Recent proposals of inducing p-wave superconductivity by sandwiching a conventional s- wave superconductor (S) with a topological insulator (TI) triggered a burst of research activity. Topological superconductors harness the inherent electron – hole symmetry of excitations in a superconductor with the helical nature of the electronic states in topological materials, that may lead to Majorana zero energy states. Various theoretical models focused on possible pairing symmetries of the proximity induced superconducting order in topological layers. Several experimental groups already realized S-TI interfaces and S-TI-S junctions, and studied the Josephson current across them. Few papers have reported unusual Shapiro steps consistent with the formation of the p-wave correlations. However, measurements that could provide the definitive evidence of the p-wave superconductivity should be phase-sensitive, to discriminate between the p- and other (s-, d-...) pairings symmetries.
In this work we report a new type of oscillations of the critical Josephson current in magnetic field observed in the Nb-Bi2Te2.3Se0.7-Nb junctions [1,2]. The ultra-short period ∼ 1 Oe of these oscillations and their sharply peaked shape reflect the resonant transmission via Andreev bound states with the ultra-fine ∼ 1 μeV interlevel spacing. We argue that the ultra- fine oscillations revealed in our S-TI-S devices is the direct consequence of the p-wave superconducting order induced at S-TI contacts.
 V.S. Stolyarov, D. Roditchev, V.L Gurtovoy, S.N. Kozlov, D.S. Yakovlev, O.V. Skryabina, V.M. Vinokur, A.A. Golubov, submitted to Nature Physics.