Magnetic Field Enhancement of Cooper Pair Cotunneling in InSb Nanowire Josephson Quantum Dot Devices

Magnetic fields will suppress superconductivity via the spin paramagnetic effect and the orbital effect. However, the observations of magnetic-field-enhanced superconductivity in superconductor nanowires and thin films were reported by several groups. Although the origin of the field-enhanced superconductivity still remains ambiguous, it is predicted that the interaction between the Cooper pairs and the magnet impurities in the superconductors plays a crucial role. Here, we have realized superconductor-sandwiched InSb nanowire quantum dot devices, in which the dot manifests itself as a magnetic impurity when it is occupied by an odd number of quasiparticles. The spin -1/2 quantum dot interacts with the superconductor leads via the spin-exchange effect, i.e., the Kondo correlation. We have observed an magnetic field enhanced Cooper pair cotunneling in each of the Kondo valleys that are associated with proper Kondo temperatures, which cannot be explained by magnetic field induced quantum phase transition. The corresbonding dot-lead coupling, bias voltage and temperature dependence measurements are systematically performed. The Josephson quantum dot supplies us a platform for the investigation of the subtle relation between Cooper pair and magnetic impurities in superconductors.