Interaction Driven Subgap Spin Exciton in the Kondo InsulatorSmB6

Abstract

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...

In-gap collective mode spectrum of the topological Kondo insulatorSmB6

“…This resolution-limited effect indicates the absence of intrinsic damping, a consequence of the mode's energy being below the charge gap where no quasiparticle decay channels are present. Transport experiments reveal the charge gap to be ≈ 19 meV, while the collective mode is observed in neutron experiments at ≈ 14 meV 15 .…”

“…We present the detailed theoretical analysis of the mode's dispersion that was able to reproduce the recent INS measurement with a reasonable quantitative accuracy 15 .…”

“…The observable bandwidth of f electrons is set by |B| 2 t f 3 ∼ 1.0 meV, and the 19 meV hybridization bandgap is controlled by the energy scale |B|V 0 ∼ 70.4 meV of the same order of magnitude 54 . The renormalized quasiparticle band-structure corresponds to that of a strong topological insulator 15 .…”

“…The quasiparticle spectrum of SmB 6 is significantly renormalized by interactions in a temperaturedependent manner 13 . Even more strikingly, inelastic neutron scattering (INS) 5,9,14,15 has identified a coherent collective mode in SmB 6 . The energy of this mode is smaller than the quasiparticle excitation gap (charge gap), and thus protects it from decay.…”

Effects of spin excitons on the surface states of SmB6 : A photoemission study

“…The difference spectra shows that this feature has its intensity derived from the X and Y -points, and is of surface origin. We identify this peak as a resonance in the surface electronic density of states caused by the coupling of the surface states to the in-gap spin-exciton excitations [28].Spinexcitons are magnetic excitations that occur in the gap of paramagnetic Kondo Insulators [29,30,31], and have been observed in bulk SmB 6 with excitation energies of 12 to 14 meV [32,33,34,35]. The intensity of the spin-exciton peak seen in neutron scattering experiments [34] increases and the peak width decreases with decreasing temperature.…”