[Paper Review] Current percolation model for the special resistivity behavior observed in Cu-doped Apatite

Qiang Hou, Wei Wei+4 more|arXiv (Cornell University)|Aug 10, 2023

Theoretical and Computational Physics16 references4 citations

TL;DR

This study proposes a current percolation model based on Cu/Pb conductive channels to explain the diverse resistivity behaviors in Cu-doped apatite (Pb10−xCux(PO4)6O), including near-zero resistivity and abrupt drops. Despite some resistivity anomalies, no Meissner effect or definitive superconducting signature was observed, leading to the conclusion that superconductivity in this system remains unconfirmed.

ABSTRACT

Since the initial report of the potential occurrence of room-temperature superconductivity under normal pressure [arXiv: 2307.12008], there has been significant interest in the field of condensed matter physics regarding Cu-doped Apatite (Pb10-xCux(PO4)6O). In this study, we performed temperature-dependent resistivity measurements on the synthesized Pb10-xCux(PO4)6O samples. The structure of the sample was confirmed to match the reference literature through X-ray diffraction analysis. Remarkably, we observed four distinct types of resistivity behaviors within samples from the same pellet: (1) A semiconductor-like behavior characterized by a decrease in resistivity as the temperature is lowered. (2) A gradual reduction in resistivity, reaching an exceptionally small value that falls below the resolution limits of our measurement equipment. (3) An abrupt drop in resistivity to a low value at ~ 250 K. (4) An almost linear reduction in resistivity exhibiting a transition at approximately 7 K (possibly associated with Pb). Following a thorough compositional analysis, we proposed a current percolation model, based on the formation of a Cu/Pb current channel, to elucidate the observed special resistivity behaviors. It is important to note that the Meissner effect was not observed in our magnetization measurements. Consequently, we reached the conclusion that the presence of superconductivity in Cu-doped Apatite has yet to be substantiated.

Motivation & Objective

• To investigate the origin of anomalous resistivity behaviors in Cu-doped apatite (Pb10−xCux(PO4)6O) following the controversial claim of room-temperature superconductivity.
• To determine whether the observed resistivity drops and low resistivity values are due to superconductivity or alternative physical mechanisms.
• To clarify the role of Cu doping and structural inhomogeneities in generating diverse electrical responses within the same sample.
• To rule out superconductivity by examining magnetization and Meissner effect signatures.

Proposed method

• Performed temperature-dependent resistivity measurements using the standard four-probe method on a physical property measurement system (PPMS).
• Conducted X-ray diffraction (XRD) to confirm the crystal structure of synthesized Pb10−xCux(PO4)6O samples.
• Used energy dispersive X-ray spectroscopy (EDS) for elemental composition analysis to assess Cu and Pb distribution.
• Carried out magnetization measurements via vibrating sample magnetometer (VSM) to search for the Meissner effect and superconducting signatures.
• Proposed a current percolation model based on the formation of Cu/Pb conductive pathways to explain resistivity anomalies.
• Analyzed M-H loops and ZFC/FC magnetization curves to detect ferromagnetic or superconducting ordering.

Experimental results

Research questions

• RQ1 What causes the four distinct resistivity behaviors observed in the same Cu-doped apatite pellet?
• RQ2 Can the observed near-zero resistivity be attributed to superconductivity or alternative mechanisms?
• RQ3 Is there any evidence for the Meissner effect or magnetic levitation in the samples?
• RQ4 How do Cu and Pb cations contribute to the formation of conductive pathways in the apatite lattice?
• RQ5 To what extent do structural inhomogeneities or impurities explain the resistivity anomalies?

Key findings

• Four distinct resistivity behaviors were observed in the same pellet: semiconductor-like decrease, near-zero resistivity below measurement resolution, abrupt drop at ~250 K, and linear decrease with a transition near 7 K.
• XRD confirmed the main phase as Pb10−xCux(PO4)6O, with minor impurities detected in elemental analysis.
• No Meissner effect was observed in magnetization measurements, and no superconducting-like M-H loops were detected.
• Magnetization data revealed a ferromagnetic hysteresis loop across 10–250 K, with kinks at 10 K, 50 K, and 80 K, indicating possible magnetic ordering.
• The resistivity drop at ~250 K correlates with a kink in the ZFC curve, possibly due to a structural or magnetic transition.
• The absence of a Meissner signal and the presence of ferromagnetic components rule out superconductivity as the origin of the resistivity anomalies.