Exotic ground state of CeRu4Sn6 investigated by means of specific-heat and NMR

The tetragonal compound CeRu4Sn6 is a narrow-gap semi-metal and shows no phase transition down to 50 mK. We performed field-sweep 119Sn NMR measurements at different frequencies (47, 70, 100 and 119 MHz) down to 2 K as well as specific-heat measurements in magnetic fields up to 10 T and temperatures down to 65 mK. The spin-lattice-relaxation rate 1/T1 in the high-T range (300 K to 20 K) is field independent and could be consistently fitted with a model for a narrow gap semi-metal (1/T1∝T exp(-Δ/kBT)) with a gap of Δ/kB≅30 K. This confirms the results from resistivity and thermopower measurements. Below 10 K, 1/T1 becomes strongly field dependent, whereas higher fields lead to a stronger reduction of 1/T1. The low-T range could be fitted with an activated behavior (1/T1∝exp(-Δ/kBT)), where Δ/kB increases with increasing field. This scenario is strongly supported by specific-heat investigations. Both results can be described consistently by assuming a double Cauchy-Lorentz function for the residual in-gap density of states N(E). The ground state is governed by these correlated states which show Zeeman-type splitting in magnetic fields.