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.
- New instrument at BESSY II: The OÆSE endstation in EMILA new instrument is now available at BESSY II for investigating catalyst materials, battery electrodes and other energy devices under operating conditions: the Operando Absorption and Emission Spectroscopy on EMIL (OÆSE) endstation in the Energy Materials In-situ Laboratory Berlin (EMIL). A team led by Raul Garcia-Diez and Marcus Bär showcases the instrument’s capabilities via a proof-of-concept study on electrodeposited copper.
- Green hydrogen: A cage structured material transforms into a performant catalystClathrates are characterised by a complex cage structure that provides space for guest ions too. Now, for the first time, a team has investigated the suitability of clathrates as catalysts for electrolytic hydrogen production with impressive results: the clathrate sample was even more efficient and robust than currently used nickel-based catalysts. They also found a reason for this enhanced performance. Measurements at BESSY II showed that the clathrates undergo structural changes during the catalytic reaction: the three-dimensional cage structure decays into ultra-thin nanosheets that allow maximum contact with active catalytic centres. The study has been published in the journal ‘Angewandte Chemie’.
- An elegant method for the detection of single spins using photovoltageDiamonds with certain optically active defects can be used as highly sensitive sensors or qubits for quantum computers, where the quantum information is stored in the electron spin state of these colour centres. However, the spin states have to be read out optically, which is often experimentally complex. Now, a team at HZB has developed an elegant method using a photo voltage to detect the individual and local spin states of these defects. This could lead to a much more compact design of quantum sensors.