Energy gap of the ground state of CeNiSn caused by local and long-range magnetic-moment interactions

Abstract

We have studied the thermal expansion of the gapped intermetallic heavy-fermion compounds CeNiSn, ${\mathrm{CeNi}}_{0.9}$${\mathrm{Cu}}_{0.1}$Sn, and ${\mathrm{Ce}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$NiSn (x=0.03,0.1) between 0.5 K and 25 K; and the transport (3T60 K) and magnetic (0.1T8 K) properties of CeNiSn. The magnetic heavy-fermion compound ${\mathrm{CeNi}}_{0.9}$${\mathrm{Cu}}_{0.1}$Sn displays an antiferromagnetic (AF) transition at ${\mathit{T}}_{\mathrm{AF}}$\ensuremath{\sim}1.5 K; CeNiSn exhibits a gapped (${\mathit{E}}_{\mathit{g}}$\ensuremath{\sim}2--8 K) and possibly ``weakly'' magnetic ground state T0.5 K; ${\mathrm{Ce}}_{0.97}$${\mathrm{La}}_{0.03}$NiSn shows ${\mathit{T}}_{\mathrm{AF}}$\ensuremath{\sim}6 K, and finally ${\mathrm{Ce}}_{0.9}$${\mathrm{La}}_{0.1}$NiSn does not display heavy-fermion or gapped behavior. We have investigated the transition between the above regimes. The comparison between the thermal-expansion data of CeNiSn, ${\mathrm{Ce}}_{0.97}$${\mathrm{La}}_{0.03}$NiSn, and single-crystalline CeNiSn shows that gap formation may be unstable against the weak magnetic transitions with ${\mathit{T}}_{\mathrm{AF}}$\ensuremath{\sim}6 K and ${\mathit{T}}_{\mathrm{AF}}$\ensuremath{\sim}2 K. The anomalous negative thermal expansion of CeNiSn that is observed below 1 K allows us to propose a simple picture for the electron spectrum near the Fermi level.