Evidence for Near Ambient Superconductivity in The Lu-N-H system

Superconductivity in the vicinity of room temperature has the potential to revolutionize both numerous technologies and our understanding of condensed matter. Zero electrical resistance and expulsion of magnetic field below critical temperature <i>T</i>_care crucial tests of superconductivity. Previous work from our group has established near-room temperature superconductivity but only at megabar pressures (e.g., Ref. 1). Recently reported evidence for superconductivity at ambient <i>P-T</i> conditions in nitrogen-doped lutetium hydride (Lu-N-H) is promisingbut controversial² Our group has conducted independent measurements on the material synthesized by the methods described in Ref. 2. Four-probe electrical measurements on selected samples in diamond anvil cells show abrupt and reproducible loss of resistance at well-defined critical temperatures and pressures. Magnetic susceptibility measurements show reproducible signatures of field expulsion at similar critical temperatures. The <i>T</i>_cvalues from electrical resistance and magnetic susceptibility for these samples agree and are consistent with the previously reported data.² On the other hand, other samples prepared with similar procedures exhibit no measureable <i>T</i>_c. but instead show evidence for anomalous metal-insulator transitions that have been well-studied in lanthanide hydrides at ambient pressure (e.g., Ref. 4). Our measurements thus provide direct evidence for near ambient superconductivity in one or more Lu-N-H phases while some phases in the material are not superconducting. We also conducted first-principles DFT and DFT+U calculations to further understand the remarkable properties of these materials. Supercell calculations starting with N-doped <i>Fm-</i>3<i>m</i> LuH₃ reveal configurations such as Lu₈H_23−xN that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (E_F).⁵ These electronic properties are present when N substitutes H in the octahedral interstices of Fm3m LuH₃. These structures also exhibit an interconnected metallic hydrogen network, a common feature of high-<i>T</i>_chydride superconductors. Electronic property systematics gives an estimate of <i>T</i>_c for one structure that is well above the critical temperatures predicted for structures considered previously. DFT+U has an especially strong effect on one of the structures considered, enhancing the vHs and flat bands near E_F. These results provide a basis for understanding the electronic properties observed for nitrogen-doped lutetium hydride. Additional work is needed to fully characterize the material, optimize its synthesis, stabilize it at ambient pressure, and accurately determine the range of critical temperatures possible.<br/><br/>1. M. Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019).<br/>2. N. Dasenbrock-Gammon et al., Nature 615, 244-250 (2023).<br/>3. N. P. Salke et al., arXiv:2306.06301.<br/>4. J. Shinar et al., Phys. Rev. Lett. 64, 563-566 (1990).<br/>5. A. Denchfield et al. arXiv:2305.18196.