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.

Topological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.

IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance. The multiple close-to-degenerate (or "convergent") valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure-of-merit in the SnSe single crystal. Up to date, there has been only theoretical calculations but no experimental observation of this particular electronic band structure. In this paper, using Angle-Resolved Photoemission Spectroscopy, we performed a systematic investigation on the electronic structure of SnSe.We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab-initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe for the first time, and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe.Thermoelectric materials could directly convert heat (many times wasted) to electrical power and therefore are of critical importance in energy industry [1][2][3][4][5][6][7]. The conversion efficiency of thermoelectric materials is quantified by the dimensionless figure of merit, = 2 ⁄ ( : Seebeck coefficient, : electrical conductivity, : total thermal conductivity, including contributions from both electrons and phonons, : temperature). Recently, singlecrystalline SnSe, a binary IV-VI semiconductor compound containing non-toxic and earthabundant elements, shows a record high ZT of ~2.6 at 923 K (along the b axis of the roomtemperature orthorhombic unit cell) [8] and the device figure of merit ~1.34 from 300-773 K when hole-doped [9], much higher than that of typical high-performance thermoelectric materials [10-15]. These excellent thermoelectric performances can be attributed to both the relatively low thermal conductivity (~0.7 Wm -1 K -1 at 300 K for the pristine samples) [8] as well as the very high Seebeck coefficient (~160 μVK -1 at 300 K with carrier density of ∼4×10 19 cm -3 ) and power factor ( 2 , ~40 μWcm -1 K -2 at 300K) [9].While the low thermal conductivity is attributed to the giant anharmonic and anisotropic bondings [8,16,17], the high Seebeck coefficient and power factor are deeply rooted in the electronic band structure of SnSe. It has been proposed that SnSe bears an electronic structure with relatively small effective mass (thus high mobility) [8,18,19] and multiple close-to-degenerate ("convergent") valence bands [9,20,21]. As the temperature increases, the carriers are thermally distributed over several convergent bands of similar energy, resulting in the enhanced Seebeck coefficient [22,23]. Besides, the most outstanding electrical conductivity and power factor along the b axis among three axes of SnSe are thought to benefit from particular "pudding-mold-like" band [24][25][26][27][28]. However, although many the...

The quantum spin Hall (QSH) effect is widely studied as a novel quantum state in condensed matter physics over the past decade. Recently, it is predicted that the transition metal pentatelluride XTe5 (X = Zr, Hf) has a large bandgap in its bulk form and a single layer of XTe5 is a QSH insulator candidate. However, the topological nature of the bulk material is still under debate because it is located close to the phase boundary of a strong topological insulator and a weak topological insulator (WTI). Here, using angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM)/scanning tunneling spectroscopy, we systematically studied the electronic structures of bulk HfTe5. Both the large bulk bandgaps and conductive edge states in the vicinity of the step edges in HfTe5 were observed, strongly suggesting a WTI phase in bulk HfTe5. Moreover, our STM experiment for the first time reveals the bulk band bending due to the broken symmetry near the step edge, making it an ideal platform for studying the development of edge states in the WTI and QSH insulator.

, KHgX are insulating in the bulk but possess robust gapless surface states (see Fig. 1a), forming the unique "hourglass Fermions"on the (010) surface. The surface fermion contains four branches (quadruplets) dispersions and unbreakable zigzag chain-like patterns 49 (Fig. 1b). These surface states can also be understood as two copies of surface states of weak topological insulators 53 . The nonsymmorphic glide mirror protects these two copies from annihilating with each other inside the mirror plane. In order to visualize the intriguing hourglass fermion surface states and confirm the nonsymmorphic topological insulator nature of KHgX, ARPES is the natural experimental tool.In this work, we report comprehensive ARPES study on the electronic structures of KHgSb on both (001) and (010) Basic information of KHgSbHigh quality KHgSb crystals was synthesized by the flux method (see Appendix for details).The crystal structure of KHgSb is shown in Fig. 1c with space group P63/mmc and the lattice constants a=b=4.78 Å , c=10.225 Å . The in-plane Hg and Sb atoms show strong bonding, forming in-plane honeycomb lattices. The off-plane K atoms sit above the center of each honeycomb, sandwiched loosely by the two adjacent layers and serve as their inversion center.The natural cleavage surface is along the (001) and (010) surface (parallel to the diagonal of the in-plane honeycomb and preserves the glide reflection). The bulk Brillouin zone (BZ) and the surface BZs of both (001) and (010) surfaces are shown in Fig. 1d. Fig. 1e, f illustrate the broad Fermi surface mapping across multiple BZs on the (001) and (010) surfaces, respectively, confirming the cleavage surfaces with correct lattice parameters (the momentum direction of kx is defined along Γ − X; ky along Γ ̅ − K ̅ and kz along Γ − Z, respectively, see Fig. 1d). The core level photoemission spectra (Fig. 1g) show sharp characteristic Sb4d, K3p and Hg5d levels. The electronic structures on the (001) surfaceWe first focus on the electronic structures on the (001) According to the band structure calculation 49, KHgSb is a fully gapped nonsymmorphic crystalline topological insulator with no surface states residing on the (001) surfaces. In order to prove this, we tuned the Fermi surface by introducing potassium atoms in situ onto the sample surface so that the absorbed potassium atoms on the surface would donate free electrons.After K-dosing, we could clearly observe the bottom of the conduction band and the top of the valence band simultaneously along both (Fig. 2g(i)-(ii)). An indirect band gap of ~200 meV is observed with no signatures of in-gap surface states (Fig. 2h), agreeing with the calculations (Fig. 2e). The electronic structures on the (010) surfaceNext we demonstrate the detailed electronic structure on the (010) surface in Figure 3. Fig. 3a shows the stacking CECs. After inspecting CECs ranging from Eb=0~400meV, one could observe (see Fig. 3a,b) the quasi-one-dimensional Fermi surface disperses into two pieces when going to higher binding energies until ...