Bosons with long range interactions on two-leg ladders in artificial magnetic fields

Abstract

Motivated by experiments exploring the physics of neutral atoms in artificial magnetic fields, we study the ground state of bosons interacting with long range dipolar interactions on a two-leg ladder. Using two complimentary variational approaches, valid for weak interactions, we find rich physics driven by the long range forces. Generically, long range interactions tend to destroy the Meissner phase in favor of modulated density wave phases. Nearest neighbor interactions produce a novel interleg charge density wave phase, where the total density remains uniform, but the density on each leg of the ladder is modulating in space, out-of-phase with one another. At weak magnetic fields, next nearest neighbor interactions lead to a fully modulated biased ladder phase, where all the particles are on one leg of the ladder, and the density is modulating in space. This state simultaneously breaks $Z_{2}$ reflection symmetry and $U(1)$ symmetry associated with translation in real space. For values of the flux near $ϕ= π$, we find that a switching effect occurs for arbitrarily weak interactions, where the density modulates in space, but the chiral current changes sign on every plaquette. Arbitrarily weak attractive interactions along the rungs destroy the Meissner phase completely, in favor of a modulated density wave phase. Varying magnetic field produces a cascade of first order transitions between modulated density wave states with different wave-vectors, which manifests itself as discrete jumps in the chiral current. Polarizing the dipoles along the ladder direction yields a region of phase space where a stable biased ladder phase occurs even at arbitrarily weak magnetic fields. We discuss the experimental consequences of our work, in particular, how the interleg charge density wave can manifest itself in recent experiments on bosons in synthetic dimensions.