The Apparent Inner Calm of Quantum Materials

For the first time, physicists from UNIGE, Grenoble Alpes University, CEA and CNRS in Saclay and Grenoble have confirmed a theory on topological phase transitions, a field of research initiated by the Nobel Prize in physics 2016.

quantum materials
The helical structure of BACOVO: the oxygen atoms, represented in red, are organized in octahedra around the cobalt atoms located at their center. The blue arrows represent the small moments carried by the cobalt atoms, ordered anti-ferromagnetically along the helical chain. © CEA / CNRS / UGA

The transitions between different phases of matter are part of our everyday life: the freezing water thus passes from the liquid state to the solid state. But some of these transitions may have a different nature, related to the properties of so-called topological excitations, which make all the particles act collectively. The BACOVO compound, unknown to the general public, is a one-dimensional quantum material on which researchers from the University of Geneva (UNIGE) and CEA, CNRS and UGA, in collaboration with scientists from neutron ILL and PSI. They discovered in this material a new topological phase transition, not governed by one, but by two types of topological excitations. In addition, they could also select which one would dominate the other thanks to a magnetic field and thus control the transition. Their research is to be discovered in the journalNature Physics .

Researchers relied on the work of the 2016 Nobel Prize in Physics, awarded to physicists David Thouless, Duncan Haldane and Michael Kosterlitz, who predict that a set of topological excitations in a quantum material is likely to induce a phase transition . Many theories have been developed on these topological excitations, including the possibility of confronting two in one and the same material. But is it possible? And if so, what would happen? The researchers were able to provide the first experimental confirmation of the theory of the existence of two simultaneous games of topological excitations and their competition. A small revolution in the mysterious world of quantum properties.

The theory and experience intimately linked

Researchers from CEA, CNRS and UGA were working on a one-dimensional anti-ferromagnetic material with special properties called BACOVO (BaCo2V2O8). “We have done different experiments on BACOVO, an oxide characterized by its helical structure, explain in chorus Béatrice Grenier, Sylvain Petit and Virginie Simonet, researchers at CEA, CNRS and UGA. But we were confronted, in our experimental results, with a mysterious phase transition “. This is why their team used Thierry Giamarchi, professor in the Department of Physics of Quantum Matter of the Faculty of Science of UNIGE. “From their results, we have established theoretical schemes capable of interpreting them,” explains the Geneva physicist.

Creation of the “standard model”

The objective was to understand how the quantum properties of BACOVO work, especially its topological excitations. “For that, we used neutron scattering. This means that we are sending a neutron beam on the material. Neutrons behave like little magnets that interact with those of BACOVO according to a strategy “disrupt to reveal”, which allows us to understand their properties, “explains Quentin Faure, PhD student at the Nèel Institute of the CNRS and at the Nanosciences Institute. and cryogenics (CEA / UGA). When the model developed by UNIGE matches the experience, it becomes the “standard model” of the material. “And indeed, the model that we established with Shintaro Takayoshi predicted exactly the result of the experiment!”, Rejoices Thierry Giamarchi.

A material with unexpected properties

But this experiment led to a discovery that scientists did not anticipate. “After establishing the” standard model “of BACOVO, we observed unexpected properties,” enthuses Shintaro Takayoshi, a researcher at the Department of Physics of Quantum Matter in the Faculty of Science of UNIGE. Indeed, once placed in a magnetic field, BACOVO develops a second set of topological excitations that is in competition with the first, confirming theories of the 70s-80s built thanks to the field opened by the work of the Nobélisés. “In addition to proving the existence of this confrontation of two sets of topological excitations within the same material, unheard of, we have been able to experimentally control which game dominates the other”, adds the researcher from Geneva. . A first !

What was initially a theoretical hypothesis has become a verified experience. Thanks to a thorough analysis of BACOVO, the physicists of the team have proved that two sets of topological excitations come into direct confrontation in the same material and control its state. Depending on the dominant game, this confrontation leads to a quantum phase transition. In addition, scientists have been able to control which game wins, allowing them to adjust the state of BACOVO’s material at leisure. “These results open up a whole range of possibilities in the quest for quantum physics,” concludes Thierry Giamarchi. Of course, we are still at the fundamental level,

Source : University of Geneva