Design principles and anomalous nonlinear optics of two-dimensional intrinsic Janus structures

Two-dimensional Janus structures have garnered growing attention across multidisciplinary fields. Despite extensive theoretical and experimental efforts, a fundamental design principle for intrinsic Janus materials remains elusive. Here, we introduce a first-principles alloy theory based on cluster expansion, incorporating strong cation-mediated anion-pair repulsion and refined short-range cluster-cluster interactions, to elucidate the formation mechanism of intrinsic Janus structures with a distorted 1T phase amid competing phases. Our approach explains why intrinsic Janus structures are observed in RhSeCl and BiTeI—composed of elements from different groups—and predicts a broad range of 1T-like intrinsic Janus candidates ready for synthesis. Notably, in RhSeCl, we reveal that intrinsic Janus materials can exhibit anomalous second-harmonic generation (SHG) driven by a distinct quantum geometric effect stemming from lattice and chemical-potential mirror asymmetry. Furthermore, an unconventional skin effect emerges in finite-thickness RhSeCl, featuring a hidden bulk SHG response. Our theory paves the way for ab initio design of intrinsic Janus materials, accelerating progress in Janus science.