Chiral Mott insulator with staggered loop currents in the fully frustrated Bose-Hubbard model

Abstract

Motivated by experiments on Josephson junction arrays in a magnetic field and ultracold interacting atoms in an optical lattice in the presence of synthetic orbital magnetic fields, we study the "fully frustrated" Bose-Hubbard model and quantum XY model with half a flux quantum per lattice plaquette. Using Monte Carlo simulations and the density matrix renormalization group (DMRG) method, we show that the phase diagram of such kinetically frustrated bosons displays three phases on a two-leg ladder: a weakly interacting chiral superfluid phase with staggered loop currents which spontaneously break time-reversal symmetry, a conventional Mott insulator at strong coupling, and a remarkable "Chiral Mott Insulator" (CMI) with staggered loop currents sandwiched between them at intermediate correlation. We discuss how the CMI state may be viewed as an exciton condensate or a vortex supersolid, propose and study a Jastrow variational wavefunction which captures its correlations, present DMRG results for the boson momentum distribution across the phase diagram, and consider various experimental implications of our phase diagram. Finally, we discuss generalizations to a staggered flux Bose-Hubbard model and a two-dimensional version of the CMI with edge currents in weakly coupled ladders.