# Direct observation of multiple topological phases in the iron-based superconductor Li(Fe,Co)As

## TL;DR

This study directly observed multiple topological phases in the iron-based superconductor Li(Fe,Co)As using high-resolution ARPES, revealing coexistence of topological states and superconductivity, and demonstrating tunability via Co doping.

## Contribution

It provides the first direct observation of topological insulator and Dirac semimetal phases in Li(Fe,Co)As and shows their tunability, linking topological states with superconductivity in iron-based superconductors.

## Key findings

- Identified topological insulator and Dirac semimetal phases near the Fermi level.
- Demonstrated tuning of topological states via Co doping.
- Showed coexistence of multiple topological phases with superconductivity.

## Abstract

Topological insulators/semimetals and unconventional iron-based superconductors have attracted major attentions in condensed matter physics in the past 10 years. However, there is little overlap between these two fields, although the combination of topological states and superconducting states will produce more exotic topologically superconducting states and Majorana bound states (MBS), a promising candidate for realizing topological quantum computations. With the progress in laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) with very high energy- and momentum-resolution, we directly resolved the topological insulator (TI) phase and topological Dirac semimetal (TDS) phase near Fermi level ($E_F$) in the iron-based superconductor Li(Fe,Co)As. The TI and TDS phases can be separately tuned to $E_F$ by Co doping, allowing a detailed study of different superconducting topological states in the same material. Together with the topological states in Fe(Te,Se), our study shows the ubiquitous coexistence of superconductivity and multiple topological phases in iron-based superconductors, and opens a new age for the study of high-Tc iron-based superconductors and topological superconductivity.