Strongly suppressed proximity effect and ferromagnetism in topological insulator/ferromagnet/superconductor thin film trilayers of Bi2Se3/SrRuO3/underdoped YBa2Cu3Ox: a possible new platform for Majorana nano-electronics

We report properties of a topological insulator-ferromagnet-superconductor trilayers comprised of thin films of 20 nm thick Bi 2 Se 3 on 10 nm SrRuO3 on 30 nm YBa 2 Cu 3 O x . As deposited trilayers are underdoped and have a superconductive transition with T c onset at 75 K, zero resistance at 65 K, T Cueri at 150 K and T * of about 200 K. Further reannealing under vacuum yields the 60 K phase of YBa 2 Cu 3 O x which still has zero resistance below about 40 K. Only when 10 × 100 microbridges 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 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 YBa 2 Cu 3 O x grains via the topological and ferromagnetic cap layers. Comparing our results to those of a reference trilayer (RTL) with the topological Bi 2 Se 3 layer substituted by a non-superconducting highly overdoped 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 RTL. This strong suppression could originate in lattice and Fermi levels mismatch as well as in short coherence length and unfavorable effects of strong spin-orbit coupling in Bi 2 Se 3 on the d-wave pairing of YBa 2 Cu 3 O x . Proximity induced edge currents in the SRO/YBCO layer could lead to Majorana bound states, a possible signature of which is observed in the present study as zero bias conductance peaks.