This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd$^{ 3+}$-doped SmB$_{ 6}$ 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 ($\Delta H$) within 5.3 K $\leq T \leq$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the ESR microwave frequencies used ($\sim$10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd$^{ 3+}$ $S$-states to the breathing mode of the SmB$_{ 6}$ cage, and can be simulated by a random distribution of the 4$^{ th}$ 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 $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{ 6}$ ($\Delta\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB$_{ 6}$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB$_{ 6}$ by coupling the Gd$^{ 3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.
Categories: cond-mat.str-el cond-mat.mtrl-sci