Low-temperature properties of CeRu₄Sn₆ from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state

The combination of low-temperature specific heat and nuclear-magnetic- resonance (NMR) measurements reveals important information on the ground-state properties of CeRu₄Sn₆, which has been proposed as a rare example of a tetragonal Kondo insulator (KI). The NMR spin-lattice-relaxation rate 1/ T₁ deviates from the Korringa law below 100 K signaling the onset of an energy gap Δ E_g1 / k_B ≈30K. 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 γ=C (T) /T: in zero field, γ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at γ≈0.6J/ K² mol. Upon increasing field, this maximum is shifted to higher temperatures with an overall reduction in γ, suggesting a residual density of states at the Fermi level developing a spin (pseudo-)gap Δ E_g2. 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/meVf.u.-can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting this model, incorporating a Ce magnetic moment of μ=Δ E_g1 / μ₀ H≈1 μ_B, to our data of both specific heat and NMR 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 γ in CeRu₄Sn₆ 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 systems.