Topological insulators are materials characterized by dissipationless, spin-polarized surface states resulting from nontrivial band topologies. Recent theoretical models and experiments suggest that SmB 6 is the first topological Kondo insulator, in which the topologically nontrivial band structure results from electron-electron interactions via Kondo hybridization. Here, we report that the surface conductivity of SmB 6 increases systematically with bulk carbon content. Further, addition of carbon is linked to an increase in n-type carriers, larger low-temperature electronic contributions to the specific heat with a characteristic temperature scale of T Ã ¼ 17 K, and a broadening of the crossover to the insulating state. Additionally, x-ray absorption spectroscopy shows a change in Sm valence at the surface. Our results highlight the importance of phonon dynamics in producing a Kondo insulating state and demonstrate a correlation between the bulk thermodynamic state and the low-temperature resistance of SmB 6 .

Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. We find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.PACS numbers: 71.10. Li, 71.27.+a, 75.30.Mb Recent theoretical work suggests SmB 6 could be a topological Kondo insulator (TKI), with an insulating bulk at low temperatures and a topologically protected metallic surface [1][2][3][4][5][6][7] that was previously ascribed to impurities [8]. Because strong electron-electron interactions produce the insulating state, the surface may support exotic correlated physics [9][10][11].Experimental investigations [12][13][14][15][16][17][18], particularly spinresolved angle-resolved photo-emission spectroscopy (ARPES) [19], have provided compelling evidence that SmB 6 is a TKI. However, information about the band structure within ≈ 50 meV of the Fermi level is limited due to the polar surface, multiplet structure, and strong correlations. In this energy range the magnetic neutron scattering is sensitive to the renormalized band structure through the imaginary part of the momentum (Q) and energy ( ω) dependent generalized susceptibility.In this Letter, we present a comprehensive measurement of the inelastic magnetic neutron scattering cross section covering the full Brillouin zone of SmB 6 for energies below 50 meV. We pair our experimental results with a slave-boson treatment of an Anderson Hamiltonian, and discuss how pseudonesting conditions for the renormalized band structure can be examined to corroborate a topologically nontrivial band structure for SmB 6 . The low energy magnetic neutron scattering cross section for SmB 6 is dominated by a resonant mode near 14 meV with bandwidth < 2 meV. Previous publications reported intensity at R [ ( 1 2 1 2 1 2 )], and investigated it versus temperature and doping [20][21][22][23][24][25]. Here, we show the mode is also intense near the X [( 1 2 00)] point and present, albeit dramatically weaker, beyond the first zone. Through this mulitzone mapping, we provide evidence for an anomalous 5d form factor for the weakly dispersing mode, and develop a minimal band structure based on dominant third neighbor hopping. The hybridized tight-binding model goes beyond early two-band theoretical treatments [26,27] by allowing f -electron fluctuations as appropriate for a mixed valence compound and provides a link between the wave vector dependence of the magnetic neutron scattering and band inversion in Kondo insu...

Recent theoretical and experimental findings suggest the long-known but not well understood low temperature resistance plateau of SmB6 may originate from protected surface states arising from a topologically non-trivial bulk band structure having strong Kondo hybridization. Yet others have ascribed this feature to impurities, vacancies, and surface reconstructions. Given the typical methods used to prepare SmB6 single crystals, flux and floating-zone procedures, such ascriptions should not be taken lightly. We demonstrate how compositional variations and/or observable amounts of impurities in SmB6 crystals grown using both procedures affect the physical properties. From X-ray diffraction, neutron diffraction, and X-ray computed tomography experiments we observe that natural isotope containing (SmB6) and doubly isotope enriched (154Sm11B6) crystals prepared using aluminum flux contain co-crystallized, epitaxial aluminum. Further, a large, nearly stoichiometric crystal of SmB6 was successfully grown using the float-zone technique; upon continuing the zone melting, samarium vacancies were introduced. These samarium vacancies drastically alter the resistance and plateauing magnitude of the low temperature resistance compared to stoichiometric SmB6. These results highlight that impurities and compositional variations, even at low concentrations, must be considered when collecting/analyzing physical property data of SmB6. Finally, a more accurate samarium-154 coherent neutron scattering length, 8.9(1) fm, is reported.