$d$-wave superconductivity as a model for diborides apart MgB$_2$

E. F. Talantsev

doi:10.3390/sym15040812

paper

$d$ -wave superconductivity as a model for diborides apart MgB $_{2}$

arXiv:2301.03357 · doi:10.3390/sym15040812

Abstract

Recently, Pei et al. (arXiv2105.13250) reported that ambient pressure $β$ -MoB $_{2}$ exhibits a phase transition to $α$ -MoB $_{2}$ (space group: $P 6/ mmm$ ) at pressure P~70 GPa and this high-pressure phase is a high-temperature superconductor exhibited $T_{c} = 32 K$ at P~110 GPa. Despite $α$ -MoB $_{2}$ has the same crystalline structure as ambient pressure MgB $_{2}$ 2 and the $T_{c}$ 's of $α$ -MoB $_{2}$ and MgB $_{2}$ are very close, the first principles calculations showed that in $α$ -MoB $_{2}$ the states near the Fermi level, $ε_{F}$ , are dominated by the $d$ -electrons of Mo atoms, while in MgB $_{2}$ the $p$ -orbitals of boron atomic sheets dominantly contribute to the states near the $ε_{F}$ . More recently, Hire et al. (arXiv2212.14869) reported that the $P 6/ mmm$ -phase can be stabilized at ambient pressure in $N b_{1 - x} M o_{x} B_{2}$ solid solutions, and these ternary alloys exhibit $T_{c} = 8 K$ . In addition, Pei et al. (Sci. China-Phys. Mech. Astron. 65, 287412 (2022)) showed that compressed WB $_{2}$ exhibits $T_{c} = 15 K$ at P~121 GPa. Here, we analyzed experimental data reported for $P 6/ mmm$ -phases of $N b_{1 - x} M o_{x} B_{2}$ (x = 0.25; 1.0) and highly-compressed WB $_{2}$ , and showed that these three phases exhibit $d$ -wave superconductivity. We also deduced the gap-to-transition temperature ratio for these three phases. We found that $N b_{0.75} M o_{0.25} B_{2}$ exhibits high strength of nonadiabaticity, which is quantified by the ratio of $T_{θ} / T_{F} = 3.5$ , which is by one order of magnitude exceeds the ratio in MgB $_{2}$ , $α$ -MoB $_{2}$ , WB $_{2}$ , pnictides, cuprates, and highly-compressed hydrides.

23 pages, 9 figures

d-wave superconductivity as a model for diborides apart MgB2​

Abstract

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$d$ -wave superconductivity as a model for diborides apart MgB $_{2}$