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.) Sn_1-xPb_x(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, ZrSe_0.9, ZrTe, and Hf_0.92Se) 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.

Angle-resolved photoemission spectroscopy (ARPES) and torque magnetometry (TM) measurements have been carried out to study the electronic structures of a correlated topological insulator (TI) candidate YbB₆. We observed clear surface states on the [001] surface centered at the Γ and M points of the surface Brillouin zone. Interestingly, the fermiology revealed by the quantum oscillation of TM measurements agrees excellently with ARPES measurements. Moreover, the band structures we observed suggest that the band inversion in YbB₆ happens between the Yb_5d and B_2p bands, instead of the Yb_5d and Yb_4f bands as suggested by previous theoretical investigation, which will help settle the heavy debate regarding the topological nature of samarium/ytterbium hexaborides.

We have successfully grown an arsenopyrite marcasite type RhSb₂ single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb₂ shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with n_e = 8.62×10¹⁸ cm^-3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb₂. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.