Quantum phases of a one-dimensional Majorana-Bose-Hubbard model

TL;DR

This paper explores the phase diagram of a 1D Majorana-Bose-Hubbard model with phase fluctuations, revealing Mott-insulating, Luttinger liquid, and gapless phases through numerical DMRG analysis.

Contribution

It introduces a detailed numerical study of phase transitions in a 1D Majorana-Bose-Hubbard model considering phase fluctuations, mapping it to a generalized Bose-Hubbard model.

Findings

Identification of three distinct phases: Mott-insulating, Luttinger liquid, and a gapless phase.

Characterization of phase transitions as Kosterlitz-Thouless or Ising types.

Discovery of a second gapless phase with nonlocal string correlations.

Abstract

Majorana zero modes (MZM-s) occurring at the edges of a 1D, p-wave, spinless superconductor, in absence of fluctuations of the phase of the superconducting order parameter, are quintessential examples of topologically-protected zero-energy modes occurring at the edges of 1D symmetry-protected topological phases. In this work, we numerically investigate the fate of the topological phase in the presence of phase-fluctuations using the density matrix renormalization group (DMRG) technique. To that end, we consider a one-dimensional array of MZM-s on mesoscopic superconducting islands at zero temperature. Cooper-pair and MZM-assisted single-electron tunneling, together with finite charging energy of the mesoscopic islands, give rise to a rich phase-diagram of this model. We show that the system can be in either a Mott-insulating phase, a Luttinger liquid (LL) phase of Cooper-pairs or a second gapless phase. In contrast to the LL of Cooper-pairs, this second phase is characterized by nonlocal string correlation functions which decay algebraically due to gapless charge-$e$ excitations. The three phases are separated from each other by phase-transitions of either Kosterlitz-Thouless or Ising type. Using a Jordan-Wigner transformation, we map the system to a generalized Bose-Hubbard model with two types of hopping and use DMRG to analyze the different phases and the phase-transitions.