Superconductivity in doped cubic silicon

Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T(c) approximately 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.     

Coexistence of superconductivity and ferromagnetism in URhGe

The discovery of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition-its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.