phenomena through van der Waals (vdW) heterostructures. However, progress has been hindered by a lack of precise control over the structural phase and interface quality of Te during ultrathin film growth. Here, we demonstrate a robust pathway for synthesizing helical Te on the superconductor NbSe2, governed by a thickness-driven phase selection mechanism. Using scanning tunneling microscopy (STM), we show that Te initially forms a kinetically trapped single-layer (1L) stripe phase. When the local thickness surpasses a critical two-layer (2L) threshold, the film spontaneously transforms into epitaxial helical Te at room temperature (RT), with the 2L islands serving as nucleation centers. Combined STM and cross-sectional scanning transmission electron microscopy (STEM) analyses confirm an atomically abrupt Te/NbSe2 vdW interface and identify a robust commensurate epitaxial relationship. This discovery provides a reproducible route to high-quality Te/superconductor heterostructures, providing a platform for future investigations into the interplay between structural chirality and superconducting proximity effects.