Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors

TL;DR

This paper reviews how scanning tunneling microscopy (STM) has advanced understanding of Fe-based high-temperature superconductors by revealing pairing symmetry, symmetry breaking, and vortex states, highlighting recent progress and challenges.

Contribution

It provides a comprehensive overview of STM insights into Fe-based superconductors, emphasizing the techniques' role in uncovering pairing symmetry, symmetry breaking, and vortex phenomena.

Findings

Demonstrated variation of pairing symmetry from nodal to nodeless s± within FeTe1-xSex

Imaged C4 to C2 symmetry breaking in electronic states

Gained insights into vortex pinning and competing states in superconductors

Abstract

In the first three years since the discovery of Fe-based high Tc superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed light on three important questions. First, STM has demonstrated the complexity of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle interference (QPI) imaging and low temperature spectroscopy have shown that the pairing order parameter varies from nodal to nodeless s\pm within a single family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the electronic states of both parent and superconducting materials. As a local probe, STM is in a strong position to understand the interactions between these broken symmetry states and superconductivity. Finally, STM has been used to image the vortex state, giving insights into the technical problem of vortex pinning, and the fundamental problem of the competing states introduced when superconductivity is locally quenched by a magnetic field. Here we give a pedagogical introduction to STM and QPI imaging, discuss the specific challenges associated with extracting bulk properties from the study of surfaces, and report on progress made in understanding Fe-based superconductors using STM techniques.