Fig. 5: Josephson phase measurements.

a False-color SEM image of SQUID #04 (scale bar: 2 μm), consisting of an SFS JJ with a Cr₂Ge₂Te₆ barrier and an intrinsic reference JJ formed by two NbSe₂. The area of the SQUID loop (17.5 μm²) is the sum of the empty parts (~5.9 μm²) and the screening areas (~11.6 μm²). b Differential resistance dV/dI (log-scale) map of SQUID #04 as a function of bias current I and out-of-plane magnetic field measured at 1.85 K, showing a SQUID oscillation. All the dV/dI curves are sweeping from zero to positive bias. The position of zero magnetic field calibrated by a NbSe₂ SQUID on the same substrate chip is indicated by the purple line in this plot. c Critical current I_c oscillations of SQUID #04 (black rectangles) with the period of 0.118 mT, consistent with the SQUID loop’s area. The phase φ of it is about 148.6°, obtained by a fit to I_c with the sine relation (black lines). The red circles show the I_c oscillations of the NbSe₂ SQUID for calibration with the period of 0.209 mT and the red lines are the fitted results. d Temperature dependence of I_c of SQUID #04 at zero magnetic field. The thickness of the Cr₂Ge₂Te barrier is ~3.5 nm measured by AFM. The nonmonotonic change of I_c implies an incomplete 0-π transition in SQUID #04 with a φ ground-state phase at low temperature.