By Gufei Zhang, D-IAS Assistant Professor at NanoSYD, Mads Clausen Institute, SDU.
Superconductivity, or infinite conductivity, appears as a result of formation of quantum condensate of Cooper pairs making possible a dissipationless flow of charged bosons. At first sight, superconductivity should be provided by well established conducting state, but in reality the best conductors (Ag, Au) do not superconductor at all, while, instead, superconductivity gains a lot from being situated in the vicinity of the insulator-metal transition induced by doping, e.g., doped copper oxides and iron pnictides . Another interesting puzzle, related to the origin of superconductivity, is whether the insulating state results from the direct localization of Cooper pairs or a two-step process where the destruction of Cooper pairs is followed by the localization of single quasiparticles . Lab-grown boron-doped diamond, increasingly being recognized as a piece of jewelry for science and technology, provides a powerful platform for answering these questions [3-9]. To demonstrate the quantum confinement and coherence of charge carriers in nanogranular diamond, I will brief on our electrical transport and direct local measurements on this material. The talk will be focused on a series of intriguing bosonic anomalies such as the bosonic insulating state and anisotropic superconductivity [4-6,8,9]. These anomalies are interpreted in the framework of the quantum confinement and coherence of single quasiparticles and Cooper pairs in the presence of granular disorder. Our data unveil the percolative nature of the electrical transport in nanodiamond and reveal the essential role of grain boundaries in determining the electronic properties of this material.