Mean-Field Analysis of Spinor Bosons in Optical Superlattices

TL;DR

This paper analyzes the phase diagram of spinor bosons in optical superlattices using a mean-field approach, revealing complex phase transitions, the impact of interactions, and tunneling resonances.

Contribution

It introduces a mean-field method that exactly treats intra-cell dynamics to study phase transitions in spinor bosons in superlattices, including effects of spin interactions and Zeeman fields.

Findings

Mott-insulating and superfluid phases identified.

Transitions are second-order for spinless, first- or second-order for spin-1 bosons.

Energy offsets cause tunneling resonances that persist into the superfluid phase.

Abstract

We study the ground-state phase diagram of spinless and spin-1 bosons in optical superlattices using a Bose-Hubbard Hamiltonian that includes spin-dependent interactions. We decouple the unit cells of the superlattice via a mean-field approach and take into account the dynamics within the unit cell exactly. The system supports Mott-insulating as well as superfluid phases. The transitions between these phases are second-order for spinless bosons and second- or first-order for spin-1 bosons. Anti-ferromagnetic interactions energetically penalize high-spin configurations and elongate all Mott lobes, especially the ones corresponding to an even atom number on each lattice site. We find that the quadratic Zeeman effect lifts the degeneracy between different polar superfluid phases leading to additional metastable phases and first-order phase transitions. Finally, we show that an energy offset between the two sites of the unit cell induces a staircase of single-atom tunneling resonances which surprisingly survives well into the superfluid regime.