Detecting the topological winding of superconducting nodes via Local Density of States

TL;DR

This paper proposes a method using STM measurements near impurities to detect the topological winding of nodes in 2D superconductors, providing a practical way to observe subtle topological features.

Contribution

It extends a previous technique to 2D nodal superconductors, establishing conditions for STM measurements to reveal topological winding via impurity scattering.

Findings

Spin-polarized STM can detect winding of nodes in NbSe2.

Impurity choice influences the measurement of topological invariants.

Robust vortices in momentum space indicate topological winding.

Abstract

Many systems are topologically trivial in the bulk, but still have non-trivial wavefunctions locally in the Brillouin zone. For example, in a small-gap Dirac material the Berry curvature is strongly peaked, but cancels over the full Brillouin zone, while in semimetals and in nodal superconductors there may be a lower-dimensional winding topology associated to the nodes. Experimentally, it is difficult to directly observe such topology. We consider general bulk Hamiltonians with nodes and chiral symmetry, extending to them the method developed in Dutreix et al. [Nature, 574(7777):219-222 (2019)], which in particular detected the winding around Dirac cones in graphene using charge modulations around an impurity. We apply our method to nodal superconductors in 2d, in presence of a (non)magnetic impurity, measured by standard or spin-polarized STM tip. We derive general conditions on the impurity scattering and on the STM tip, expressed in terms of their preference among the two chiralities, for when the measurement near the impurity captures the winding difference between any chosen pair of (Bogoliubon) Dirac cones. We emphasize the robustness of observing vortices in momentum space, in contrast to dislocations in real space, in STM data. Testing the conditions on the topological nodal superconductor proposed for monolayer NbSe2 under an in-plane magnetic field, we find that spin-polarized STM on a magnetic impurity can detect the winding of each of the 12 nodes. We conclude that a judicious choice of impurity can be a powerful tool to determine topological quantities in 2d superconducting systems as well as any nodal chiral system.