Deciphering the Enigma of Cu-Doped Lead Apatite (LK-99): Structural Insights, Electronic Properties, and Implications for Ambient-Pressure Superconductivity

TL;DR

This study uses quantum simulations to analyze the structural and electronic properties of Cu-doped lead apatite LK-99, clarifying its insulating and semiconducting behaviors, and addressing conflicting experimental results regarding its superconductivity potential.

Contribution

It provides detailed insights into the structural distortions and electronic states of LK-99 caused by Cu and S doping, advancing understanding of its properties relevant to high-temperature superconductivity.

Findings

Base compounds are insulating with large band gaps.

Cu doping causes structural distortion and semiconducting behavior.

S doping reduces the band gap and alters electronic states.

Abstract

The most recent discovery, the Cu-doped lead apatite LK-99, is a proposed room-temperature superconductor operating under ambient pressure. However, this discovery has brought a slew of conflicting results from different scientific groups. While some observed the absence of electrical resistance, others could not confirm any signs of superconductivity in LK-99. Here, we investigate the structural and electronic properties of LK-99 and its antecedent compounds through quantum mechanics (QM) and QM-based molecular dynamics (QM-MD) simulations. Our study elucidates the insulating nature of base compounds, Pb$_{10}$(PO$_4$)$_6$O and Pb$_{10}$(PO$_4$)$_6$(OH)$_2$, spotlighting their large band gaps. Notably, Cu doping in LK-99 disrupts its symmetry, yielding a distorted ground-state crystal structure with a triclinic P1 symmetry and CuO$_4$ square coordination. Such alterations predispose LK-99 to exhibit semiconducting behaviors, characterized by a flat band above the Fermi energy, arising from Cu-3d and O-2p orbitals. In addition, the S doping sustains the triclinic P1 symmetry but leads to a significantly reduced band gap, with a band emerging primarily from Cu-3d and S-3p orbitals. These findings are important in understanding LK-99's structural and electronic properties and provide a strategic compass for the development of high-T$_C$ superconductors.