Observation of topological order in a superconducting doped topological insulator

Experimental observation of topological order in three-dimensional bulk solids has recently led to a flurry of research activity ^1-21 . Unlike the two-dimensional electron gas or quantum Hall systems, three-dimensional topological insulators can harbour superconductivity and magnetism, making it possible to study the interplay between topologically ordered phases and broken-symmetry states. One outcome of this interplay is the possible realization of Majorana fermions-quasiparticles that are their own antiparticles-on topological surfaces, which is of great interest in fundamental physics ^9-13,22-24 . Here we present measurements of the bulk and surface electron dynamics in Bi ₂ Se ₃ doped with copper with a transition temperature T _c up to 3.8 K, observing its topological character for the first time. Our data show that superconductivity occurs in a bulk relativistic quasiparticle regime where an unusual doping mechanism causes the spin-polarized topological surface states to remain well preserved at the Fermi level of the superconductor where Cooper pairing takes place. These results suggest that the electron dynamics in superconducting Bi ₂ Se ₃ are suitable for trapping non-Abelian Majorana fermions. Details of our observations constitute important clues for developing a general theory of topological superconductivity in doped topological insulators.