X-ray absorption spectroscopy measurements of iron based chalcogenides

   Iron based chalcogenides (IBC) is a group of compounds where the presence of superconductivity (SC), together with relatively simple crystal structure, predestinates them as ideal candidates for both experimental and theoretical investigations. IBC are quasi two-dimensional in their crystal structure, which consists of two alternating layers purely composed of iron or a chalcogen. These form trilayers separated by Van der Waals gaps. FeSe was found to be superconducting below a critical temperature of Tc = 8 K . The pairing mechanism is believed to be unconventional, i.e. not phonon-mediated. In recent years, the interest in IBC was further triggered by a discovery that the SC transition temperature of FeSe can be enhanced above 100 K when a mono layer of FeSe is deposited on SrTiO3. For an overview see e.g.
   FeTe is an adjacent compound to FeSe. Although bulk FeTe itself is non superconducting, mixed alloys FeTe1-xSex reach superconducting properties with transition temperatures higher than those of pure FeSe (14.5 K at x =0.5). Thus, also the understanding of magnetism and electronic properties of FeTe is important to reveal mechanisms of superconductivity in iron based chalcogenides. For this, we conducted an X ray absorption spectroscopy (XAS) experiment at the Swiss Light Source in Villigen using FeTe single crystals. Utilizing X-rays under different polarized conditions, we derived X ray magnetic circular dichroism (XMCD) and X-ray magnetic linear dichroism (XMLD). XMCD signals were scaling with bulk magnetization following a drop connected with a formation of an antiferromagnetic state below 80 K. On the other hand, XMLD signal increased at the same time being maximal at lowest temperatures (1.6 K). XMLD data will be briefly discussed with ongoing theoretical calculations.
   In my talk, I will also mention recent XAS measurements of FeSe mono-layer islands [6] grown on a topological insulator surface of Bi2Se3.