Slow crystalline electric field fluctuations in the Kondo lattice SmB6

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd3+-doped SmB6 single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd3+ ESR linewidth (Delta H) within 5.3 K <= T <= 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the used ESR microwave frequencies (similar to 10GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd3+S states to the breathing mode of the SmB6 cage and can be simulated by a random distribution of the 4th CEF parameter, b(4), that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential T dependence of Delta H above T similar or equal to 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB6 (Delta similar or equal to 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB6. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB6 by coupling the Gd3+ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism. (AU)