Transition metal d-electron oxides with an odd number of electrons per unit cell are expected to form metals with partially occupied energy bands, but exhibit in fact a range of behaviors, being either insulators, or metals, or having insulator-metal transitions. Traditional explanations involved predominantly electron-electron interactions in fixed structural symmetry. The present work focuses instead on the role of symmetry breaking local structural motifs. Viewing the previously observed V-V dimerization in VO₂ as a continuous knob, reveals in density functional calculations the splitting of an isolated flat band from the broad conduction band. This leads past a critical percent dimerization to the formation of the insulating phase while lowering the total energy. In VO₂ this transition is found to have a rather low energy barrier approaching the thermal energy at room temperature, suggesting energy-efficient switching in neuromorphic computing. Interestingly, sufficient V-V dimerization suppresses magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, forming a metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.

每个晶胞具有奇数电子的过渡金属d电子氧化物预计会形成具有部分占据能带的金属，但实际上表现出一系列行为，要么是绝缘体，要么是金属，要么具有绝缘体-金属过渡。传统的解释主要涉及固定结构对称性中的电子-电子相互作用。目前的工作重点是对称性打破局部结构图案的作用。将先前观察到的 VO ₂中的 VV 二聚化视为连续的旋钮，在密度泛函计算中揭示了孤立的平带与宽导带的分裂。这导致超过临界百分比二聚化以形成绝缘相，同时降低总能量。在 VO ₂中，发现这种转变具有相当低的能垒，接近室温下的热能，这表明神经形态计算中的节能切换。有趣的是，足够的VV二聚化会抑制磁性，导致非磁性绝缘态，而当二聚化减少时会出现磁性，形成金属态。这项研究为设计具有对称破缺诱导平带的新型功能量子材料开辟了道路。