Tuning MgB_2(0001) surface states

TL;DR

This study uses density functional theory to analyze and demonstrate how surface states on MgB_2(0001) can be tuned via surface termination and charging, impacting its electronic and superconducting properties.

Contribution

It provides a detailed theoretical analysis of surface state localization and energy shifts on MgB_2(0001) surfaces, revealing how surface termination and doping can control these states.

Findings

B {\sigma}_1 surface state is localized in the top B layer.

Charging the B layer shifts and fills surface states, affecting their localization.

Adding Mg or Li terminates shifts the spz state and modifies its character.

Abstract

Surface state localization and hybridization on B, Mg, and Li terminated MgB_2(0001) surfaces are studied within density functional theory (DFT). For the B terminated surface we find a low energy B {\sigma}_1 surface state, a spz surface state, and B {\sigma}_2 and {\sigma}_3 quantum well states, which are 90% localized in the topmost B layer. Our results demonstrate that by charging the B atomic layer, either by changing the surface termination or through electro-chemical doping, the B {\sigma} surface states are shifted down in energy, filled, and delocalize through hybridization as they cross the bulk MgB_2 bulk bands. On the other hand, the spz surface state should be shifted up in energy, emptied, and gain an increasingly metallic s character by adding a Mg, Mg^{+1}, or Li terminating atomic layer. These results clearly show both the robust nature of MgB_2(0001) surface states, and how their localization and energy range may be tuned by surface termination and charging, with implications for the superconducting and plasmonic behaviour of MgB_2.