Revisiting 2π phase slip suppression in topological Josephson junctions

Current state-of-the-art devices for detecting and manipulating Majorana fermions commonly consist of networks of Majorana wires and tunnel junctions. We study a key ingredient of these networks - a topological Josephson junction with charging energy - and we pinpoint crucial features for device implementation. The phase-dependent tunneling term contains both the usual 2π-periodic Josephson term and a 4π-periodic Majorana tunneling term representing the coupling between Majoranas on both sides of the junction. In nontopological junctions when the charging energy is small compared to the Josephson tunneling scale, the low-energy physics is described by 2π phase slips. By contrast, in a topological junction, due to the 4π periodicity of the tunneling term, it is usually expected that only 4π phase slips are possible while 2π phase slips are suppressed. However, we find that if the ratio between the strengths of the Majorana assisted tunneling and the Josephson tunneling is small, as is likely to be the case for many setups, 2π phase slips occur and may even dominate the low-energy physics. In this limit, one can view the 4π phase slips as a pair of 2π phase slips with arbitrarily large separation. We provide an effective description of the system in terms of 2π and 4π phase slips valid for all values of the tunneling ratio. Comparing the spectrum of the effective models with numerical simulations, we determine the crossover between the 4π phase slip regime to the 2π phase slip dominated regime. We also discuss the role of the charging energy as well as the implications of our results on the dissipative phase transitions expected in such a system.