Wohlleben Effect and Emergent π junctions in superconducting Boron doped Diamond thin films

Diamond is an excellent wide-bandgap electrical insulator. However, boron (B) doping is known to induce superconductivity in diamonds. We have performed electrical transport and magnetic measurements under pressure with doping concentrations of (1.4 and 2.6) × 1021 cm−3, in a temperature range 2 - 10 K and present two interesting effects in superconducting B doped diamond (BDD) thin films: (i) Wohlleben effect (paramagnetic Meissner effect, PME) and (ii) pressure-induced spin glass-like susceptibility anomaly. PME, a low field anomaly in inhomogeneous superconductors, could arise from flux trapping, flux compression, or for non-trivial reason such as emergent Josephson π junctions. The joint occurrence of PME and spin glass type anomalies points to the possible emergence of π junctions. BDD is a disordered s-wave superconductor; and π junctions could be produced by spin-flip scattering of spin ½ moments when present at weak superconducting regions. A frustrated network of 0 and π junctions will result in a distribution of spontaneous equilibrium supercurrents, a spin glass (phase glass) state. Anderson localized spin ½ spinons embedded in a metallic fluid (two-fluid model) could create π junction by spin-flip scattering. Our findings are consistent with the presence of π junctions, invoked to explain the observation of certain resistance anomaly in BDD.