Two-dimensional Intrinsic Janus Structures: Design Principle and Anomalous Nonlinear Optics

TL;DR

This paper introduces a first-principles alloy theory to design intrinsic Janus two-dimensional structures, explaining their formation, predicting new materials, and revealing their nonlinear optical properties such as anomalous second-harmonic generation.

Contribution

It develops a novel alloy theory based on cluster expansion to understand and predict intrinsic Janus 2D materials with unique optical properties.

Findings

Explains formation of intrinsic Janus structures like RhSeCl and BiTeI.

Predicts a wide range of 1T-like intrinsic Janus materials for synthesis.

Reveals anomalous second-harmonic generation and a new skin effect in RhSeCl.

Abstract

Two-dimensional Janus structures have garnered rapidly growing attention across multidisciplinary fields. However, despite extensive theoretical and experimental efforts, a principle for designing intrinsic Janus materials remains elusive. Here, we propose a first-principles alloy theory based on cluster expansion, incorporating a strong repulsive interaction of a cation-mediated anion-pair cluster and refined short-range cluster-cluster competitions, to unravel the formation mechanism of intrinsic Janus structures with a distorted 1T phase among numerous competing phases. Our theory not only explains why intrinsic Janus structures are accidentally observed in RhSeCl and BiTeI which are composed of alloyed elements from different groups, but also accurately predicts a wide range of 1T-like intrinsic Janus materials that are ready for synthesis. Intriguingly, as demonstrated in the case of RhSeCl, we reveal that intrinsic Janus materials can exhibit anomalous second-harmonic generation (SHG) with a distinct quantum geometric effect, originating from strong lattice and chemical-potential mirror asymmetry. Furthermore, a novel skin effect unexpectedly emerges in finite-thickness RhSeCl, accompanied by a hidden SHG effect within the bulk region. Our theory paves the way for the ab initio design of intrinsic Janus materials, significantly accelerating progress in Janus science.