Fig. 1: Junctions’ structure.

a A tunnel junction whose barrier is ferroelectric can be reversibly switched between a high and low-conductance states by the application a voltage pulse of amplitude V_pol, which sets the remnant ferroelectric polarisation direction towards one electrode or the opposite. b Scheme of a superconductor/ferroelectric/superconductor tunnel junction based on a YBa₂Cu₃O_7-δ/BiFeO₃ heterostructures on which a micrometric Mo₈₀Si₂₀ contact is made through an aperture across an insulating photo-resist overlayer. c (Top) EELS linescan acquired while scanning the electron beam across the stacking, moving a distance d from the YBCO into the MoSi layer along the direction of the cyan arrow. The signal intensity is indicated by the colour scale (in arbitrary units). The edges of interest include the O K edge near 528 eV, Fe L_2,3 with onset near 709 eV or the Ba M_4,5, near 781 eV. Principal Component Analysis was used to remove random noise. (Bottom, left) Atomic resolution HAADF image of the YBCO/ BFO/ MoSi stack. Then horizontal dashed lines highlight the approximate locations of the BFO top and bottom interfaces (Bottom, right) Normalised integrated intensities for the O, Fe and Ba signals in blue, red and black, respectively. The vertical dashed lines highlight the approximate locations of the BFO top and bottom interfaces. In all panels a cyan arrow shows the direction of the linescan.