Federico Mazzola

An international research group has discovered a new state of matter characterized by the existence of a quantum phenomenon called chiral current. These currents are generated on an atomic scale by a cooperative movement of electrons, unlike conventional magnetic materials whose properties originate from the quantum characteristic of an electron known as spin and their ordering in the crystal. Chirality is a property of extreme importance in science, for example, it is fundamental also to understand DNA. In the quantum phenomenon discovered, the chirality of the currents was detected by studying the interaction between light and matter, in which a suitably polarized photon can emit an electron from the surface of the material with a well-defined spin state. The discovery significantly enriches our knowledge of quantum materials, on the search for chiral quantum phases and on the phenomena that occur at the surface of materials. The discovery of the existence of these quantum states may pave the way for the development of a new type of electronics that employs chiral currents as information carriers in place of the electron’s charge. Furthermore, these phenomena could have an important implication for future applications based on new chiral optoelectronic devices, and a great impact in the field of quantum technologies for new sensors, as well as in the biomedical and renewable energy fields. Born from a theoretical prediction, this study directly and for the first time verified the existence of this quantum state, until now enigmatic and elusive, thanks to the use of the Italian Elettra synchrotron. Until now, knowledge about the existence of this phenomenon was limited to theoretical predictions for some materials. Its observation on the surfaces of solids makes it extremely interesting for the development of new ultra-thin electronic devices. The research group, which includes national and international partners including the Ca’ Foscari University of Venice, the SPIN Institute, the CNR -Materials Officina Institute and the University of Salerno, investigated the phenomenon of a material already known to the scientific community for its electronic properties and superconducting spintronics applications, but the discovery has a broader scope, being much more general and applicable to a vast range of quantum materials. These materials are revolutionizing quantum physics and the current development of new technologies, with properties that go far beyond those described by classical physics. The study was published in the prestigious journal Nature. Full link: https://www.nature.com/articles/s41586-024-07033-8

The study centres on Co3Sn2S2, a magnetic Weyl kagome system. The kagome lattice, a hexagonal network of interconnected triangles, has been a subject of intense scientific interest due to its distinctive geometry and potential for hosting topological phases. Co3Sn2S2, in particular, exhibits intriguing properties such as high-electron density flat bands, Dirac states, and the presence of Fermi arcs. These are unique properties of the electronic band structure that offer exciting possibilities for novel electronic and quantum devices. The researchers investigated, led by Federico Mazzola, the surface terminations of Co3Sn2S2, focusing on how different terminations affect the connectivity of Weyl points, which are monopoles of Berry curvature responsible for anomalous Hall effects and topological surface states. Scanning tunnelling microscopy (STM) studies had suggested the presence of multiple surface terminations in Co3Sn2S2, and this research aimed to clarify their impact on electronic properties. By utilizing micro-angle-resolved photoelectron spectroscopy (micro-ARPES) and first-principles calculations, the team measured the energy-momentum spectra and Fermi surfaces for different surface terminations of Co3Sn2S2. The results revealed that the type of termination significantly influenced the electronic properties and topological features of the material. Notably, the researchers observed termination-dependent Fermi arcs that connect Weyl points in distinct ways, suggesting that the surface environment plays a crucial role in shaping the material’s topological connectivity. The study’s findings have broad implications for the field of materials science and quantum electronics. The ability to control topological properties by manipulating surface electrostatic potentials opens up new avenues for designing responsive magnetic spintronics devices and harnessing the unique electronic characteristics of materials like Co3Sn2S2. This research represents a significant step toward understanding and harnessing topological phases in materials, paving the way for innovative applications in fields such as spintronics, superconductors, and low-voltage electronics. The study’s combination of experimental and theoretical approaches provides a comprehensive view of the interplay between surface terminations and topological properties, offering valuable insights for future material design and engineering. The full article is available at the following link: https://doi.org/10.1021/acs.nanolett.3c02022

In an international collaboration involving experiments and theory, the topological wrapping of electrons in matter, which refers to the curvature of the space in which they exist and move, has been measured in a new class of materials. This new discovery could lead to a deeper understanding of quantum materials, which are materials where the collective behavior of the electrons that make them up generates new properties that can only be interpreted using the laws of quantum mechanics. These materials are expected to have future applications in various technological fields, from renewable energies to biomedicine, and from electronics to quantum computers. Using advanced experimental techniques that utilize light generated by a particle accelerator called a synchrotron, and thanks to modern modeling techniques of material behavior, it has been possible to measure the wrapping of electrons for the first time, in relation to the concept of topology. Just as a soccer ball and a donut have different topological properties related to their shapes (for example, the donut has a hole while the soccer ball does not), the behavior of electrons in materials is influenced by certain quantum properties that determine their wrapping, similar to how the trajectory of light in the universe is modified by the presence of stars, black holes, dark matter, and dark energy, which bend space and time. Although this particular characteristic of electrons has been known for many years, no one had been able to directly measure this “topological wrapping” until now. By exploiting an effect known as circular dichroism (from the Greek “di-” meaning two and “chroma” meaning color), which means that materials absorb light differently depending on its polarization, it has been possible to obtain this measurement for the first time. In addition to using this particular experimental technique available only at a synchrotron source, the synergy with theoretical analysis has been particularly relevant. In fact, the theoretical researchers on the team employed sophisticated quantum simulations made possible only through the use of powerful supercomputers, thus guiding the experimental colleagues towards the specific region of the material’s electronic properties where it was possible to measure the effect related to circular dichroism. Achieving this feat was the result of an international collaboration of scientists, including CNR-IOM in Trieste, the University of Bologna, Ca’ Foscari University in Venice, the University of Milan, the University of Würzburg (Germany), the University of St. Andrews (UK), Boston College, and the University of Santa Barbara (United States). The class of materials that the team focused on is called Kagome materials, named after their close resemblance to the pattern of bamboo threads in a traditional Japanese basket. These materials are revolutionizing quantum physics due to their magnetic, topological, and superconducting properties. This new discovery promises to revolutionize the way quantum materials will be studied in the future, thereby opening the doors to new developments in quantum technologies. The study has been published in the journal Nature Physics, D. Di Sante et al. “Flat band separation and robust spin Berry curvature in bilayer kagome metals.” The full article is available at the following link: https://www.nature.com/articles/s41567-023-02053-z