Kinetic magnetism in triangular moiré materials

Abstract

Magnetic properties of materials ranging from conventional ferromagnetic metals to strongly correlated materials such as cuprates originate from Coulomb exchange interactions. The existence of alternate mechanisms for magnetism that could naturally facilitate electrical control has been discussed theoretically1–7, but an experimental demonstration8 in an extended system has been missing. Here we investigate MoSe2/WS2 van der Waals heterostructures in the vicinity of Mott insulator states of electrons forming a frustrated triangular lattice and observe direct evidence of magnetic correlations originating from a kinetic mechanism. By directly measuring electronic magnetization through the strength of the polarization-selective attractive polaron resonance9,10, we find that when the Mott state is electron-doped, the system exhibits ferromagnetic correlations in agreement with the Nagaoka mechanism.

Tunable exciton valley-pseudospin orders in moiré superlattices

“…Our pump–probe measurements establish WSe 2 /WS 2 moiré superlattice as a spin-½ Bose–Hubbard model, which has been predicted to host a ground state of FM- xy order at v ex = 1 as the virtual hopping of bosons gives rise to an FM in-plane super-exchange interaction 36 , 44 . Upon further doping, the kinetic energy of extra bosons would favor an FM- z order, similar to Nagaoka ferromagnetism in Fermi–Hubbard model 19 , 43 , 45 . We reveal the essential physics near v ex = 1 using a phenomenological spin-½ XXZ model on a triangular lattice, where the effect of adding excitons to the v ex = 1 correlated insulator is captured by a z -exchange interaction J z that increases with doping (see Methods: “Theoretical phase diagram” and Supplementary Note 1 ).…”

“…Our observations provide strong evidence of phase transitions from a (transient) xy order to FM- z order driven by both exciton filling and magnetic field. This can be naturally understood in a spin-½ Bose–Hubbard model from competitions between the super-exchange effect and Nagaoka-type kinetic ferromagnetism 19 , 43 . Our pump–probe measurements establish WSe 2 /WS 2 moiré superlattice as a spin-½ Bose–Hubbard model, which has been predicted to host a ground state of FM- xy order at v ex = 1 as the virtual hopping of bosons gives rise to an FM in-plane super-exchange interaction 36 , 44 .…”

“…Along with the strong light-matter interaction and unique optical selection rules 5 , 6 , correlated excitons in semiconducting moiré systems hold promises for novel applications in photonics and valleytronics 7 – 9 . However, while various magnetic orderings and correlated phases of electrons are reported, such as correlated insulator 10 – 14 , superconductivity 15 – 17 , intrinsic 18 , 19 or exciton-mediated ferromagnetism 20 , and fractional quantum anomalous Hall states 21 – 24 ; correlated phases of excitons remain unexplored until very recently 25 – 28 , and exciton “magnets” with “spin” orders have not been demonstrated.…”

Hybrid Moiré Excitons and Trions in Twisted MoTe2–MoSe2 Heterobilayers

“…14 This, in turn, provides all-optical means to probe the physics of correlated charges in moirélattices. 15,16 In this work, we demonstrate that MoTe 2 −MoSe 2 heterobilayers realize canonical moirésystems with nearresonant conduction bands. The large mismatch of ∼7% between the lattice constants of MoTe 2 and MoSe 2 layers renders the system robust against mesoscopic lattice reconstruction, 17 thus supporting moiréinterference phenomena down to collinear alignment in parallel (R-type) and antiparallel (H-type) limits.…”

“…In MoSe 2 –WS 2 heterobilayers, in particular, near-resonant conduction bands enhance the hybridization among interlayer and intralayer excitons and give rise to a multiplicity of moiré excitons with sizable oscillator strength . This, in turn, provides all-optical means to probe the physics of correlated charges in moiré lattices. , …”