Slow crystalline electric field fluctuations in the Kondo lattice SmB$_{6}$

Abstract

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd 3+doped SmB6 single crystals at X-and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd 3+ ESR linewidth (∆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 ESR microwave frequencies used (∼10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd 3+ S-states to the breathing mode of the SmB6 cage, and can be simulated by a random distribution of the 4 th CEF parameter, b4, 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 ∆H above T 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB6 (∆ 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 Gd 3+ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.