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).

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.

arö


You might also be interested in

  • 14 parameters in one go: New instrument for optoelectronics
    Science Highlight
    21.02.2024
    14 parameters in one go: New instrument for optoelectronics
    An HZB physicist has developed a new method for the comprehensive characterisation of semiconductors in a single measurement. The "Constant Light-Induced Magneto-Transport (CLIMAT)" is based on the Hall effect and allows to record 14 different parameters of transport properties of negative and positive charge carriers. The method was tested now on twelve different semiconductor materials and will save valuable time in assessing new materials for optoelectronic applications such as solar cells.
  • Sodium-ion batteries: How doping works
    Science Highlight
    20.02.2024
    Sodium-ion batteries: How doping works
    Sodium-ion batteries still have a number of weaknesses that could be remedied by optimising the battery materials. One possibility is to dope the cathode material with foreign elements. A team from HZB and Humboldt-Universität zu Berlin has now investigated the effects of doping with Scandium and Magnesium. The scientists collected data at the X-ray sources BESSY II, PETRA III, and SOLARIS to get a complete picture and uncovered two competing mechanisms that determine the stability of the cathodes.
  • BESSY II: Molecular orbitals determine stability
    Science Highlight
    07.02.2024
    BESSY II: Molecular orbitals determine stability
    Carboxylic acid dianions (fumarate, maleate and succinate) play a role in coordination chemistry and to some extent also in the biochemistry of body cells. An HZB team at BESSY II has now analysed their electronic structures using RIXS in combination with DFT simulations. The results provide information not only on electronic structures but also on the relative stability of these molecules which can influence an industry's choice of carboxylate dianions, optimizing both the stability and geometry of coordination polymers.