(Nanowerk News) They are seen as a beacon of hope for the energy-efficient and high-tech electronics of the future: topological quantum materials. One of its properties is the conduction of spin-polarized electrons on its surface – even though it is actually non-conductive inside. To illustrate: In a spin-polarized electron, the intrinsic angular momentum, i.e. the direction of the particle’s rotation (spin), is not purely randomly aligned.
To distinguish topological materials from conventional ones, scientists usually study their surface currents. However, the topology of the electron is closely related to the wave nature of quantum mechanics and its spin. This connection has now been proven directly through the photoelectric effect – a phenomenon in which electrons are released from a material, such as a metal, with the help of light.
This research has been published in (Natural Physics, “Flat band separation and strong Berry spin curvature in metal kagome bilayer”).
Visualizing electron topology with “3D glasses”
Prof Giorgio Sangiovanni, founding member of ct.qmat in Würzburg and one of the theoretical physicists on the project, likens the discovery to using 3D glasses to visualize the electron’s topology. As he explains: “Electrons and photons can be described in quantum mechanics as waves and particles. Therefore, the electron has a spin that we can measure thanks to the photoelectric effect.”
To do this, the team used circularly polarized X-ray light – light particles that have torsions. Sangiovanni elaborates: “When a photon encounters an electron, the signal coming from the quantum material depends on whether the photon has right or left polarization. In other words, the orientation of the electron spin determines the relative strength of the signal between the left and right polarized rays. Therefore, this experiment can be considered like polarized glasses in a 3D cinema, where light rays of different orientations are also used. Our ‘3D glasses’ make the electron topology visible.”
Led by the Würzburg-Dresden Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matters – this groundbreaking experiment, along with its theoretical description, is the first successful attempt at topological characterization of quantum materials. Sangiovanni pointed out the important role of particle accelerators in the experiment, stating: “We need a synchrotron particle accelerator to generate this special X-ray light and to create the ‘3D cinema’ effect.”
Quantum matter, particle accelerators and supercomputers
The journey to this monumental success spanned three years for the researchers. Their starting point is TbV6Sn6 kagome metal, a quantum material. In this particular class of materials, the atomic lattice has a mixture of triangular and honeycomb lattice in a structure reminiscent of Japanese basket weave. Kagome Metal plays an important role in ct.qmat material research.
“Before our experimental partners can start their synchrotron experiments, we need to simulate the results to make sure we are on the right track. In the first step, we construct a theoretical model and run calculations on a supercomputer,” said Dr. Domenico di Sante, project leader and theoretical physicist, who is also an associate member of the Würzburg Center for Collaborative Research (SFB) 1170 ToCoTronics.
The findings from the measurements align with theoretical predictions, allowing the team to visualize and confirm the metallic kagome topology.
International research network
The research project involved scientists from Italy (Bologna, Milan, Trieste, Venice), England (St. Andrews), United States (Boston, Santa Barbara), and Würzburg. The supercomputer used for the simulations is in Munich, and the synchrotron experiments are carried out in Trieste. “The findings of this study perfectly illustrate the extraordinary results that theoretical and experimental physics can produce when working together,” Prof. Sangiovanni concluded.