Percolation-induced resistivity drop in lutetium dihydride with controllable electrical conductivity over six orders of magnitude

The recent report of near-ambient superconductivity in the nitrogen-doped lutetium hydride has attracted considerable attention. Subsequent follow-up studies confirmed the pressure-induced color changes in both N-free and N-doped LuH_2 but failed to reproduce superconductivity. It remains a puzzle why the samples in the original report exhibited pronounced resistance anomaly reminiscent of the superconducting transition. Here, we show that percolation of metallic grains with high conductivity through the insulating surfaces in cold-pressed LuH_2 samples can occasionally produce sharp resistance drops, which even display magnetic field and/or current dependences but stay far from zero resistance. The insulating surface of LuH_2 grain should be attributed to the modification of hydrogen stoichiometry or the contamination by oxygen/nitrogen, resulting in an increase of resistance by over six orders of magnitude. Such an effect is more significant than that discovered recently in LaH_3± x , which may indicate that LuH_2 can be a potential superionic conductor. Our results call for caution in asserting the resistivity drops as superconductivity and invalidate the background subtraction in analyzing the corresponding resistance data.