D -Wave Superconducting Gap Symmetry as a Model for Nb 1−x Mo x B 2 (x = 0.25; 1.0) and WB 2 Diborides.

Recently, Pei et al. (National Science Review2023, nwad034, 10.1093/nsr/nwad034) reported that ambient pressure β -MoB2 (space group: R 3 ¯ m ) exhibits a phase transition to α -MoB2 (space group: P 6 / m m m ) at pressure P~70 GPa, which is a high-temperature superconductor exhibiting T c = 32   K at P~110 GPa. Although α -MoB2 has the same crystalline structure as ambient-pressure MgB2 and the superconducting critical temperatures of α -MoB2 and MgB2 are very close, the first-principles calculations show that in α -MoB2, the states near the Fermi level, ε F , are dominated by the d-electrons of Mo atoms, while in MgB2, the p-orbitals of boron atomic sheets dominantly contribute to the states near the ε F . Recently, Hire et al. (Phys. Rev. B2022, 106, 174515) reported that the P 6 / m m m -phase can be stabilized at ambient pressure in Nb1−xMoxB2 solid solutions, and that these ternary alloys exhibit T c ~ 8   K . Additionally, Pei et al. (Sci. China-Phys. Mech. Astron. 2022, 65, 287412) showed that compressed WB2 exhibited T c ~ 15   K at P~121 GPa. Here, we aimed to reveal primary differences/similarities in superconducting state in MgB2 and in its recently discovered diboride counterparts, Nb1−xMoxB2 and highly-compressed WB2. By analyzing experimental data reported for P6/mmm-phases of Nb1−xMoxB2 (x = 0.25; 1.0) and highly compressed WB2, we showed that these three phases exhibit d-wave superconductivity. We deduced 2 Δ m (0) k B T c = 4.1 ± 0.2 for α -MoB2, 2 Δ m (0) k B T c = 5.3 ± 0.1 for Nb0.75Mo0.25B2, and 2 Δ m (0) k B T c = 4.9 ± 0.2 for WB2. We also found that Nb0.75Mo0.25B2 exhibited high strength of nonadiabaticity, which was quantified by the ratio of T θ T F = 3.5 , whereas MgB2, α-MoB2, and WB2 exhibited T θ T F ~ 0.3 , which is similar to the T θ T F in pnictides, A15 alloys, Heusler alloys, Laves phase compounds, cuprates, and highly compressed hydrides. [ABSTRACT FROM AUTHOR]