One promising avenue to study one-dimensional (1D) topological phases is to realize them in synthetic materials such as cold atomic gases. Intriguingly, it is possible to realize Majorana boundary modes in a 1D number-conserving system consisting of two fermionic chains coupled only by pair-hopping processes [C. V. Kraus et al., Phys. Rev. Lett. 111, 173004 (2013)10.1103/PhysRevLett.111.173004]. It is commonly believed that significant interchain single-particle tunneling necessarily destroys these Majorana modes, as it spoils the Z2 fermion-parity symmetry that protects them. In this Rapid Communication, we present a mechanism to overcome this obstacle, by piercing a (synthetic) magnetic π flux through each plaquette of the Fermi ladder. Using bosonization, we show that in this case there exists an exact leg-interchange symmetry that is robust to interchain hopping, and acts as fermion parity at long wavelengths. We utilize density matrix renormalization group and exact diagonalization to verify that the resulting model exhibits Majorana boundary modes up to large single-particle tunnelings, comparable to the intrachain hopping strength. Our work highlights the unusual impacts of different topologically trivial band structures on these interaction-driven topological phases, and identifies a distinct route to stabilizing Majorana boundary modes in 1D fermionic ladders.
Bibliographical noteFunding Information:
C.C. thanks M. D. Schulz for discussions. Part of the simulation is developed from the alps package  . C.C. and F.J.B. are supported by NSF-DMR 1352271 and by the Sloan Foundation FG-2015-65927. W.Y. and Y.C. are supported by the National Natural Science Foundation of China (Grants No. 11625416, No. 11474064, and No. 11274069). C.S.T. is supported by the Robert A. Welch Foundation under Grant No. E-1146.