Disorder brings out quantum physical talents
The Dirac cone is typical for topological insulators and is practically unchanged on all 6 images (ARPES measurements at BESSY II). The blue arrow additionally shows the valence electrons in the volume. The synchrotron light probes both and can thus distinguish the Dirac cone at the surface (electrically conducting) from the three-dimensional volume (insulating). © HZB
Quantum effects are most noticeable at extremely low temperatures, which limits their usefulness for technical applications. Thin films of MnSb2Te4, however, show new talents due to a small excess of manganese. Apparently, the resulting disorder provides spectacular properties: The material proves to be a topological insulator and is ferromagnetic up to comparatively high temperatures of 50 Kelvin, measurements at BESSY II show. This makes this class of material suitable for quantum bits, but also for spintronics in general or applications in high-precision metrology.
Quantum effects such as the anomalous quantum Hall effect enable sensors of highest sensitivity, are the basis for spintronic components in future information technologies and also for qubits in quantum computers of the future. However, as a rule, the quantum effects relevant for this only show up clearly enough to make use of them at very low temperatures near absolute zero and in special material systems.
Quantum effects up to 50 K
Now, an international team led by HZB physicist Prof. Dr. Oliver Rader and Prof. Dr. Gunther Springholz, University of Linz, has observed two particularly important physical properties in thin films of MnSb2Te4: Such doped structures are robust topological insulators and also ferromagnetic up to almost 50 Kelvin. "According to the theoretical considerations published so far, the material should be neither ferromagnetic nor topological," says Rader. "However, we have now experimentally demonstrated exactly these two properties."
Disorder makes the difference
The group combined measurements of spin- and angle-resolved photoemission spectroscopy (ARPES) and magnetic X-ray circular dichroism (XMCD) at BESSY II, examined the surfaces with scanning tunnelling microscopy (STM) and spectroscopy (STS), and carried out further investigations. "We can see that the additional manganese atoms have led to a certain disorder. This explains why the theoretical observation came to a different result - the theory assumed an ideally ordered structure, which is not realised" says Rader.
The properties are extraordinarily robust and occur up to a temperature of just under 50 K, which is three times higher than the best ferromagnetic systems before (see Nature, 2019). This makes this material an interesting candidate for spintronics and even qubits.
- BESSY II: How intrinsic oxygen shortens the lifespan of solid-state batteriesAlthough solid-state batteries (SSBs) demonstrate high performance and are intrinsically safe, their capacity currently declines rapidly. A team from the TU Wien, Humboldt-University Berlin and HZB has now analysed a TiS₂|Li₃YCl₆ solid-state half-cell in operando at BESSY II using a special sample environment that allows for non-destructive investigation under real operating conditions. Data obtained by combination of soft and hard X-ray photoelectron spectroscopy (XPS and HAXPES) revealed a new degradation mechanism that had not previously been identified in solid-state batteries. They have gained some surprising insights, particularly regarding the harmful role played by intrinsic oxygen. This study provides valuable information for improving design and handling of such batteries.
- Spintronics at BESSY II: Real-time analysis of magnetic bilayer systemsSpintronic devices enable data processing with significantly lower energy consumption. They are based on the interaction between ferromagnetic and antiferromagnetic layers. Now, a team from Freie Universität Berlin, HZB and Uppsala University has succeeded in tracking, for each layer separately, how the magnetic order changes after a short laser pulse has excited the system. They were also able to identify the main cause of the loss of antiferromagnetic order in the oxide layer: the excitation is transported from the hot electrons in the ferromagnetic metal to the spins in the antiferromagnet.
- Environmental Chemistry at BESSY II: Radicals in waterwaysHow do radicals form in aqueous solutions when exposed to UV light? This question is important for health research and environmental protection, for example with regard to the overfertilisation of water bodies by intensive agriculture. A team at BESSY II has now developed a new method of investigating hydroxyl radicals in solution. By using a clever trick, the scientists gained surprising insights into the reaction pathway.