Anharmonic rattling vibrations effects in the ESR of Er³⁺doped SmB₆Kondo insulator

We report X-band Electron Spin Resonance (ESR) experiments on ≈ 0.2% and ≈ 0.7 % Er3+ doped SmB6 at low temperature (4 K - 40 K). The crystal field ground state of Er3+ in SmB6 is a Γ8 quartet with a nearby Γ6 excited doublet. The angular dependence of the resonances is not consistent with transitions between pure cubic crystal field states. The data were interpreted in terms of a dynamic Jahn-Teller (JT) effect by a coupling to Γ3 vibrational modes, which we propose to originate from the rattling of the small Er3+ ions in the large SmB6 cage. Our data show an anisotropic pair of E and E’ resonances at g ≈ 4.4 and two nearly isotropic signals at g ≈ 5.8, one intense and narrow (A vibrational mode) and the other broad and faint, which we attribute to Er3+ ions at lattice sites which are strongly affected by strain, defects and/or extrinsic Al impurities that inhibits the JT effects. Our results are in general consistent with those previously reported by Sturm et al. In addition to the angular dependence of the lines, we discuss the intensities, g-values and the linewidths of the Er3+ transitions as a function of temperature.