Condensed matter physics: topological defects discovered in amorphous solids

An international team led by  Alessio Zaccone, professor of Mathematical Methods in Physics at the University of Milan, has demonstrated the existence of topological defects in the amorphous state of matter, which includes all structurally disordered systems, such as human cells, or other essential materials like glass and polymers. This discovery has been recently described in a study published in Nature Communications.

An amorphous material is a solid whose molecules and atoms form disordered structures, meaning they do not occupy regular and well-defined positions in space. This is the opposite of what happens in crystals, whose ordered structure facilitates their mathematical description, as well as the identification of those "defects", which practically control the physical properties of crystals, such as their plastic yielding and melting, or the way an electric current propagates through them.

Of particular importance are topological defects, which are mathematically described as points of singularity within an ordered pattern, and around which the integral of a certain quantity changes its value after a full loop around the defect. Well known examples of topological defects, which control their mechanical properties are vortices, anti-vortices, solitons and dislocations. In amorphous systems such as glasses, or a random network of neuronal connections, topological defects were observed for the first time only in 2021, thanks to computer simulations of glassy materials conducted by professor Zaccone and his research team. Later on, other teams in China, France and the USA replicated the observation in different glassy systems.

Now, thanks to particular methods of numerical analysis applied to the treatment of experimental data from video microscopy, professor Zaccone and his international team have managed to clearly identify the topological defects in an amorphous colloidal glass, created by randomly assembling colloidal particles that interact via a magnetic field. The experimental data were collected at the University of Konstanz (Germany).

"The experimental demonstration of the existence of topological defects in disordered systems is a turning point in condensed matter physics, because it paves the way for the possibility to rationally control and manipulate the physical properties of amorphous materials and systems, with implications for artificial intelligence, the nervous system of living beings and even the large-scale structure of the cosmos", said professor Zaccone.

The study, which is part of professor Zaccone's Multimech project, was made possible thanks to an ERC Consolidator grant, as well as a grant funded by the US Army Research Office.