Percolation-induced resistivity drop in cold-pressed LuH2

TL;DR

This study investigates how surface modifications and grain size affect the resistivity of cold-pressed LuH2, revealing that resistivity drops are due to percolation effects rather than superconductivity, emphasizing caution in interpreting such drops.

Contribution

It demonstrates that resistivity drops in cold-pressed LuH2 are caused by percolation effects from insulating grain surfaces, not superconductivity, challenging previous interpretations.

Findings

Resistivity varies with grain size and surface conditions.

Resistivity drops are due to percolation, not superconductivity.

Surface insulating layers influence electrical behavior.

Abstract

The stoichiometric bulk LuH2 is a paramagnetic metal with high electrical conductivity comparable to simple metals. Here we show that the resistivity of cold-pressed (CP) LuH2 samples varies sensitively upon modifying the grain size or surface conditions via the grinding process, i.e., the CP pellets made of commercially purchased LuH2 powder remain metallic but exhibit thousands of times higher resistivity, while additional grinding of LuH2 powders in air further enhances the resistivity and even results in weakly localized behaviors. For these CP samples, interestingly, we can occasionally observe abrupt resistivity drops at high temperatures, which also show dependences on magnetic fields and electrical current. Measurements of variable-temperature XRD, magnetic susceptibility, and specific heat exclude the possibilities of structural, magnetic, and superconducting transitions for the observed resistivity drops. Instead, we tentatively attribute these above observations to the presence of insulating layers on the grain surface due to the modification of hydrogen stoichiometry or the pollution by oxygen/nitrogen. Percolation of the metallic grains through the insulating surfaces can explain the sudden drop in resistivity. The present results thus call for caution in asserting the resistivity drops as superconductivity and invalidate the background subtraction in analyzing the resistivity data.