Tunable exciton valley-pseudospin orders in moir\'e Bose-Hubbard model

TL;DR

This paper demonstrates a tunable exciton-based Bose-Hubbard model in a moiré superlattice, revealing unique magnetic orders and phase transitions driven by exciton filling and magnetic fields, with implications for quantum materials.

Contribution

It introduces a novel realization of the spin-1/2 Bose-Hubbard model using excitons in a moiré superlattice and explores their magnetic phase behavior.

Findings

Evidence of in-plane ferromagnetic order of exciton valley pseudospin.

Transition to out-of-plane ferromagnetic order with increased exciton filling and magnetic field.

The phase diagram differs from fermionic systems and aligns with a simple phenomenological model.

Abstract

Spin and charge are the two most important degrees of freedom of electrons. Their interplay lies at the heart of numerous strongly correlated phenomena including Hubbard model physics and high temperature superconductivity. Such interplay for bosons, on the other hand, is largely unexplored in condensed matter systems. Here we demonstrate a unique realization of the spin-1/2 Bose-Hubbard model through excitons in a semiconducting moir\'e superlattice. We find evidence of a transient in-plane ferromagnetic (FM-$xy$) order of exciton spin - here valley pseudospin - around exciton filling $\nu_{ex}$ = 1, which transitions into a FM-$z$ order both with increasing exciton filling and a small magnetic field of 10 mT. The phase diagram is different from the fermion case and is qualitatively captured by a simple phenomenological model, highlighting the unique consequence of Bose-Einstein statistics. Our study paves the way for engineering exotic phases of matter from spinor bosons, as well as for unconventional devices in optics and quantum information science.