Superconductivity in diamond

Abstract

Diamond is an electrical insulator well known for its exceptional hardness. It also conducts heat even more effectively than copper, and can withstand very high electric fields 1 . With these physical properties, diamond is attractive for electronic applications 2 , particularly when charge carriers are introduced (by chemical doping) into the system. Boron has one less electron than carbon and, because of its small atomic radius, boron is relatively easily incorporated into diamond 3 ; as boron acts as a charge acceptor, the resulting diamond is effectively hole-doped. Here we report the discovery of superconductivity in boron-doped diamond synthesized at high pressure (8-9 GPa) and temperature (2,500-2,800 K).Electrical resistivity, magnetic susceptibility, specific heat and field-dependent resistance measurements show that boron-doped diamond is a bulk, type-II superconductor below the superconducting transition temperature T c ≈4 K; superconductivity survives in a magnetic field up to H c2 (0)≥3.5 T. The discovery of superconductivity in diamond-structured carbon suggests that Si and Ge, which also form in the diamond structure, may similarly exhibit superconductivity under the appropriate conditions.With their potential for electronic applications as microchip substrates, high efficiency electron emitters, photodetectors and transistors, diamond and carrier-doped diamond have been studied extensively 3-6 . The extremely short covalent bonds of carbon atoms in diamond give diamond many of its desirable properties, but also constrain geometrically which dopants can be incorporated and their concentration. Because of its small atomic radius compared to other potential dopants, boron is readily incorporated into the dense (1.763×10 23 atoms cm −3 ) diamond lattice. Boron dopes holes into a shallow acceptor level close to the top of the valence band that is separated from the conduction band of diamond by E g ≈5.5 eV. Electrical transport studies of B-doped diamond, including high-pressure synthesized crystals and CVD (chemical vapour deposition) films, find that low boron concentrations n≈10 17 -10 19 cm −3 give a semiconducting conductivity with an activation energy of ~0.35 eV (refs 7-11). Increasing the concentration to 10 20 cm −3 gradually decreases the activation energy 9,10 , and for n≥10 20 cm −3 , the electrical conductivity acquires metallic-like behaviour near room temperature [8][9][10][11] that signals an insulator-metal transition near this concentration. A metallic-like conductivity has not been found, however, at low temperatures for any presently available B concentration, which has reached (2-3)×10 21 cm −3 (refs 8-11).We have studied B-doped diamond synthesized by reacting B 4 C and graphitic carbon at pressure, 8-9 GPa, and temperature, 2,500-2,800 K, for ~5 s. Under these conditions, polycrystalline diamond aggregates 1-2 mm in size formed at the interface between graphite and B 4 C. All the diamond aggregates had a metal-like lustre. Scanning electron microscopy (SEM) showed that the di...

Direct observation of a dispersionless impurity band in hydrogenated graphene

“…(1) can be well understood since hydrogen adsorption effectively removes π bonds (sp 3 C-H bond). Concerning (2), the feature appears between 284.3 eV and 285.3 eV, as indicated by the arrows in fig. 1(c).…”

Resonating Valence Bond Mechanism of Impurity Band Superconductivity in Diamond

“…Diamond has been reported [1] to become a superconductor upon high boron doping with a T c ≈ 4 K. This remarkable discovery of superconductivity in an uncompensated p-type semiconductor has possible implications for basic science and technology. As an example, shallow dopant states in Si and Ge play a key role in modern solid state electronics; from physics point of view they offer a testing ground for various ideas such as Anderson and Mott localization and their interplay.…”

Electric-field-driven hole carriers and superconductivity in diamond

“…This new class of superconductors with directional covalent bonding, most likely electron-phonon mediated, consists not only of doped diamond, but also other doped semiconductors 3 . In the 1960's, it was predicted that doping a semiconductor with many-valley features in the band structure would induce an effective interaction to overcome the Coulomb repulsion 4,5 ; this mechanism was confirmed in subsequent experiments on reduced SrTiO 3 6 and Ge 1−x Te 7 , albeit with rather low transition temperatures, T c , of at most 0.5 K. Nevertheless, the 2004 discovery [8][9][10] of superconductivity in boron-doped diamond has excited interest in this new class of superconductors, which now also extends to silicon 11 and SiC 12,13 . In doped diamond, the concentration of B dopants exceeded a critical value responsible for a metal-insulator transition, which eventually induced superconductivity around 7-9 K with carrier densities of ∼10 21 cm −38,14 .…”