CeRu${}_4$Sn${}_6$: heavy fermions emerging from a Kondo-insulating state

Abstract

The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu${}_4$Sn${}_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $\Delta E_{g}1/k_B\simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient $\gamma =C(T)/T$ : In zero field, $\gamma$ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at $\gamma =0.6$ J/molK${}^2$. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap $\Delta E_{g}2$. A simple model, based on two narrow quasiparticle bands located at the Fermi level - which cross the Fermi level in zero field at 0.022 states/meV f.u. - can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting our data of both specific heat and NMR to the model, incorporating a Ce magnetic moment of $\mu =\Delta E_{g}1/{\mu }_{0H}\simeq 1{\mu }_B$, leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced $\gamma$ in CeRu${}_4$Sn${}_6$ and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of system

11 pages, 13 figures