Pb induces superconductivity in Bi2Se3 analyzed by point contact spectroscopy

Some topological insulators become superconducting when doped with Cu and Pd. Superconductivity in a non-superconductor may be induced by proximity effect: i.e. Contacting a non-superconductor with a superconductor. The superconducting macroscopic wave function will induce electronic pairing into the normal compound. In the simplest topological insulator, Bi$_2$Se$_3$, superconductivity may be induced with Pb. We studied with point contact junctions formed by contacting Bi$_2$Se$_3$ crystals and Pb, glued and pressed with silver paste and/or applying low heat to the Pb to improve the contact. Junctions were formed with a thin tungsten plated gold W(Au) wire as one electrode, and the other $Bi_2Se_3$ and Pb. We study the characteristics of the electron coupling; the transition temperature,$T_C$, evolution with temperature of the energy gap, $\Delta$, and 2$\Delta$/K$_B$T$_C$ ratio. The superconductor Bi$_2$Se$_3$-Pb behaves different as explained in the classical BCS theory. In BCS a superconductor is only weak or strong coupled, depending on the electronic interaction that form the electronic condensate. This differentiation is given by the size of the mentioned ratio. BCS Typical values are 3.53 to about 4.3 for weak or strong coupling limits respectively. In this study performed in Bi$_2$Se$_3$-Pb we found different values to the normal ones from 10 to 23, indicating very strong limit. Those values never have been observed in other superconductor. The transition temperatures found varies from 2.7 to 7 K. This information and other results will be presented in this paper.