Superconductivity of the grain boundaries in boron-doped nanocrystalline diamond

The signature of the anisotropic superconducting order parameter (Delta) in heavily boron-doped nanocrystalline diamond (BNCD) films is demonstrated from the low-temperature resistivity and magnetoresistance measurements. Due to the presence of boron acceptors predominantly at the well-aligned grain boundaries, Rashba-type spin-orbit coupling can arise which influences the superconducting properties of these films. The one-dimensional (1D) filamentary channels of the grain boundaries suggest the modulation of the Delta which explains the peaks observed in the temperature-dependent resistance. This also explains the oscillatory magnetoresistance as a function of the magnetic fields and their angle dependence. From the observed superlattice-like microstructure of the BNCD films, a possible mechanism for creating Fulde-Ferrel and Larkin-Ovchinnikov (FFLO)-type state and chiral vortex lines from the superposition of multiple (Andreev) bound states is discussed. Overall, the interface states of the diamond films can be explained by the well-known Su-Schrieffer-Heeger "soliton" model which is supported by the observation of a zero-bias conductance peak.