Nature: Wu Shiwei research group and collaborators of Fudan University have made important progress in the field of two-dimensional nonlinear optics of magnetic materials

In recent years, two-dimensional magnetic materials have become a hot research topic in the world They can keep the spontaneous magnetization to the thickness of single protocell layer, which provides a new research platform for people to understand and control the low-dimensional magnetism, and also opens up a new direction for the research and development of two-dimensional magnetism and spin electronics devices, which has important application value in new photoelectric devices, spin electronics devices, etc Although the ferromagnetic properties of two-dimensional magnetic materials have been studied, the experimental study of the antiferromagnetic state is a big problem in the field because it does not have macroscopic magnetization, the material system as a whole does not show magnetism to the outside, and the sample is thin and small To solve this problem, recently, Wu Shiwei group of Fudan University and Xu Xiaodong group of Washington University have combined to observe the nonreciprocal second harmonic nonlinear optical response from the interlayer antiferromagnetic structure in the two-dimensional magnetic material double-layer chromium triiodide, and revealed the relationship between the interlayer antiferromagnetic coupling of chromium triiodide and the van der Waals stacking structure The related research results were published in the journal Nature (DOI: 10.1038 / s41586-019-1445-3) under the title of "giant nonreciprocal second harmonic generation from antiferromagnetic bilayer CRI 3" The second harmonic response induced by interlayer antiferromagnetism, which is observed in the research work, excites the team members because they know that it is of great significance in the research of two-dimensional materials and nonlinear optics "The meaning lies first and foremost in its uniqueness." According to Wu Shiwei, the second harmonic studied in the field of two-dimensional materials up to now is mostly caused by the symmetry breaking of the lattice structure "Symmetry breaking means breaking symmetry For example, a person's left and right hands are mirror symmetry If a finger is injured, the mirror symmetry will be broken." However, the nonreciprocal second harmonic generated by the magnetic structure is different from the former in essence and is very novel in principle Because there is no macroscopic magnetic moment in antiferromagnetic materials, it is difficult to produce macroscopic and measurable response to external physical excitation, and it is often unable to do anything for two-dimensional antiferromagnetic materials with only a few atomic layers "In the past, this problem was like a place where the light couldn't shine There was no way to deal with it However, such a "dark" state can now be "bright" by means of the second harmonic It's also the beauty of introducing a classic approach to a new field " Wu Shiwei felt this This second harmonic process is highly sensitive to the symmetry of the magnetic structure of the material, which opens up a broad research space for the research of two-dimensional magnetic materials At the same time, the research team found that the second harmonic signal of the double-layer antiferromagnetic chromium triiodide has more than three orders of magnitude improvement in the response coefficient compared with the previously known magnetic second harmonic signal (such as chromium oxide Cr2O3), and ten orders of magnitude higher than the second harmonic generated by the conventional ferromagnetic interface Using this strong second harmonic signal, the team was able to reveal the symmetry of the protocell stacking structure of the double-layer chromium triiodide According to Wu Shiwei, chromium triiodide belongs to monoclinic crystal system at high temperature, which changes into rhombohedral crystal system due to structural phase transition at low temperature The difference between them is the interlayer translation of van der Waals interaction (a kind of interaction between atoms or molecules, which is much weaker than chemical bond interaction) However, in the limit of oligo layer, the lattice stacking structure at low temperature is still controversial In the experiment, the team used a polarized light to measure the polarization of materials in different directions in space By measuring the second harmonic signal of polarization polarization, it was found that the stacking structure of the material and the monoclinic lattice has mirror face symmetry It is consistent with the theoretical calculation results recently published in the world, and provides new experimental evidence and research means for the study of the relationship between the stacking structure of two-dimensional materials and the ferromagnetic and antiferromagnetic coupling between layers Innovative R & D experimental system to achieve basic research breakthrough The antiferromagnetic materials detected by the research team in the experiment only have two protocell layer thickness (thickness below 2 nm), but under this condition, neutron scattering and other measurement methods are difficult to work To solve this problem, based on the accumulation in the field of nonlinear optics of two-dimensional materials in the past years, the team used the optical second harmonic method to detect the magnetic structure and related characteristics of two-dimensional magnetic materials The optical second harmonic process is highly sensitive to the symmetry of the system The detection method of optical second harmonic starts from the symmetry of the system and can detect the antiferromagnetism of the system sensitively Different from the usual experimental method of detecting magnetism, it does not depend on the macroscopic magnetism of the material, but on the symmetry breaking caused by the microscopic magnetic structure In the antiferromagnetic state, the magnetic structure of double-layer chromium triiodide not only breaks the time inversion symmetry, but also breaks the space inversion symmetry, resulting in a strong nonreciprocal second harmonic response When the system rises above the transition temperature, or when an out of plane magnetic field is applied, the symmetry of the magnetic structure changes, and the second harmonic signal disappears From 2017 to now, two years of cooperation have watered today's results The team first made preliminary measurements using the existing non liquid helium variable temperature micro optical scanning imaging system in the laboratory, but due to the absence of magnetic field in the system, many key experimental measurements were limited In order to solve this problem, the members of the research group overcome the difficulties Using a set of non liquid helium room temperature hole superconducting magnet, a set of non liquid helium variable temperature and strong magnetic field micro optical scanning imaging system was independently developed and built With the help of the new system, optical measurement under strong magnetic field was realized, and the detection of key data was completed It is reported that the research team also includes Yao Wang, Professor of Hong Kong University, Xiao Di, Professor of Carnegie Mellon University, Cao Ting, Professor of Washington University, Michael McGuire, researcher of Oak Ridge National Laboratory, and Liu Weitao, Chen Zhanghai, Gao Chunlei, Professor of Fudan University Wu Shiwei and Xu Xiaodong are the corresponding authors, and sun Zeyuan and Yi Yangfan, PhD students of Fudan University, are the co first authors The research work is supported by NSFC, major research plans of the Ministry of science and technology, special plans for key research and development and other project funds.