Electric-field-driven hole carriers and superconductivity in diamond

First-principles calculations of electric-field-driven superconductivity at the hydrogenated diamond (110) surface are presented. While the hydrogens on the surface effectively maintain the intrinsic $s{p}^3$ covalent nature of diamond, the hole carriers induced by an external negative electric field $(E$-field) lead to a metallic surface region. Importantly, the concentration of hole carriers, confined within a few carbon layers of thickness $\sim$5–10 Å below the surface, exceeds 10${}^{21}$ cm${}^{-3}$, which is larger than the critical hole density responsible for superconductivity in the boron-doped diamond, while the calculated electron-phonon coupling constants are comparable in magnitude, suggesting the possibility of superconductivity with enhanced critical field.

Figure 1

Atomic structure of the hydrogenated diamond (110) surface, (a) side view and (b) top view. The single slab consists of 13 carbon layers (gray circles) and is terminated by hydrogens (black small circles) on both sides of the slab. (c) Two-dimensional Brillouin zone of the present calculations, corresponding to the unit cell represented by the dotted rectangle in (b).

Figure 2

(a) Calculated band structure of the hydrogenated diamond (110) surface in zero $E$-field. (b) Partial density of states (PDOS) in the carbon muffin-tin spheres at the surface and third layers, [C(1) and C(3)], and the terminating hydrogen (H). Band structures for $E$-fields of (c) $-$0.5 V/Å and (d) $-$1.0 V/Å. In (c) and (d), the reference zero energy is set to the Fermi energy ($E_{\mathrm{F}}$) located below the top of the valence band at $\Gamma$. In the insets, Fermi surfaces with a hole pocket, centered at $\Gamma$, are shown in a rectangular zone bounded by J/2 and J’/2.

Figure 3

(a) Calculated planar-averaged induced hole density. (b) Partial density of states at $E_F$ in the muffin-tin spheres. (c) Electrostatic potential along the $z$ axis in $E$-fields of $-$0.5 (broken line) and $-$1.0 V/Å (solid line). In (c), arrows indicate the positions of hydrogen (H) and the first carbon layer [C(1)].