Topological twists in electronic band structures become noticeable at the boundaries of materials. At interfaces or surfaces, the electronic wavefunctions need to smoothly connect from one side to another. The new edge states that emerge are protected by the topological invariants on both sides of the boundary. SmB6 has been widely studied as an example in which heavy f-electrons and d-electrons interact, giving rise to the formation of a low temperature Kondo insulating state. The formation of the Kondo state coincides with a change in total parity of the occupied bands, resulting in an extra topological twist in the bandstructure. This is expected to give rise to topological edge states and possibly provides an explanation for a saturation of the resistivity at low temperature.
We show using optical spectroscopy that the formation of the Kondo state goes hand in hand with subtle shifts of optical spectral weight over extremely large energy ranges (of order U≈ 7 eV). Our results give an experimental estimate of the effective Coulomb interaction, but our detailed analysis also provides evidence that the low energy properties (such as the size of the energy gap) are set by the same energy scale.