First order transition in Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O ($0.9<x<1.1$) containing Cu$_2$S (2308.04353v1)

Abstract: Lee et al. reported that the compound LK99, with a chemical formula of Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O ($0.9<x<1.1$), exhibits room-temperature superconductivity under ambient pressure. In this study, we investigated the transport and magnetic properties of pure Cu$_2$S and LK-99 containing Cu$_2$S. We observed a sharp superconducting-like transition and a thermal hysteresis behavior in the resistivity and magnetic susceptibility. However, we did not observe zero-resistivity below the transition temperature. We argue that the so-called superconducting behavior in LK-99 is most likely due to a reduction in resistivity caused by the first order structural phase transition of Cu$_2$S at around 385 K, from the $\beta$ phase at high temperature to the $\gamma$ phase at low temperature.

• The paper shows that a first-order β-γ phase transition in Cu₂S causes a 3–4 order magnitude resistivity drop in LK-99 samples.
• It employs solid-state synthesis, x-ray diffraction, 4-probe resistivity, and SQUID magnetometry to detail the impurity's impact.
• The findings challenge room-temperature superconductivity claims by emphasizing the need for precise impurity characterization.

Analysis of Cuprous Sulfide in Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$ and Implications for Reported Superconductivity

The paper authored by Zhu et al. presents a rigorous examination of the purported superconductivity of the compound known as LK-99, with a chemical formula of Pb$_{10-x}$Cu$_x$PO$_4$)$_6$ (with 0.9 < x < 1.1), previously suggested to exhibit room-temperature superconductivity under ambient pressure by Lee et al. This paper methodically investigates the transport and magnetic properties of Cu$_2$S and its influence on LK-99, ultimately questioning the superconductivity claims.

Key Findings

Zhu et al. conducted a detailed analysis with a focus on the role of Cu$_2$S, an impurity in the LK-99 samples. The paper identifies a first-order structural phase transition in Cu$_2$S from the β phase to the γ phase around 385 K, a phenomenon that drastically reduces resistivity by 3–4 orders of magnitude. This resistivity drop is comparable to the one reported for LK-99 and suggests that the superconductivity-like behavior attributed to LK-99 is a spurious result stemming from the behavior of the Cu$_2$S impurity.

Key experimental outcomes demonstrate a sharp transition in resistivity in both pure Cu$_2$S and LK-99 mixed with viable quantities of Cu$_2$S. However, in these experiments, zero-resistivity, a definitive haLLMark of superconductivity, was not observed below the transition temperature. Magnetic susceptibility measurements further add evidence; they present a marked thermal hysteresis without the typical characteristics associated with a superconducting transition.

Methodological Framework

The research synthesized LK-99 samples with varying Cu$_2$S concentrations using solid-state reactions, further subjected to structural characterization via x-ray diffraction and resistivity measurements through the standard 4-probe method. Additionally, the temperature-dependent magnetic properties were analyzed using a SQUID magnetometer.

Discussion and Implications

The findings of Zhu et al. provide a substantial basis for challenging the claims of room-temperature superconductivity in LK-99. The paper suggests that the phenomena originally ascribed to superconductivity are likely due to structural phase transitions in the ubiquitous impurity, Cu$_2$S. This revelation has key implications for the field of superconductivity research. It emphasizes the necessity for precise impurity characterization and synthesis conditions in such materials to accurately discern intrinsic superconducting properties.

Moreover, the observed wide thermal hysteresis and lack of zero-resistivity indicate a phase transition behavior not consistent with a second-order superconducting transition. As such, these results call for a reevaluation of earlier findings when Cu$_2$S impurities are present.

Future Perspectives

The paper significantly informs future investigations into room-temperature superconductors, highlighting the critical role of exhaustive material characterization. The authors recommend further studies focusing on the precise measurement of thermal hysteresis in resistivity and susceptibility to distinguish potential superconducting phases from impurity-driven behaviors. Given the extensive implications of room-temperature superconductors, such rigorous scrutiny is imperative to advance both theoretical understanding and practical application in the field.

In conclusion, Zhu et al.’s work underscores the importance of cautious interpretation of superconducting phenomena, particularly regarding impurity influences, and contributes a critical perspective to ongoing research in high-temperature superconductors.