Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

TL;DR

This study demonstrates that interface engineering in Pb/Si(111) systems can significantly control the effective mass of quantum well states, with experimental and theoretical evidence linking lattice constant variations to effective mass changes.

Contribution

It provides a detailed analysis of how interface-induced lattice constant changes affect the effective mass of quantum well states in Pb films, combining experimental and density functional theory approaches.

Findings

Effective mass is reduced by a factor of three with interface engineering.

Lattice constant variations of about 2% influence the effective mass.

Anomalous dependence of effective mass on lattice constant due to orbital overlap and hybridization.

Abstract

The in-plane effective mass of quantum well states in thin Pb films on a Bi reconstructed Si(111) surface is studied by angle-resolved photoemission spectroscopy. It is found that this effective mass is a factor of three lower than the unusually high values reported for Pb films grown on a Pb reconstructed Si(111) surface. Through a quantitative low-energy electron diffraction analysis the change in effective mass as a function of coverage and for the different interfaces is linked to a change of around 2% in the in-plane lattice constant. To corroborate this correlation, density functional theory calculations were performed on freestanding Pb slabs with different in-plane lattice constants. These calculations show an anomalous dependence of the effective mass on the lattice constant including a change of sign for values close to the lattice constant of Si(111). This unexpected relation is due to a combination of reduced orbital overlap of the 6p_z states and altered hybridization between the 6p_z and 6p_xy derived quantum well states. Furthermore it is shown by core level spectroscopy that the Pb films are structurally and temporally stable at temperatures below 100 K.