Quasiparticle Chirality and Electron-Phonon Interactions Drive Novel Material Phases.

Research demonstrates a linear relationship between electronic coupling and electron-phonon interaction strength in distorted structures exhibiting Kekulé bond order. This yields spatially varying pairing strength, creating domains of enhanced coupling, and clarifies phonon-mediated phenomena, notably superconductivity, offering avenues for material engineering. (Kekulé order refers to an alternating bond pattern).

Graphene, a single-atom-thick sheet of carbon, continues to reveal complex behaviours beyond its initially predicted exceptional electronic properties. Recent research focuses on distortions within graphene’s normally symmetrical lattice, specifically the emergence of ‘bond density waves’ which create a ‘Kekulé-ordered’ structure, altering how electrons interact with atomic vibrations, known as phonons. Understanding these ‘electron-phonon’ interactions is crucial for manipulating graphene’s electronic characteristics and potentially enhancing superconductivity. Dominik Szczęsny, from the Institute of Physics at Jan Długosz University in Częstochowa, investigates these local correlations within Kekulé-ordered graphene, presenting a distance-dependent framework to describe electronic structure and revealing a linear relationship between electronic coupling and the strength of electron-phonon interactions. This work, detailed in the article ‘Electron-phonon coupling in Kekulé-ordered graphene’, demonstrates how induced distortions create spatially varying pairing strengths, offering insights into phonon-mediated phenomena and potential avenues for materials engineering.

Researchers investigate the influence of structural distortions on the electronic properties of graphene, with a particular focus on inducing and controlling superconductivity. Graphene, a single-layer sheet of carbon atoms arranged in a honeycomb lattice, exhibits exceptional electronic characteristics, but realising its full potential requires precise manipulation of its atomic structure. Distortions, such as bond density waves – periodic modulations in the bond lengths – and Kekulé order – a pattern of alternating bond lengths creating localised bonds – significantly alter the material’s behaviour.

The team develops a theoretical framework to analyse graphene’s electronic structure locally, accounting for these distortions and their impact on electron-phonon interactions. Phonons represent quantised vibrations within the crystal lattice, and their interaction with electrons is crucial for superconductivity. The strength of this interaction, known as electron-phonon coupling, directly correlates with the electronic coupling within the material, as determined by the framework. This allows researchers to predict how specific distortions affect the likelihood of Cooper pairs – pairs of electrons that carry supercurrent – forming.

Crucially, the distortions introduce a non-uniform distribution of pairing strength across the graphene sheet. This creates domains where electron-phonon coupling is enhanced, increasing the probability of superconductivity in those regions. The research utilises computational methods, primarily tight-binding calculations, to model these complex interactions. Tight-binding is an approximation method used in condensed matter physics to calculate the electronic structure of materials, simplifying the Schrödinger equation to focus on interactions between atoms. Both established and modern techniques are employed, including optimisation algorithms such as the Levenberg-Marquardt algorithm, used to refine the calculations and achieve accurate results.

The ultimate aim is to engineer graphene materials with tailored superconducting properties by precisely controlling the spatial distribution of electron-phonon coupling. Recent publications in 2024, alongside the continued work of established researchers in the field, demonstrate the ongoing nature of this investigation and its potential to advance materials science. The ability to manipulate these distortions offers a pathway towards creating novel superconducting materials with applications in diverse fields, including energy transmission and quantum computing.