Unveiling Electronic Correlation and the Ferromagnetic Superexchange Mechanism in the van der Waals Crystal CrSiTe 3

The recent discovery of intrinsic ferromagnetic order in the atomically thin van der Waals crystal $\mathrm{Cr}X{\mathrm{Te}}_{3}$ ($X=\mathrm{Si}$, Ge) stimulates intensive studies on the nature of low-dimensional magnetism because the presence of long-range magnetic order in two-dimensional systems with continuous symmetry is strictly prohibited by thermal fluctuations. By combining advanced many-body calculations with angle-resolved photoemission spectroscopy we investigate ${\mathrm{CrSiTe}}_{3}$ single crystals and unveil the pivotal role played by the strong electronic correlations at both high- and low-temperature regimes. Above the Curie temperature (${T}_{c}$), Coulomb repulsion ($U$) drives the system into a charge transfer insulating phase. In contrast, below ${T}_{c}$ the crystal field arranges the $\mathrm{Cr}\text{\ensuremath{-}}3d$ orbitals such that the ferromagnetic superexchange profits, giving rise to the bulk ferromagnetic ground state with which the electronic correlations compete. The excellent agreement between theory and experiment establishes ${\mathrm{CrSiTe}}_{3}$ as a prototype low-dimensional crystal with the cooperation and interplay of electronic correlation and ferromagnetism.