How substrates influence superconductivity through moiré phonons

The interplay between the electron and phonon dynamics in 2D materials is a complex and fascinating subject. By examining the behaviour of NbSe₂ on graphene, a new study has provided new insights into how substrates influence superconductivity through moiré phonons.

In recent years, the study of two-dimensional (2D) materials has transformed the field of condensed matter physics. These materials, often only a few atoms thick, exhibit remarkable properties that differ drastically from their bulk counterparts. Among the most exciting of these is a class of materials known as superconductors, which can conduct electricity without any resistance at low temperatures. One material in particular, NbSe₂ (niobium diselenide), has been a subject of intense research due to its ability to become superconducting at temperatures close to absolute zero. In this article, we will explore a recent study ¹ that investigates the role of graphene as a substrate for a single layer of NbSe₂, revealing how the combination of these materials could lead to new insights in electron behaviour and phonon dynamics, ultimately contributing to advancements in superconductivity. The scientists used advanced techniques to observe these interactions, focusing particularly on a phenomenon known as “moiré phonons.”

What are moiré phonons?

To understand the role of moiré phonons, we first need to understand a bit about the materials involved. Graphene is a single layer of carbon atoms arranged in a honeycomb lattice, and NbSe₂ is a superconductor that can also exist as a thin monolayer. When these two materials are placed on top of one another, their atomic structures can sometimes mismatch in a specific, repeating pattern known as a “moiré pattern.” This is similar to the way two pieces of mesh might form an interference pattern when placed on top of each other.

The study focuses on the phonons—vibrations of atoms within a material—within this moiré pattern. These vibrations, or moiré phonons, are thought to be localized at the interface between NbSe₂ and graphene, and they can significantly impact the material’s properties.

Phonons play a critical role in how electrons move and interact within a material. The electron-phonon coupling is essential for understanding many of the material’s properties, including its superconductivity. The way electrons and phonons interact can either enhance or suppress superconductivity, so understanding this coupling is vital for designing better superconducting materials.

In the case of NbSe₂, when it is placed on graphene, the study found that the moiré phonons at the interface contribute significantly to the electron-phonon coupling. The team measured the strength of the electron-phonon coupling in both bulk and monolayer NbSe₂. In bulk NbSe₂, the electron-phonon coupling constant (which measures the strength of the interaction between electrons and phonons) was found to be 0.76. In contrast, for the monolayer of NbSe₂ on graphene, this coupling constant was weaker, at 0.55. This weaker coupling suggests that the graphene layer affects the way electrons interact with the vibrations of the NbSe₂ atoms.

A superstructure and phonon dynamics

The researchers observed that the NbSe₂ monolayer forms a unique superstructure when placed on graphene. This superstructure, a (9×9) pattern, aligns with the graphene lattice. This alignment creates a periodic interaction between the two materials that influences the electron behaviour.

The study provided detailed measurements of the phonon dynamics in NbSe₂ when it is placed on graphene. They found that while the typical phonons behave as expected, there is a soft, dispersionless branch of phonons at 1.7 meV. These are attributed to the moiré phonons and are localized at the interface between the two materials. This new phonon branch is crucial in understanding how the moiré pattern influences the properties of the material.

The role of graphene

The research also calculated the superconducting critical temperature (Tₐ) of the NbSe₂ monolayer on graphene. According to their calculations, the lower electron-phonon coupling in this system corresponds to a lower superconducting critical temperature—around 1.56 K, which is consistent with experimental transport measurements.

Graphene, while not itself superconducting, appears to have a significant influence on the behaviour of NbSe₂ when they are combined. The study highlights that graphene’s unique electronic properties, such as its high conductivity and flexibility, can affect the way electrons move through NbSe₂. These effects are magnified by the moiré phonons that form at the interface between the two materials. These findings could pave the way for future research into how substrates like graphene could be used to control or enhance the properties of other superconducting materials.

This study is an important step in understanding how 2D materials like NbSe₂ can interact with other materials at the atomic level. By studying the electron-phonon interactions in these systems, researchers can gain insights into how to better control the behaviour of electrons in superconducting materials.

Additionally, understanding the role of moiré phonons could lead to new techniques for tuning the properties of materials to suit specific applications, such as in quantum computing or energy-efficient electronics, where superconductivity plays a crucial role.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.