D. Santos-Cottin, University of Fribourg, Milan Orlita, LNCMI-Grenoble and Ana Akrap, University of Fribourg.
Weyl semimetals are an exciting new group of materials, showing unique signatures in their transport and optical behavior, inherited from their distinct topological features. The presence of nodes in the electronic band structure of Weyl semimetals makes their electrons behave as if they are massless, and this leads to a number of interesting properties. Material scientists have been searching for their experimental realizations ever since the first discoveries. One such proposed Weyl semimetal was EuCd2As2, which was described as a magnetic Weyl semimetal in various computational studies based on density functional theory (DFT) calculations, but also several experimental works. In a magnetic Weyl semimetal, it would be possible to manipulate the topological properties using a small magnetic field. Without an external magnetic field, a small gap separates the conduction and valence bands. But in a magnetic field, the two bands overlap, creating a Weyl semimetal. In a broader context, the idea is to use the material’s magnetic structure to control its topology.
However, in a new study an international research team has studied this material in great detail. Most surprisingly, the widely investigated material EuCd2As2, turned out not to be a Weyl semimetal after all, but rather a magnetic semiconductor. These new results directly contradict about 30 published papers, both theoretical and experimental, that claimed that EuCd2As2 was a Weyl semimetal. Beyond the ground state of EuCd2As2, the main message is that the condensed-matter community has to be more careful when making conclusions mostly based on first principle calculations.
The key development for the new experiments was synthesizing high-quality samples of EuCd2As2. Previously, all the investigated samples had metal-like resistivity. The new samples showed activated behavior of the resistivity, which is characteristic of semiconductors. The ability to prepare such pure samples allowed for more accurate magnetic and electric measurements than in previous studies. To achieve cleaner crystals, their careful crystal synthesis used very pure starting materials, in particular, extremely clean europium. Several different experimental techniques were used: electronic transport, optical spectroscopy, and excited-state photoemission spectroscopy. The goal was to determine the ground state of EuCd2As2. The material was studied at various temperatures and using infrared spectroscopy under an external magnetic field up to 16 T (see figure). All the experiments led to the same conclusion: the compound unmistakably behaves as a magnetic semiconductor – it combines antiferromagnetic behavior with activated electrical conductivity, and a band gap of 0.77 eV. An external magnetic field strongly impacts the band gap and the transport properties. Applying 2 T is enough to decrease the band gap by 125 meV. However, in contradiction to many previous studies, the material never ceases to behave as a semiconductor, even under a strong magnetic field. The coveted magnetic Weyl semimetal phase simply isn’t there.
How is it possible that so many studies could get the basic properties of EuCd2As2 so wrong? One of the main reasons is that europium has electrons in its f orbitals, leading to strong electron-correlation effects. Such localized electrons become notoriously difficult for DFT to simulate. The ab-initio community has taken note of this curious discrepancy. Most importantly, the positive takeaway is the almost forgotten power of magneto-optical spectroscopy, a technique that was widely used in the past to learn about semiconductors.
Figure: (a) Near-infrared transmission showing the interband absorption edge at low fields, B < 1 T. (b) Color plot of relative magnetotransmission, TB/TAVR, in a broad energy range and up to 2 T. (c) Magnetotransmission TB/T0 and (d) its first derivative, d/dE[TB/T0], in a broad energy and magnetic field range.
EuCd2As2: A Magnetic Semiconductor, D. Santos-Cottin, I. Mohelský, J. Wyzula, F. Le Mardelé, I. Kapon, S. Nasrallah, N. Barišić, I. Živković, J.R. Soh, F. Guo, K. Rigaux, M. Puppin, J.H. Dil, B. Gudac, Z. Rukelj, M. Novak, A.B. Kuzmenko, C.C. Homes, Tomasz Dietl, M. Orlita, and Ana Akrap, Phys. Rev. Lett. 131, 186704 (2023).
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.186704
Contact: milan.orlita@lncmi.cnrs.fr, ana.akrap@unifr.ch
