[Paper Review] On the Experimental Evidence for Possible Superconductivity in LK99

Harishchandra Singh, A. Gautam+9 more|arXiv (Cornell University)|Aug 12, 2023

Physics of Superconductivity and MagnetismPhysics and Astronomy3 citations

TL;DR

This study investigates the claim of room-temperature superconductivity in LK-99 by synthesizing Cu- and Ni-doped lead apatite compounds. Despite the presence of Cu2S impurities, transport and magnetization measurements reveal insulating behavior, no flux expulsion, and paramagnetic response—providing no evidence for superconductivity, contradicting earlier reports of room-temperature levitation and zero resistance.

ABSTRACT

The desire to create an energy efficient world is bound to be incomplete without the discovery of a room temperature superconductor at ambient pressure. A recent report on the room-temperature ambient-pressure superconductor has inspired scientists to study the Cu doped Lead apatite named as LK-99. Here, we have synthesized Cu doped LK-99 and Ni-doped LK-99 compounds and studied their temperature dependent transport and magnetization behavior. In spite of the presence of impurity phase Cu$_2$S, the temperature dependent resistance shows an insulating nature of the sample. The radio frequency penetration depth measurement unveils the absence of diamagnetic flux expulsion in this sample. The temperature dependent ac susceptibility measurements reveal the paramagnetic nature of the Ni doped LK-99.

Motivation & Objective

• To verify the reported room-temperature superconductivity in LK-99 through controlled synthesis and comprehensive characterization.
• To determine whether the observed electrical and magnetic responses in LK-99 are due to superconductivity or impurity phases such as Cu2S.
• To assess the role of Cu and Ni doping in inducing superconducting behavior in lead apatite.
• To apply rigorous experimental criteria—resistivity below 1 µV/cm, diamagnetic flux expulsion, and ZFC/FC splitting—for identifying true superconductivity.

Proposed method

• Solid-state synthesis of PbSO4, Cu3P, Ni3P, and lanarkite precursors using high-temperature reactions at 725–925 °C.
• Sealed-tube synthesis of Cu- and Ni-doped lead apatite via reaction of lanarkite with Cu3P or Ni3P at 925 °C for 24 hours under vacuum.
• Powder X-ray diffraction (PXRD) using a Bruker D8 Advance diffractometer to confirm phase purity and identify impurities such as Cu and Cu2S.
• Temperature-dependent resistivity measurements from 150 K to 300 K to assess metallic or insulating behavior.
• Tunnel diode oscillator technique for measuring radio frequency penetration depth to detect diamagnetic flux expulsion.
• AC susceptibility measurements on Ni-doped LK-99 to probe magnetic transitions and distinguish superconducting from paramagnetic responses.

Experimental results

Research questions

• RQ1 Does Cu-doped lead apatite (LK-99) exhibit true superconductivity at ambient pressure and room temperature?
• RQ2 Can the reported magnetic levitation and zero-resistance behavior in LK-99 be attributed to superconductivity or to impurity phases like Cu2S?
• RQ3 What is the nature of the electrical transport and magnetic response in LK-99 and Ni-doped LK-99 across a wide temperature range?
• RQ4 To what extent do the experimental signatures—resistivity, penetration depth, and ac susceptibility—support the presence of a superconducting transition?

Key findings

• The temperature-dependent resistivity of LK-99 shows an exponential decrease from 150 K to 300 K, indicating insulating behavior rather than metallic or superconducting conduction.
• Penetration depth measurements across 250–300 K show no significant frequency shift, ruling out diamagnetic flux expulsion and thus excluding superconductivity.
• AC susceptibility measurements on Ni-doped LK-99 reveal a linear decrease with increasing temperature, confirming a paramagnetic response rather than a superconducting transition.
• PXRD analysis confirms the presence of Cu2S and unreacted Cu as impurities in the Cu-doped LK-99 sample, which may contribute to misleading electrical and magnetic responses.
• The absence of ZFC/FC splitting and failure to meet the 1 µV/cm resistance criterion further exclude superconductivity in the synthesized samples.
• No evidence of bulk superconductivity is observed, despite theoretical predictions of flat bands and enhanced Coulomb interactions upon Cu doping.