Paving the way for a new type of material the properties of which can be controlled and modified at will

A team of researchers from the Politecnico di Milano and the University of Rostock (Germany) has discovered and observed in the laboratory a new type of phase transition in a bizarre quasi-crystal made of laser light. The discovery was recently published in the renowned journal Nature and could pave the way for a holistic understanding of the inner workings of complex or engineered materials and their use in advanced phase-controlled materials-based applications.

Stefano Longhi, who works in the Physics Department of the Politecnico di Milano, said: "The discovery of this new phase transition in quasi-crystals represents a breakthrough in the understanding of some fundamental phenomena of quantum matter. It may also pave the way for the development of a new technology and type of material unlike anything we have seen before, the properties of which we will be able to simultaneously control and modify at will. It would be a new form of matter much more flexible and controllable than the one we currently know about."

A typical example of phase transition is that observed in winter: small deviations from a temperature of 0°C determine whether water exists in its liquid form or as solid ice and snow.  There are also less perceptible, and therefore less familiar, transitions between states (or phases) of matter that can be just as significant to the operation of many commonly used devices, from computer chips to cell phones: for example the ability or not of a material to conduct electricity, or to exchange energy or particles with the surrounding environment. 

Using state-of-the-art optical technologies, the team of experimental physicists recently made a quasi-crystal of light (quasi-crystals are structures that are not perfectly ordered, like crystals, but not completely disordered and are among the rarest structures in nature) and demonstrated that seemingly independent properties in this bizarre material are actually intimately linked and can jointly undergo sudden change.

To study the characteristics of these fascinating materials, a quasi-crystal was emulated in the laboratory with laser light that propagates in an intertwined manner in kilometre-long optical fibres. The complex dynamics of light in these fibres closely mirrors the quantum motion of electrons in the quasi-crystal.

During the study of light propagation in these systems, a triple phase transition was discovered, in which the topological properties, conductivity, and energy exchange between the quasi-crystal and its surroundings change abruptly but at the exact same time.