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.
- Disorder creates new properties in compound semiconductorsAn international research team has demonstrated that the intrinsic disorder of the compound semiconductor CuInSnS₄ can be exploited to influence its optical properties. While the atomic vibrations also sense the local disorder, their response is averaged over many different local environments and therefore appear isotropic, as expected for a cubic crystal. In contrast, the optical excitations, known as excitons, are much more sensitive to the local arrangement of atoms. Surprisingly, they show a direction-dependent optical response even though the average crystal structure is cubic. These findings shed new light on the relationship between disorder and material properties, opening up new options for targeted 'disorder engineering' in optoelectronic and photocatalytic devices.
- Perovskite solar cells: Predictions of long-term stabilityReliable statements about the long-term stability of perovskite solar cells are still difficult to make. However, a new study by Dr Carolin Ulbrich’s team, published in the renowned journal Joule, highlights which methods are useful for this purpose and identifies areas where further research is needed.
- Superconducting TES array X-ray spectrometer goes into operation at BESSY IIEurope's first and only TES-spectrometer at a synchrotron source is now in operation at BESSY II, developed within a collaboration between the HZB, the MPI-CEC (Mühlheim-an-der-Ruhr, Germany) and the NIST (Boulder CO, USA). The photon detection efficiency of the new instrument exceeds that of wavelength-dispersive X-ray emission spectrometers by a factor of 100 to 1000. It will be used to investigate the electronic properties of atomically thin layers, nanostructures and highly diluted atomic and molecular samples. The team is looking forward to receiving exciting research proposals from the user community.