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

TL;DR

This study investigates the temperature-dependent ESR linewidth in Gd-doped SmB$_{6}$, revealing slow CEF fluctuations linked to the insulator-metal transition and hybridization gap, suggesting complex charge and valence fluctuation dynamics.

Contribution

It demonstrates the presence of slow crystalline electric field fluctuations in SmB$_{6}$ and models their effects on ESR linewidth in relation to phase transition and hybridization gap.

Findings

Inhomogeneous ESR linewidth broadening occurs between 5.3 K and 12.0 K.

Linewidth inhomogeneity correlates with the insulator-to-metal transition.

Exponential temperature dependence of linewidth suggests excitation energy near the hybridization gap.

Abstract

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.