Chiral topological superconducting state with Chern number $\mathcal{C} =-2$ in Pb$_3$Bi/Ge(111)

TL;DR

This paper models Pb$_3$Bi/Ge(111) as a candidate for chiral topological superconductivity with Chern number -2, highlighting the roles of Rashba spin-orbit coupling and Zeeman field in realizing Majorana modes.

Contribution

It develops a tight-binding model capturing key electronic features and predicts conditions for topological superconductivity and Majorana edge modes in Pb$_3$Bi/Ge(111).

Findings

System can be tuned into a chiral topological superconductor with Chern number -2.

Two chiral Majorana edge modes propagate along the same boundary.

Physically realistic conditions for observing Majorana modes are discussed.

Abstract

Materials realization of chiral topological superconductivity is a crucial condition for observing and manipulating Majorana fermions in condensed matter physics. Here we develop a tight-binding description of Pb$_3$Bi/Ge(111), identified recently as an appealing candidate system for realizing chiral $p$-wave topological superconductivity [Nat. Phys. 15, 796 (2019)]. We first show that our phenomenological model can capture the two main features of the electronic band structures obtained from first-principles calculations, namely, the giant Rashba splitting and type-II van Hove singularity. Next, when the $s$-wave superconducting property of the parent Pb system is explicitly considered, we find the alloyed system can be tuned into a chiral topological superconductor with Chern number $\mathcal{C} = -2$, resulting from the synergistic effect of a sufficiently strong Zeeman field and the inherently large Rashba spin-orbit coupling. The nontrivial topology with $\mathcal{C} = -2$ is further shown to be detectable as two chiral Majorana edge modes propagating along the same direction of the system with proper boundaries. We finally discuss the physically realistic conditions to establish the predicted topological superconductivity and observe the corresponding Majorana edge modes, including the influence of the superconducting gap, Land\'{e} $g$-factor, and critical magnetic field. The present study provides useful guides in searching for effective $p$-wave superconductivity and Majorana fermions in two-dimensional or related interfacial systems.