Another article by researchers from Instituto Superior Técnico was published on 11 October 2024 in Nature Physics. Pedro Ribeiro, a researcher at the Center of Physics and Advanced Materials Engineering (CeFEMA), Flávio Riche and Miguel Gonçalves, Técnico PhD students when writing the article, were involved in the study of twisted bilayer graphene, a graphene material, and its optical and electrical properties.
The work, titled “Incommensurability enabled quasi-fractal order in 1D narrow-band moiré systems”, is the result of a collaboration within the Laboratory of Physics for Materials and Emergent Technologies (LaPMET) and involved researchers Eduardo Castro (University of Porto) and Bruno Amorim (University of Minho), also part of LaPMET.
Moiré materials are usually formed by superimposing two or more layers of materials in two dimensions with a slight angular deviation between them. This mismatch in the position of the two layers causes a new pattern to emerge, a ‘moiré pattern’, which can give the material new electronic and optical properties. Different patterns can be created by varying the rotation angle between the two layers, giving rise to various phases of matter.
In recent years, moiré materials have been a point of interest in condensed matter physics due to experimental observations of ‘exotic phases of matter’, in which peculiar properties such as superconductivity (a phenomenon in which electric current passes through the material without resistance) can be observed.
In the study of these materials, previous theoretical work assumed that the patterns repeated periodically, an approximation that simplified the problem. In contrast, the work published in Nature Physics studies the existence of exotic properties that can be observed without this approximation, taking into account the ‘quasiperiodicity’ of the material – the pattern ‘almost’ repeats, but never did it.
‘The fact that it’s a quasiperiodic system [and no periodicity is assumed] makes it much more susceptible to showing one of these exotic phases [in which the material can become a superconductor or an insulator],’ explains Pedro Ribeiro. ‘Basically, we proved that, under certain conditions, quasiperiodicity favours the appearance of these phases’, adds the CeFEMA researcher. ‘The message we want to convey in this paper is that we don’t think quasiperiodicity is a detail – it’s actually a fundamental ingredient – and this article is a step towards proving this is true.’
Pedro Ribeiro emphasises the merit of the Técnico students involved in the study. ‘Técnico is important because it produces very good students’, he shares, referring in particular to Miguel Gonçalves and Flávio Riche, the students who signed the article written during their PhDs in Physics at Técnico.
