Is CeNiSn a Kondo Semiconductor

Cerium intermetallic compounds can have different ground states, depending on the hybridization between f-electrons and conduction electrons. CeNiSn is usually classified as a Kondo semiconductor, originally because of the enhanced electrical resistivity at low temperatures [1]. But when high quality samples became available, the low-temperature resistivity turned out to be metallic [2]. Tunnel spectroscopy provides a direct access to the electronic density of states (EDOS) [3]. Using mechanically controllable break junctions (MCBJ), Ekino et al. [4] observed dI/dV spectra with ∼ 10 meV broad zero-bias (ZB) minima. They assumed — without further experimental evidence — that their junctions were in the tunnel regime, and interpreted the ZB minima as being due to a gap in the EDOS. These ZB minima were found to be suppressed in magnetic fields B ≥ 14 T only along the a-axis, indicating as a crossover from a pseudogap to a metallic heavy-fermion state [5]. Our investigation of MCBJs of CeNiSn, both in the metallic (direct contact) and in the vacuum-tunneling regime, is based on three CeNiSn single crystals with long sides in the a, b, and c direction of the orthorhombic crystal lattice, respectively. Magnetic fields up to 8 T could be applied perpendicular to the long side of the sample (perpendicular to current flow). For further details see Refs. [6, 7].