Evidence of near-ambient superconductivity in a N-doped lutetium hydride

Abstract

The absence of electrical resistance exhibited by superconducting materials would have enormous potential for applications if it existed at ambient temperature and pressure conditions. Despite decades of intense research efforts, such a state has yet to be realized. At ambient pressures, cuprates are the material class exhibiting superconductivity to the highest critical superconducting transition temperatures (Tc), up to about 133?K . Over the past decade, high-pressure ‘chemical precompression’of hydrogen-dominant alloys has led the search for high-temperature superconductivity, with demonstrated Tc approaching the freezing point of water in binary hydrides at megabar pressures. Ternary hydrogen-rich compounds, such as carbonaceous sulfur hydride, offer an even larger chemical space to potentially improve the properties of superconducting hydrides. Here we report evidence of superconductivity on a nitrogen-doped lutetium hydride with a maximum Tc of 294?K at 10?kbar, that is, superconductivity at room temperature and near-ambient pressures. The compound was synthesized under high-pressure high-temperature conditions and then—after full recoverability—its material and superconducting properties were examined along compression pathways. These include temperature-dependent resistance with and without an applied magnetic field, the magnetization (M) versus magnetic field (H) curve, a.c. and d.c. magnetic susceptibility, as well as heat-capacity measurements. X-ray diffraction (XRD), energy-dispersive X-ray (EDX) and theoretical simulations provide some insight into the stoichiometry of the synthesized material. Nevertheless, further experiments and simulations are needed to determine the exact stoichiometry of hydrogen and nitrogen, and their respective atomistic positions, in a greater effort to further understand the superconducting state of the material.

Main

Dense elemental hydrogen has long been predicted to be a very-high-temperature superconductor, yet the extremely high pressures required have presented challenges in confirming those superconducting phases. The superhydride materials offer the promise of retaining the superconducting properties of dense elemental hydrogen but at much lower pressures. The prediction of a 220–235-K superconducting transition temperature (Tc) in CaH6 at 150?GPa and the watershed discovery of a 203-K Tc for H3S at 155?GPa have instigated a materials discovery boon in which, at present, almost all possible binary systems of high-pressure hydride systems have been modelled. The recent observation of an anomalously high Tc in YH6 showed that high-temperature superconductivity can be achieved with lower hydrogen content and more modest pressures than previously understood. As the main discoveries have all been at greater than megabar pressures, the goal has shifted to further lowering the pressure required, with a focus on the vast sample space of ternary hydride compounds. One direction is a third, light element acting as a dopant in the metal hydrides, which is predicted to have two main beneficial effects. First, there is a predicted increase in Tc as seen in proposed examples such as critical temperatures approaching 500?K in the Li–Mg–H system, although still in the megabar regime, and virtual crystal approximation simulations and recent experimental evidence indicating an increase of the transition temperature by at least 25?K from doping the LaH10 framework. Second, the addition of a third element can greatly enhance the stability of a hydrogen-rich lattice, thereby lowering the pressure range over which it is stable. LaBH8 is predicted to be stable down to 20–40?GPa while maintaining its high-temperature superconductivity, and a metal–boron–carbon clathrate is predicted to retain its superconducting properties at ambient pressures. The presence of stability combined with increasing Tc by introducing the third element opens the possibility of pushing the hydride superconductors to higher values of Tc at sub-megabar pressures.

As there is an overwhelming amount of phase space unexplored by simulation in ternary rare-earth hydrides, rational chemical design is needed at present to identify the next candidate material. The La and Y binary superhydrides are predicted and measured to adopt similar high-pressure stoichiometries and phases with the Y-based ones exhibiting higher Tc at equivalent pressures. The smaller size of the Y3+ cation offers a simple chemical rationale for this behaviour. However, the Sc hydrides with an even smaller ionic radius are predicted to have completely different structures and lower Tc . Owing to the lanthanoid contraction, the lanthanoids heavier than Dy offer comparable or smaller trivalent ionic radii than Y but with the complication of f electrons. Although the 4f electrons in lanthanoid compounds are often atom-localized and semivalent at ambient conditions, the inherent magnetism of partially occupied 4f states or migration towards the Fermi level under pressure could be detrimental to the superconducting properties. Although synthesis efforts for the high-pressure YbHx system have produced structures distinct from its La and Y counterparts, probably owing to the transfer of d electrons to unoccupied f states, predictions indicate that hydrides of the two heaviest lanthanoids should be able to achieve Tc?≥?145?K by a megabar owing to the strong electron correlation of the 4f electrons near the Fermi level. Causes of the high Tc achieved in the sub-megabar regime are believed to be twofold. First, the over half-filled valence 4f states suppress the phonon softening and second they provide some enhancement to the electron–phonon coupling relative to the transition metal (Y and La) rare earths. Combining the benefits of light atom doping and the presence of 4f electrons in the valance states should increase the stability of a hydrogen-rich rare-earth hydride to lower pressures while potentially enhancing its superconducting properties.

In this paper, we present experimental evidence of superconductivity at 294?K and 10?kbar pressure in a ternary lutetium–nitrogen–hydrogen compound in which the combination of a full 4f shell along with the electron donation and chemical pressure of the nitrogen drive the Tc and pressure stability of nitrogen-doped lutetium hydride into the near-ambient regime. The measured superconducting properties are the observation of zero resistance, a.c. magnetic susceptibility and d.c. magnetic susceptibility with zero field and field cooling, magnetization M–H curve, heat capacity, voltage–current (V–I) curves and the reduction of Tc under an external magnetic field with an upper critical magnetic field of about 88?tesla based on the Ginzburg–Landau (GL) model at zero temperature . The composition and structure are explored with elemental analysis, EDX measurements, XRD, Raman spectroscopy and density functional theory (DFT) simulations.

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Sources: Nature volume 615, pages244–250 (2023)

Published: 08 March 2023

DOI: 10.1038/s41586-023-05742-0

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