Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spinorbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by nonsymmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.

Crystal structures of bismuth oxychalcogenide compounds Bi2O2X and Bi2OX2 (X = S, Se, and Te).

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In this paper, BiCuSeO single crystals are successfully grown by the flux method under different growth temperature (690 °C, 730 °C and 775 °C). The crystal surface morphology, microstructure, chemical composition and electrical property are systematically characterized. By changing the growth temperature, the growth mechanism evolution, from dislocation-driven spiral growth mode to two-dimensional layer-by-layer one, is observed due to the different growth supersaturation. Simultaneously, temperature-dependent resistance substantiates that the electrical property changes from semiconductor to metal. Chemical analysis proves that BiCuSeO crystals grown on higher temperatures (730 °C and 775 °C) are slightly non-stoichiometric. The present results demonstrate the possibility to modulate crystal morphology and electrical property of BiCuSeO by controlling supersaturation. This method may be applicable to similar compounds (BiCuOCh (Ch=S, Te)). 6 four-probe technique on a physical property measurement system (PPMS, Quantum Design Inc.). Results and discussion Characteristics of the as-grown BiCuSeO CrystalsThe inset of Fig. 2(a) shows a photograph of the BiCuSeO single crystal synthesized at 730 o C.One can see that the thin sheet-shape crystals, with the typical sizes of 2.5×2×0.05 mm 3 , show metallic luster. As depicted in Fig. 2(a), all reflections in the X-ray diffraction (XRD) result of BiCuSeO single crystal can be indexed as (00l). It confirms that the thin foil samples were grown along ab-plane. The Laue pattern further proves that the axis perpendicular to BiCuSeO crystal surface belongs to c-axis (see inset of Fig. 2(a)). It proves again that the growth direction of BiCuSeO is along ab-plane. SEM images of the as-grown BiCuSeO crystal and the corresponding element mapping of Bi, Cu and Se are presented in Fig. 2(b). The distribution of Bi, Cu and Se in the grown sample is homogeneous. It strongly suggests that the growth is homogeneous. The XRD patterns of BiCuSeO single crystal powders under different growth temperatures (690 °C, 730 °C and 775 °C) are shown in Fig. 2(c). All Bragg peaks can be indexed to the ZrSiCuAs-type structure and no peaks of impurity phase are detected within the instrument detection limit. The XRD pattern and Rietveld refinement of BiCuSeO-775 powders are shown in Fig. 2(d).Comparing these results, we substantiate that the experimental data (black circle) are in good agreement with the calculations (red line). The lattice parameters of this sample are a=b=3.9821Å and c=8.8744Å, similar to previous results. 12 The unit-cell parameters extracted from the Rietveld refinement are summarized in Table 1. It is demonstrated that the lattice constants a, b and c decrease obviously as the growth temperature increasing. As is well known, the BiCuSeO crystal structure (shown in the inset of Fig. 2(c)) is formed by the alternative stacking of (Cu 2 Se 2 ) 2− layers

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