Recently there have been increasingly hot debates on whether a bulk Fermi surface of chargeneutral excitations exists in the topological Kondo insulator SmB6. To unambiguously resolve this issue, we performed the low-temperature thermal conductivity measurements of a high-quality SmB6 single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to magnetic field up to 14.5 T. These results exclude the existence of fermionic charge-neutral excitations in bulk SmB6, such as scalar Majorana fermions or spinons, thus put a strong constraint on the explanation of the quantum oscillations observed in SmB6.Topological insulator is a novel quantum state of matter, and has been suggested theoretically and observed experimentally [1][2][3][4][5]. Like the edge channel found in the quantum Hall system, the strong spin-orbit coupling in a three dimensional topological insulator leads to a nontrivial and robust conducting surface state. This metallic state is protected by the time-reversal symmetry. Studies on topological insulators have later stimulated the search for many other topological materials, such as topological crystalline insulators, Weyl and Dirac semimetals, and topological Kondo insulator [6][7][8][9]. Especially, interaction effect could play an important role in topological Kondo insulators and render exotic physics in them.As one of the most historical heavy-fermion (HF) materials, SmB 6 has been studied for more than 50 years [10,11] and was recently shown to be a topological Kondo insulator [12]. For decades, the low-temperature conductivity in SmB 6 remains puzzling: its resistivity shows insulating behavior down to a few Kelvins but saturates down to the lowest temperature upon further reducing the temperature. This puzzle was successfully resolved by recent transport experiments, which show that the material is a bulk insulator but with a metallic surface [13,14], consistent with the theoretical prediction that SmB 6 is a topological Kondo insulator [9,[15][16][17]. At high temperatures, transport properties are dominated by thermal excitations in the insulating bulk, and thus insulating behaviors are observed. At low temperatures, however, bulk excitations vanishes because of the energy gap, and surface signals become dominant. The existence of the metallic surface states is now well established and observed in a number of experiments [18][19][20][21][22][23][24][25][26][27][28][29][30][31].Although electric transport measurements so far show that the bulk of SmB 6 has no gapless charge carriers (namely a finite charge-gap), there exist other experiments suggesting possible gapless excitations in the bulk. Especially, the recent quantum oscillation measurement claims multiple Fermi seas in the bulk of an insulating SmB 6 sample [20], in direct contradiction with transport measurements. As the electrical transport only measures the ch...

, 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 ...