Room-temperature superconductivity in boron- and nitrogen-doped lanthanum superhydride

Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride ${\mathrm{LaH}}_{10}$ has been of particular focus as the first material to exhibit a superconducting critical temperature ($T_{\mathrm{c}}$) near room temperature. Experiments indicate that the use of ammonia borane as the hydrogen source can increase the conductivity onset temperatures of lanthanum superhydride to as high as 290 K. Here we examine the doping effects of B and N atoms on the superconductivity of ${\mathrm{LaH}}_{10}$ in its fcc ($Fm\overline{3}m$) clathrate structure at megabar pressures. Doping at H atomic positions strengthens the ${\mathrm{H}}_{32}$ cages of the structure to give higher phonon frequencies that enhance the Debye frequency and thus the calculated $T_{\mathrm{c}}$. The predicted $T_{\mathrm{c}}$ can reach 288 K in ${\mathrm{LaH}}_{9.985}{\mathrm{N}}_{0.015}$ within the average high-symmetry structure at 240 GPa. The remarkably large shift in $T_{\mathrm{c}}$ to higher values produced by small degrees of chemical doping opens the prospect for the creation of still higher-temperature superconductivity in superhydrides potentially at even lower-pressure conditions.

Figure 1

(a) Optimized structure of ${\mathrm{LaH}}_{10}$ with its fcc La sublattice and ${\mathrm{H}}_{32}$ clathrate structure. (b) Electronic band structure and DOS (${\mathrm{Hartree}}^{-1}/\mathrm{spin}$) of ${\mathrm{LaH}}_{10}$ at 210 GPa. (c) Phonon dispersion and Eliashberg function ${\alpha }^2\mathrm{F}\left(\omega \right)$ of ${\mathrm{LaH}}_{10}$ at 210 GPa. The pink, blue, and green circles in (b) mark the projections of H-$s$, La-$d$, and La-$f$ orbitals, respectively. The size of the black circles in (c) is proportional to the electron-phonon coupling linewidths. The results are consistent with the original predictions [3].

Figure 2

Phonon dispersion (red lines) and Eliashberg function ${\alpha }^2\mathrm{F}\left(\omega \right)$ (red lines) of (a) ${\mathrm{La}}_{0.99}{\mathrm{H}}_{10}{\mathrm{B}}_{0.01}$, (b) ${\mathrm{LaH}}_{9.99}{\mathrm{B}}_{0.01}$ and (c) ${\mathrm{LaH}}_{9.99}{\mathrm{N}}_{0.01}$ at 210 GPa. The size of red circles is proportional to the electron-phonon coupling linewidths. In order to facilitate the comparison, the black lines in (a), (b) show the phonon disperson and ${\alpha }^2\mathrm{F}\left(\omega \right)$ of ${\mathrm{LaH}}_{10}$ at 210 GPa. The green lines in (a) show the phonon dispersion of ${\mathrm{La}}_{0.98}{\mathrm{H}}_{10}{\mathrm{B}}_{0.02}$ at 210 GPa with the imaginary modes along Γ-X. The blue lines in (c) show the phonon dispersion and ${\alpha }^2\mathrm{F}\left(\omega \right)$ of ${\mathrm{LaH}}_{9.99}{\mathrm{N}}_{0.01}$ at 240 GPa.

Figure 3

(a) DOS, $\lambda ,{\langle \omega \rangle }_{\log }$, and $T_{\mathrm{c}}$ of ${\mathrm{LaH}}_{10-x}{\mathrm{B}}_x$ and ${\mathrm{LaH}}_{10-x}{\mathrm{N}}_x$ with different doping levels at 210 GPa. (b) DOS, $\lambda ,{\langle \omega \rangle }_{\log }$, and $T_{\mathrm{c}}$ of ${\mathrm{LaH}}_{9.99}{\mathrm{B}}_{0.01}$ and ${\mathrm{LaH}}_{9.99}{\mathrm{N}}_{0.01}$ at different pressures. For each $T_{\mathrm{c}}$ curve, the upper and lower bounds are obtained by choosing μ* = 0.10 and 0.11, respectively.

Figure 4

Dependence of $T_{\mathrm{c}}$ on doping level and pressure for ${\mathrm{LaH}}_{10-x}{\mathrm{N}}_x$ with μ* = 0.10. The black dots represent the calculated data grid.