Raman spectroscopic evidence of impurity-induced structural distortion in SmB₆

Impurity-induced structural distortion in SmB₆ is suggested by Raman spectroscopy study on impurity-controlled samples. Significant differences in the Raman spectra of single crystals SmB₆(6N), synthesized with 99.9999%-pure boron, and SmB₆(3N), synthesized with 99.9%-pure boron, are detected. While no noticeable differences are detected in the X-ray diffraction of SmB₆(3N) and SmB₆(6N), all the Raman phonon modes of the T_2g, E_g, and A_1g of SmB₆(3N) are clearly broader and shift to higher wavenumbers than those of SmB₆(6N), and the T_2g and E_g modes of SmB₆(3N) show doublet features. Based on the high-pressure Raman measurements and phonon calculation in uniaxial compression model, we argue that small amount of impurities in SmB₆(3N) is enough to induce anisotropic distortion in the B₆ octahedra, leading to peculiar behaviors in Raman spectrum of SmB₆(3N). Our results may present a clue for understanding the current puzzles about SmB₆ of various origins with different impurities.