Efforts to induce insulator-to-metal transition and superconductivity in lead apatite with copper and sulfur dopants.

In mid-2023, a South Korean research group from the company Quantum Energy Research Center (QERC) claimed the discovery of superconductivity at room temperature and atmospheric pressure in polycrystalline samples of copper (II)-substituted lead apatite material called LK-99, with the chemical composition Pb_10-xCu_x(PO₄)₆O, 0.9 < x < 1.1 (PCPOO). However, superconductivity is not observed in most of the LK-99 samples synthesized by independent research groups. Later, independent researchers found that single crystals of PCPOO are electrical insulators [P. Puphal et. al., APL Mater. 11, 101128 (2023)]. Subsequently, it was hypothesized that QERC researchers had misidentified the superconductive composition in their inhomogeneous samples, and the proposed new superconductive composition requires both copper and sulfur doping i.e. Pb_10-xCu_x(P(O_1-yS_y)₄)₆O_1-zS_z (PCPOSOS), x, y, and z to be determined [Hyun-Tak Kim et al., 2024 APS March meeting abstract A16.00002]. QERC did not provide William & Mary their LK-99 samples for investigative studies. Therefore, we decided to synthesize PCPOSOS samples at William & Mary. Since pure lead apatite is a band insulator, we are systematically studying its doping with copper and sulfur in an effort to induce a Mott insulator phase and then an insulator-to-metal transition (IMT). The IMT is a prerequisite to superconductivity.