Majorana fermions in topological-insulator nanowires: From single superconducting nanowires to Josephson junctions

Signatures of Majorana fermion bound states in one-dimensional topological insulator (TI) nanowires with proximity effect induced superconductivity are studied. The phase diagram and energy spectra are calculated for single TI nanowires and it is shown that the nanowires can be in the topological invariant phases of winding numbers $W=0, \pm 1$, and $\pm 2$ corresponding to the cases with zero, one and two pairs of Majorana fermions in the single TI nanowires. It is also shown that the topological winding numbers, i.e., the numbers of pairs of Majorana fermions in the TI nanowires can be extracted from the transport measurements of a Josephson junction device made from two TI nanowires, while the sign in the winding numbers can be extracted using a superconducting quantum interference device (SQUID) setup.