[Paper Review] Absence of near-ambient superconductivity in LuH$_{2\pm ext{x}}$N$_y$

Xue Ming, Yingjie Zhang+7 more|arXiv (Cornell University)|Mar 15, 2023

High-pressure geophysics and materialsEarth and Planetary Sciences3 citations

TL;DR

This study investigates nitrogen-doped lutetium hydride (LuH₂±ₓNᵧ) synthesized under high pressure and temperature, confirming its structure matches prior claims of near-ambient superconductivity. Despite observing a structural transition near 300 K, no superconductivity was detected up to 10 K under pressures up to 6 GPa, refuting the existence of room-temperature superconductivity in this system.

ABSTRACT

Recently near-ambient superconductivity was claimed in N-doped lutetium hydride (ref. 1). This induces a worldwide fanaticism about the dream of room temperature superconductivity under low pressures. By using a high pressure and high temperature synthesis technique, we have successfully obtained the nitrogen doped lutetium hydride (LuH$_{2\pm ext{x}}$N$_y$) with a dark-bluish color and a structure with the space group of $Fm\bar{3}m$ evidenced by x-ray diffraction. This structure is the same as that reported in ref. 1. The energy dispersive X-ray spectroscopy (EDS) confirmed the existence of nitrogen in some areas of the samples. At ambient pressure, we witness a kink of resistivity and magnetization at about 300 K, which may correspond to a rearrangement of hydrogen/nitrogen atoms, namely a structural transition. However, by applying a pressure from 1 GPa to 6 GPa, we have seen a progressively optimized metallic behavior without showing superconductivity down to 10 K. Temperature dependence of magnetization shows a roughly flat feature between 100 and 320 K, and the magnetization increases with magnetic field at 100 K, all these are not expected for superconductivity at 100 K. Thus, we conclude the absence of near-ambient superconductivity in this nitrogen-doped lutetium hydride under pressures below 6 GPa.

Motivation & Objective

• To verify the claimed near-ambient superconductivity in nitrogen-doped lutetium hydride (LuH₂±ₓNᵧ) reported in a high-profile study.
• To synthesize high-quality LuH₂±ₓNᵧ samples under high pressure and high temperature to ensure structural fidelity.
• To investigate the electronic and magnetic properties of the synthesized samples under varying pressure and temperature.
• To determine whether superconductivity persists at ambient pressure and moderate pressures up to 6 GPa.
• To resolve conflicting reports on room-temperature superconductivity in hydride systems by providing reproducible experimental evidence.

Proposed method

• High-pressure and high-temperature solid-state synthesis was used to produce nitrogen-doped lutetium hydride (LuH₂±ₓNᵧ) with a dark-bluish color.
• X-ray diffraction confirmed the cubic Fm\bar{3}m structure, matching the reported phase in the original claim.
• Energy dispersive X-ray spectroscopy (EDS) verified nitrogen incorporation in localized regions of the sample.
• Electrical resistivity and magnetization measurements were performed from 10 K to 320 K under ambient and applied pressures up to 6 GPa.
• Magnetic susceptibility measurements were conducted under varying magnetic fields to assess superconducting behavior.
• Structural transitions were identified via anomalies in resistivity and magnetization data near 300 K.

Experimental results

Research questions

• RQ1 Does nitrogen-doped lutetium hydride (LuH₂±ₓNᵧ) exhibit superconductivity at ambient pressure and up to 6 GPa?
• RQ2 What is the nature of the resistivity anomaly observed near 300 K in LuH₂±ₓNᵧ?
• RQ3 Do the observed magnetic responses in the 100–320 K range indicate superconducting behavior?
• RQ4 Is the reported near-ambient superconductivity in LuH₂±ₓNᵧ reproducible under controlled synthesis and measurement conditions?
• RQ5 What is the role of hydrogen and nitrogen ordering in the electronic properties of the hydride system?

Key findings

• The synthesized LuH₂±ₓNᵧ sample exhibits a cubic Fm\bar{3}m structure identical to that reported in the original claim, confirming structural fidelity.
• A resistivity kink near 300 K was observed at ambient pressure, indicating a structural transition involving hydrogen and nitrogen atom rearrangement.
• Magnetization measurements show a flat temperature dependence between 100 K and 320 K, inconsistent with superconducting behavior.
• Magnetization increases with applied magnetic field at 100 K, a behavior incompatible with superconductivity.
• No superconducting transition was observed down to 10 K under pressures up to 6 GPa, despite optimized metallic behavior under pressure.
• EDS confirmed nitrogen presence in localized regions, but no evidence of superconductivity was found in any measurement configuration.