Measurement of the intrinsic higher harmonic current-phase relation in NbN/NiCu/NbN Josephson junctions

We report on the measurement of the intrinsic higher harmonic current-phase relation for epitaxial NbN/NiCu/NbN (SFS) Josephson junctions. We investigated the $\ensuremath{\pi}\text{\ensuremath{-}}0$ phase transition as a function of temperature for SFS Josephson junctions with a 3.2-nm-thick NiCu layer, and fitted the phase transition temperatures ${T}_{\ensuremath{\pi}\text{\ensuremath{-}}0}$ with a theoretical expression derived from the Usadel equations. We found that junctions on the same chip and with the same dimension showed different phase transition temperatures depending on the values of the fitting parameter $\ensuremath{\alpha}$. The fitting parameter $\ensuremath{\alpha}$ indicates the magnetic scattering intensity, which mainly derives from spin-flip or spin-flop scattering in the NiCu layer. At the $\ensuremath{\pi}\text{\ensuremath{-}}0$ crossover, the intrinsic positive second harmonic current-phase relation was confirmed in a junction with moderate magnetic scattering by the nonvanishing critical current, half-integer Shapiro steps, and half-periodic Fraunhofer modulation.