Topology in the correlated Fe-based superconductors

Project Information

FeTOP

Grant agreement ID: 101024210

DOI

10.3030/101024210

Project closed

EC signature date 29 March 2021

Start date 1 March 2022

End date 25 July 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 219 312,00

EU contribution

€ 219 312,00

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - FeTOP (Topology in the correlated Fe-based superconductors)

Reporting period: 2022-03-01 to 2024-02-29

Summary of the context and overall objectives of the project

Electrons in materials repel each other through the Coulomb interaction. The strength of this interaction governs many properties of the material and can be decisive in determining if the material is, e.g. a conductor or an insulator. However, the question of how the Coulomb repulsion affects the topological properties of a material - attributes that are thought to be independent of the details of the electronic behaviour - has not yet been answered.

In this project, I will study an example class of materials - known as kagome metals - that are presumed to exhibit topological properties and provide a framework within which to address this question. This constitutes an important step in understanding the fundamental properties of materials and will allow for the development of theories that elucidate the interplay between electronic interactions and topological properties. Such progress holds tremendous promise for society as a whole. Materials exhibiting topological properties can revolutionize the form and function of our modern computers by introducing new ways of storing information and by permitting the design of so-called quantum computers. In this context, the impact of the electronic Coulomb repulsion can rarely be neglected and consequently, understanding how it impacts the topological properties of a material is crucial to advancing the field to the stage where it can have a real-world impact.

The overall objective of the project is to provide a theoretical basis for understanding topological phenomena in materials that are governed by electronic interactions, so-called correlated materials. This is achieved through three complementary approaches which (1) Describe how such topological properties arise in the first place, (2) how they are affected once interactions become dominant, and (3) how their presence can be unambiguously detected.

These objectives have been addressed in a series of four peer-reviewed publications that are freely available. In summary, these describe how interactions can result in specific states of matter in the kagome metals and how specific experiments can be designed to detect these phases. Interactions lead to both superconductivity and a so-called charge order - where the electronic density increases around specific atoms in the lattice - and the publications detail the relationship between the two and how they are affected by disorder and details of the electronic structure.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The initial work on this project centered on understanding the microscopic origin of both superconductivity and charge order in kagome metals. This understanding is an essential step in describing the possible topological properties of the kagome metals. Specifically, I described the types of charge order possible in the kagome metals and how these may arise, including a detailed description of how to design experiments to unambiguously detect and distinguish the charge order present in the actual materials. Subsequently, I conducted a similar study of how superconductivity arises in these materials and how this may lead to topological phenomena. This includes detailed considerations of how superconductivity is affected by disorder and details of the electronic structure and provides important predictions for the possible superconducting states observed in the kagome metals, alongside their topological properties.

In summary, the action resulted in the following results:
(1) A detailed description of the charge orders possible in the kagome metals, their possible microscopic origin, and experimental methods to detect these orders. This has been disseminated in the peer-reviewed journal Physical Review B and is freely available. It was also presented at the Gordon Research Seminar 'Superconductivity' conference.
(2) A microscopic derivation of the possible superconducting phases of the kagome metals, including how these are affected by electronic interactions, and which ones may exhibit topological properties. This has been disseminated in the peer-reviewed journal Physical Review B and is freely available.
(3) A comprehensive treatment of how superconductivity is affected by changes in the electronic structure, in particular as a result of straining the material. This helps explain a set of puzzling experiments on the kagome metals and was published as a letter in Physical Review B and is freely available.
(4) A study of how lattice disorder impacts specific superconducting phases in the kagome lattice, including how this impacts the topologically non-trivial variants. The details of the kagome materials result in a surprising robustness of superconductivity to disorder. This was published in Physical Review B and is freely available.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project has advanced the state of the art in several key ways:
(1) It has elucidated the origin and properties of charge orders and superconductivity in the kagome metals.
(2) It has made clear that the impact of disorder in these systems is different from their square-lattice counterparts.
(3) It highlights the important impact strain can have on both the electronic degrees of freedom and on the superconducting instabilities in these systems.

These results have the potential to allow the topological aspects of the kagome metals to be exploited with wide-reaching consequences for our electronic devices and computing methods.

Calculation showing how the superconducting critical temperature is impacted by disorder.

Superconducting phase diagram for a kagome lattice as a function of interaction strengths.

Calculation of the impact of strain on the electronic structure of a kagome metal.

Illustration of the different ways charge orders can result in magnetic orders in kagome metals.