Bi(111) thin film with insulating interior but metallic surfaces

TL;DR

This study reveals that Bi(111) thin films are insulating internally but metallic on the surface, resolving a long-standing debate and suggesting potential spintronics applications.

Contribution

It provides the first clear experimental evidence that Bi(111) thin films are insulating inside yet metallic on the surface, clarifying previous controversies.

Findings

Bi thin films up to 90 nm are insulating internally

Surface metallicity persists despite insulating interior

Results clarify the temperature dependence of resistivity in Bi films

Abstract

The electrical conductance of molecular beam epitaxial Bi on BaF2(111) was measured as a function of both film thickness (4-540 nm) and temperature (5-300 K). Unlike bulk Bi as a prototype semimetal, the Bi thin films up to 90 nm are found to be insulating in the interiors but metallic on the surfaces. This result has not only resolved unambiguously the long controversy about the existence of semimetal-semiconductor transition in Bi thin film but also provided a straightforward interpretation for the long-puzzled temperature dependence of the resistivity of Bi thin films, which in turn might suggest some potential applications in spintronics.