Surface-localized topological superconductivity in nodal-loop materials: BdG analysis

TL;DR

This paper investigates surface superconductivity in nodal-line semimetals, revealing that drumhead surface states favor a surface-localized chiral p-wave pairing, with implications for surface-sensitive experiments.

Contribution

It provides a theoretical analysis showing that surface states in nodal-line semimetals naturally promote chiral p-wave superconductivity, with detailed layer-dependent gap behavior.

Findings

Chiral p-wave order is strongly enhanced at the surface layers.

Superconductivity gaps out the drumhead surface states.

Zero-energy peaks split into coherence peaks in the surface LDOS.

Abstract

We theoretically study surface superconductivity in a nodal-line semimetal by combining a minimal tight-binding model with a layer-resolved Bogoliubov-de Gennes approach. In the normal state, the model realizes a bulk nodal loop and an associated drumhead surface band in a slab geometry with open boundaries in the $z$ direction: the central layers reproduce the bulk-like density of states, whereas the surface layer exhibits a sharp zero-energy peak originating from the drumhead states. On top of this band structure we introduce chiral $p$-wave and $d_{x^2-y^2}$-wave superconducting channels and determine the layer-dependent gap amplitudes self-consistently. The chiral $p$-wave order parameter is strongly enhanced at the outermost layers and decays within only a few layers towards the interior, while the $d$-wave order parameter is more than an order of magnitude smaller on all layers. The quasiparticle dispersion and surface local density of states in the chiral $p$-wave state show that the drumhead band is efficiently gapped out and that the zero-energy peak in the normal surface spectrum is split into two coherence peaks, directly reflecting the induced superconducting gap. These results demonstrate that superconductivity driven by drumhead surface states is naturally biased toward a surface-localized chiral $p$-wave pairing symmetry and may offer qualitative guidance for interpreting surface-sensitive experiments on Pd-doped CaAgP.