Strongly suppressed superconducting proximity effect and ferromagnetism in trilayers of $\rm Bi_2Se_3$ / $\rm SrRuO_3$ / underdoped $\rm YBa_2Cu_3O_x$: A possible new platform for Majorana nano-electronics.

We report properties of topological insulator - ferromagnet - superconductor trilayers comprised of thin films of 20 nm thick $\rm Bi_2Se_3$ on 10 nm $\rm SrRuO_3$ on 30 nm $\rm YBa_2Cu_3O_x$. As deposited trilayers are underdoped and have a superconductive transition with $\rm T_c$ onset at 75 K, zero resistance at 65 K, $\rm T_{Cueri}$ at 150 K and $\rm T^*$ of about 200 K. Further reannealing under vacuum yields the 60 K phase of $\rm YBa_2Cu_3O_x$ which still has zero resistance below about 40 K. Only when $10\times 100$ micro-bridges were patterned in the trilayer, some of the bridges showed resistive behavior all the way down to low temperatures. Magnetoresistance versus temperature of the superconductive ones showed the typical peak due to flux flow against pinning below $\rm T_c$, while the resistive ones showed only the broad leading edge of such a peak. All this indicates clearly weak-link superconductivity in the resistive bridges between superconductive $\rm YBa_2Cu_3O_x$ grains via the topological and ferromagnetic cap layers. Comparing our results to those of a reference trilayer with the topological $\rm Bi_2Se_3$ layer substituted by a non-superconducting highly overdoped $\rm La_{1.65}Sr_{0.35}CuO_4$, indicates that the superconductive proximity effect as well as ferromagnetism in the topological trilayer are actually strongly suppressed compared to the non-topological reference trilayer. This strong suppression is likely to originate in strong proximity induced edge currents in the SRO/YBCO layer that can lead to Majorana bound states, a possible signature of which is observed in the present study as zero bias conductance peaks.