SPECIAL TOPIC — Topological semimetals
We report the transport properties of the CaAs3 single crystal, which has been predicted to be a candidate for topological nodal-line semimetals. At ambient pressure, CaAs3 exhibits semiconducting behavior with a small gap, while in some crystals containing tiny defects or impurities, a large “hump” in the resistivity is observed around 230 K. By applying hydrostatic pressure, the samples appear to a tendency towards metallic behavior, but not fully metallized up to 2 GPa. Further high pressure studies are needed to explore the topological characteristics for CaAs3.
We investigate the temperature-dependent infrared spectroscopy of SrMnSb2, which is a semimetal with multiple Fermi surfaces. A notable blue shift of the plasma minimum in reflectivity upon cooling indicates that the carrier density varies with temperature. In the real part of the optical conductivity σ1(ω), a linearly-increased component which extrapolates to zero conductivity at finite frequency has been identified, which suggests dispersion of gapped Dirac band structures near the Fermi level. A two-Drude model, representing two different types of carriers, is introduced to describe the real part of optical conductivity. We separate the contributions of two-Drude model in dc conductivity, and demonstrate that the transport properties of SrMnSb2 are mainly affected by the narrow-Drude quasiparticles. Compared with the similar phenomena observed in CaMnSb2 and SrMnBi2, we can infer that the two-Drude model is an appropriate approach to investigate the multiband materials in AMnSb2 and AMnBi2 families.
Based on k·p analysis and realistic tight-binding calculations, we find that time-reversal-breaking Weyl semimetals can be realized in magnetically-doped (Mn, Eu, Cr, etc.) Sn1-xPbx(Te, Se) class of topological crystalline insulators. All the Weyl points are well separated in momentum space and possess nearly the same energy due to high crystalline symmetry. Moreover, both the Weyl points and Fermi arcs are highly tunable by varying Pb/Sn composition, pressure, magnetization, temperature, surface potential, etc., opening up the possibility of manipulating Weyl points and rewiring the Fermi arcs.
Two-dimensional systems with chiral symmetry allow stable discrete band crossings (nodal points) in Brillouin zones. Here we study the local evolutions of these nodal points under chiral symmetry preserving perturbations. We find that these evolutions can be classified by different types of local k·p models around the nodal points. Several concrete examples are calculated to illustrate our results.
Recently, the non-centrosymmetric WC-type materials (i.e., MoP, ZrTe, TaN, etc) have attracted extensive interest due to the discovery of their topological properties. By means of the first-principles calculations, here we have investigated the structural, thermodynamic, elastic, and electronic properties of the WC-type MX compounds (TiS, TiSe, TiTe, ZrS, ZrSe, ZrTe, HfS, HfSe, and HfTe). Among these nine compounds, five of them (TiS, ZrS, ZrSe0.9, ZrTe, and Hf0.92Se) have been experimentally synthesized to crystallize in the WC-type structure and other four members have never been reported. Our calculations demonstrated that they are all structurally, thermodynamically, and dynamically stable, indicating that all of them should be possibly synthesized. We have also derived their elastic constants of single crystalline and their bulk and shear moduli in terms of the R. Hill approximations. Furthermore, in similarity to ZrTe, all these compounds have been theoretically derived to be topological semimetals. Whereas TiS is unique because of the coexistence of the Dirac nodal lines (DNLs) and sixfold degenerate nodal points (sixfold DNPs), the other eight members are revealed to exhibit coexisted Weyl nodes (WPs) and triply degenerate nodal points (TDNPs). Their electronic and topological properties have been further discussed.
YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-Ⅱ Weyl semimetal. However, as a representation of the new category of topological matters, the scanning tunneling microcopy (STM) results on such important material are still absent. Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples. A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure. Analysis of STM images yields the information of the index of the vicinal surface. Our results here lay a playground of future atomic scale research on YbMnBi2.