Effect of Sm valence variation on hybridization gap and in-gap excitons in SmB6 studied by Raman spectroscopy*

SmB$_6$ is a proposed topological Kondo insulator where the presence of topological nontriviality can be tuned by variations in the Sm valence. A range of samples where Sm valence was varied by increasing numbers of Sm vacancies was investigated using Raman spectroscopy over a temperature range of 10 to 300 K. We show a possibility to characterize the presence of Sm vacancies on the order of 1% by correlating with the intensity of defect-induced Raman scattering. At temperatures below 50 K features due to hybridization gap and four in-gap excitons at energies between 128 and 140 cm$^{-1}$ appear in the Raman spectra of samples with low number of Sm vacancies. The narrow 4 cm$^{-1}$ line width of the excitons is associated with their long life time due to the hybridization gap. A presence of Sm vacancies leads to an appearance of the impurity states in the gap and a respective decrease of the exciton life time. As much as estimated 1% of Sm impurities leads to quenching of the in-gap excitons.