Strain-driven electronic phase transition and quantum transport signatures in epitaxial bismuth films on silicon substrates.

Bismuth (Bi), a canonical semimetal with giant spin-orbit coupling, is a prime candidate for hosting exotic topological phases of matter. Strain engineering offers a powerful pathway to manipulate its electronic ground state, yet systematic experimental validation remains elusive. Here, we demonstrate a strain-driven electronic phase transition in epitaxial Bi films grown on Si substrates via molecular beam epitaxy (MBE). A combination of in situ electron diffraction and ex situ X-ray diffraction reveals that coherent compressive strain, induced by lattice mismatch, systematically relaxes as the film thickness surpasses a critical value of approximately 150 Å. In the highly strained regime, the films exhibit a quasi-insulating behavior, consistent with the theoretically predicted strain-induced opening of a bulk band gap-a key prerequisite for a topological phase transition. As the strain relaxes, the system reverts to its intrinsic semimetallic state. This transition is directly mirrored in the electronic transport properties, where the evolution of a low-temperature resistivity anomaly provides a clear signature of weak localization, reflecting changes in electronic coherence and disorder. Our work establishes a direct correlation between epitaxial strain, electronic structure, and quantum transport phenomena in Bi, providing a comprehensive experimental framework for exploring strain-engineered topological states in quantum materials.