Prediction and observation of an antiferromagnetic topological insulator.

Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order1. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics1, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic4 and electronic5 properties of these materials, restricting the observation of important effects to very low temperatures2,3. An intrinsic magnetic topological insulator-a stoichiometric well ordered magnetic compound-could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering  that MnBi2Te4  shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ2 topological classification; ℤ2 = 1 for MnBi2Te4, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling6-8 and axion electrodynamics9,10. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect2 and chiral Majorana fermions3.

Superconductivity of nanostructured Pb7Bi3 films doped by Ce.

By means of electrochemical deposition from electrolytes, containing salts of Pb and Bi (0.03 mol/l and 0.02 mol/l respectively) thin films of intermetallic Pb7Bi3 have been fabricated. The superconducting transition temperature of the films was measured to be around 7.8 K. The deposition of the films with thickness of 50-100 nm was performed via passing rectangular current pulses with given amplitude and length. It was shown that adding salt of Ce into the electrolyte leads to a significant growth of the Tc for the deposited films reaching its maximum at the salt concentration of 0.06 mol/I. X-ray analysis data revealed the single phase of Pb7Bi3 films with hexagonal structure (SG) having a textures parallel to (101) plane. The morphology of the film surface is characterized by nanocluster structure with typical grain size around 70-80 nm. For the films, fabricated with adding salt of Ce, together with the intermetallic phase of Pb7Bi3, the second phase containing Bi is detected. At the same time, the typical grain size is reduced to 20-30 nm. Additionally, the suppression of the superconductivity in the grown films is investigated. The influence of the composition and structure on the superconducting critical temperature is discussed for both types of the fabricated films.