A recent experiment of polycrystalline ${\mathrm{WB}}_2$ with hP3 (space-group 191, prototype ${\mathrm{MgB}}_2$) and hP12 (space-group 194, prototype ${\mathrm{WB}}_2$) structures was reported to realize 17-K superconductivity (SC) at 90 GPa, and the hP3 structure is believed to be responsible for this emergent SC. However, a microscopic understanding of what makes the hP3 structure so different from the hP12 structure and why the hP3 can feature such strong electron-phonon coupling (EPC) SC is still missing. Here, based on first-principles calculations, we found that in the hP3 structure, W $d$ orbitals contribute most to electronic occupation near the $E_{\mathrm{F}}$, and $d_{z^2}$ orbitals of two neighboring W atoms have some hybridization to form weak $\sigma$ bonds. The further EPC analysis indicates that the dominant $d_{z^2}$ states are strongly coupled with the out-of-plane phonon modes by stretching the $\mathrm{W}\text{−}\mathrm{W}\sigma$ bond, thereby yielding a large superconducting gap and high $T_{\mathrm{c}}$ of $\sim 35$ K. By contrast, for the hP12 structure, two neighboring W atoms are isolated without charge hybridization to form the covalent bonds, and, accordingly, their phonon modes become very stiffened, which cannot effectively couple to W $d$ orbital states associated with a lower $T_{\mathrm{c}}$ of $\sim 4$ K. Therefore, our findings not only provide an explanation for the emergent strong EPC SC in the hP3 structure, but also have important implications for the design of high-$T_{\mathrm{c}}$ superconductors among transition metal borides.

Close

PrevNext

View figure in article

• Electron-phonon coupling
• Electronic structure
• Superconductivity
• First-principles calculations