May 31 2021
Researchers from the Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter have gained a new understanding of the behavior of electrons in powerful magnetic fields.
The illustration shows electrons in a topological quantum metal waiting to be activated by a magnetic field. Once they start moving, they follow a spiraling helix upwards—in contrast to the previously proposed picture of electrons moving in circles in a two-dimensional plane. This creates a special effect that is the foundation for promising topological quantum phenomena. Image Credit: Jörg Bandmann.
The study results describe electric current measurements in 3D materials that indicate a quantum Hall effect—a kind of phenomenon that has so far only linked to 2D metals.
This latest 3D effect can be the basis for topological quantum phenomena, which are assumed to be specifically strong and thus potential candidates for highly robust quantum technologies. The results of the study were recently published in Nature Communications—a scientific journal.
Drs Tobias Meng and Johannes Gooth are both early career researchers in the Würzburg-Dresdner Cluster of Excellence ct.qmat that has been investigating topological quantum materials since 2019.
But the duo could hardly believe the results of the latest publication in the Nature Communications journal and claimed that electrons in the topological metal zirconium pentatelluride (ZrTe5) move only in 2D planes, even though the material is 3D.
Therefore, Drs Meng and Gooth have initiated their own research and experiments on the ZrTe5 material. While Dr Meng from the Technische Universität Dresden (TUD) designed the hypothetical model, Dr. Gooth from the Max Planck Institute for Chemical Physics of Solids developed the experiments. Seven measurements using different methods invariably led to the same conclusion.
Electrons Waiting for Their Turn
The study, performed by Drs Meng and Gooth, sheds new light on the workings of the Hall effect in 3D materials. According to the researchers, electrons travel via the metal along 3D paths; however, their electric transport can still look like 2D. This is possible in the topological metal zirconium pentatelluride because a small part of the electrons is still waiting to be stimulated by an external magnetic field.
“The way electrons move is consistent in all of our measurements, and similar to what is otherwise known from the two-dimensional quantum Hall effects. But our electrons move upwards in spirals, rather than being confined to a circular motion in planes. This is an exciting difference to the quantum Hall effect and to the proposed scenarios for what happens in the material ZrTe5,” stated Dr Meng on the genesis of the latest scientific model.
This only works because not all electrons move at all times. Some remain still, as if they were queuing up. Only when an external magnetic field is applied do they become active.
Dr Johannes Gooth, Max Planck Institute for Chemical Physics of Solids
Experiments Confirm the Model
For their experimental study, the researchers cooled the topological quantum material as low as −271 °C and then applied an external magnetic field. They subsequently conducted thermoelectric and electric measurements by supplying currents via the specimen, assessed the thermodynamics of the material by examining its magnetic properties, and finally applied ultrasound.
The researchers also looked into the inner operations of the material using X-ray, electronic, and Raman spectroscopy.
But none of our seven measurements hinted at the electrons moving only two-dimensionally. Our model is in fact surprisingly simple, and still explains all the experimental data perfectly.
Dr Tobias Meng, Head of Emmy Noether Group for Quantum Design, Technische Universität Dresden
Dr. Meng is also the leading theorist in the new project.
Outlook for Topological Quantum Materials in 3D
Discovered in 1980, the Nobel-prize-winning quantum Hall effect explains the stepwise conduction of electric current in a metal. It represents the foundation of topological physics, a domain that has undergone a surge since 2005 because of its potential for the functional materials of the 21st century. But so far, the quantum Hall effect has only been seen in 2D metals.
The scientific outcomes of the new study deepen the understanding of the behavior of 3D materials in magnetic fields. The cluster members Drs Meng and Gooth have now planned to additionally pursue this new direction of study.
We definitely want to investigate the queueing behavior of electrons in 3D metals in more detail.
Dr Tobias Meng, Head of Emmy Noether Group for Quantum Design, Technische Universität Dresden
People Involved
Apart from the members of Dr. Tobias Meng’s research team for Quantum Design at TUD, the new publication was jointly headed by researchers from Johannes Gooth’s group at the Max Planck Institute for Chemical Physics of Solids. Ultrasound measurements were carried out at Helmholtz-Zentrum Dresden-Rossendorf.
Journal Reference:
Galeski, S., et al. (2021) Origin of the quasi-quantized Hall effect in ZrTe5. Nature Communications. doi.org/10.1038/s41467-021-23435-y.