1204.3637 (Sumanta Tewari et al.)

The return of the minigap: The delicate issue of a topological gap in
semiconductor Majorana wires    [PDF]

Sumanta Tewari, T. D. Stanescu, J. D. Sau, S. Das Sarma

The excitation gap above the Majorana fermion (MF) modes at the ends of 1D topological superconducting (TS) nanowires scales with the bulk quasiparticle gap E_{qp}. This gap, also called minigap, facilitates experimental detection of the pristine TS state and MFs at experimentally accessible temperatures T << E_{qp}. Here we show that the linear scaling of minigap with E_{qp} can fail in quasi-1D wires with multiple confinement bands. TS states in such wires have an approximate chiral symmetry supporting multiple near zero energy modes at each end leading to a minigap which can effectively vanish. We show that the problem of small minigap in such wires can be resolved by forcing the system to break the approximate chiral symmetry externally with a second Zeeman field. Although experimental signatures such as zero bias peak from the wire ends is suppressed by the second Zeeman field above a critical value, such a field is required in some important parameter regimes of quasi-1D wires to isolate the topological physics of end state MFs.

View original: http://arxiv.org/abs/1204.3637