New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into the long-wavelength infrared regime and cannot be readily reached by other layered materials.

PrefaceThe recent rapid development in the field of topological materials raises expectations that these materials might allow solving a large variety of current challenges in condensed matter science, ranging from applications in quantum computing, to infra-red sensors or heterogenous catalysis. 1-8 In addition, exciting predictions of completely new physical phenomena that could arise in topological materials drive the interest in these compounds. 9,10 For example, charge carriers might behave completely different from what we expect from the current laws of physics if they travel through topologically non-trivial systems. 11,12 This happens because charge carriers in topological materials can be different from the normal type of fermions we know, which in turn affects the transport properties of the material. It has also been proposed that we could even find "new fermions", i.e. fermions that are different from the types we currently know in condensed matter systems as well as in particle physics. 10 Such proposals connect the fields of high energy or particle physics, whose goal it is to understand the universe and all the particles it is composed of, with condensed matter physics, 1 arXiv:1804.10649v1 [cond-mat.mtrl-sci] 27 Apr 2018where the same type, or even additional types, of particles can be found as so-called quasiparticles, meaning that the charge carriers behave in a similar way as it would be expected from a certain particle existing in free space.The field of topology in condensed matter physics evolved from the idea that there can be insulators whose band structure is fundamentally different (i.e. has a different topology) from that of the common insulators we know. If two insulators with different topologies are brought into contact, electrons that have no mass and cannot be back scattered are supposed to appear at the interface. These edge states also appear if a topological insulator (TI) is in contact with air, a trivial insulator. 2D TIs have conducting edge states whereas 3D TIs, which were discovered later, have conducting surface states. TIs have already been reviewed multiple times, [13][14][15][16] which is why we focus here on the newer kind of topological materials, namely topological semimetals (TSMs). Nevertheless, we will refer to TIs and their properties where relevant in the context of topological materials and to contrast them with TSMs.The term "topological semimetal" is widely used and basically includes all semimetals that exhibit some non-trivial band topology. We will describe in more detail later in this review what non-trivial topology actually means. Since TSMs are semimetals they are characterized by a gap-less band structure, i.e. they differ from normal metals by being charge balanced, meaning the amount of electrons and holes contributing to their conductivity is exactly the same. Or, phrased differently, the hole and electron pockets composing the Fermi surface are of the same size.

The fabrication of highly active and robust hexagonal ruthenium oxide nanosheets for the electrocatalytic oxygen evolution reaction (OER) in an acidic environment is reported. The ruthenate nanosheets exhibit the best OER activity of all solution‐processed acid medium electrocatalysts reported to date, reaching 10 mA cm−2 at an overpotential of only ≈255 mV. The nanosheets also demonstrate robustness under harsh oxidizing conditions. Theoretical calculations give insights into the OER mechanism and reveal that the edges are the origin of the high OER activity of the nanosheets. Moreover, the post OER analyses indicate, apart from coarsening, no observable change in the morphology of the nanosheets or oxidation states of ruthenium during the electrocatalytic process. Therefore, the present investigation suggests that ruthenate nanosheets are a promising acid medium OER catalyst with application potential in proton exchange membrane electrolyzers and beyond.

Sb 2 Si 2 Te 6 , a 2D material, exhibits an intrinsically high thermoelectric figure of merit ZT of 1.08 at 823 K. The thermoelectric performance can be further enhanced by a cellular nanostructure with ultrathin Si 2 Te 3 nanosheets covering the Sb 2 Si 2 Te 6 grains. The Si 2 Te 3 acts as a hole-transmitting electron-blocking filter and, at the same time, causes extra phonon scattering that leads to ultralow thermal conductivity and a high ZT value of 1.65 at 823 K.

Single crystalline Sn 2 Co 3 S 2 with the shandite-type structure was investigated by magnetization, magnetoresistance, Hall effect, and heat capacity measurements and by 119 Sn Mößbauer spectroscopy. Sn 2 Co 3 S 2 orders ferromagnetically at 172 K with an easy-axis magnetization of ≈1 μ B along the hexagonal c axis. The half-metallic ferromagnetic state is investigated by detailed band-structure calculations by density functional theory (DFT) methods. The magnetoresistance and the Hall effect as well as the DFT results show that ferromagnetic Sn 2 Co 3 S 2 is a compensated metal. The 119 Sn Mößbauer spectroscopic data confirm these findings. Large transferred hyperfine fields B hf up to 34.2 T are observed.