Quantum size effects, multiple Dirac cones and edge states in ultrathin Bi(110) films

TL;DR

This study experimentally verifies quantum size effects, Dirac cones, and edge states in ultrathin Bi(110) films, revealing oscillatory electronic behavior and confirming theoretical predictions about topological states and edge phenomena.

Contribution

First experimental observation of Dirac states and quantum size effects in Bi(110) films using ARPES, confirming theoretical models of topological and edge states.

Findings

Dirac states identified in odd-bilayer films

Quantum size effects cause oscillatory electronic structure

Edge states observed in ultrathin Bi(110) islands

Abstract

The presence of inherently strong spin-orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac fermion states in the odd-bilayer (BL) films, originating from dangling $p_z$ orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electronic structure of Bi(110) films grown on a high-$T_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES). We observe an oscillatory behavior of electronic structure with the film thickness and identify the Dirac-states in the odd-bilayer films, consistent with the theoretical predictions. In the even-bilayer films, we find another Dirac state that was predicted to play a crucial role in the QSH effect. In the low thickness limit, we observe several extremely one-dimensional states, probably originating from the edge-states of Bi(110) islands. Our results provide a much needed experimental insight into the electronic and structural properties of Bi(110) films.