Quantum Matter Seminar
Professor Zhao's group is interested in probing, understanding, and eventually controlling emergent electronic and magnetic phases in quantum materials, including both three-dimensional (3D) bulk and two-dimensional (2D) atomic and moiré crystals. In particular, her group focuses on the 3D and 2D materials with strong electron-electron interactions and/or strong spin-orbit coupling. Her primary experimental tools are a broad set of optical spectroscopy and microscopy techniques, including linear and nonlinear, static and ultrafast, elastic and inelastic ones. If you are interested in our research, please explore Professor Zhao's group website (Zhao Research Group) for more information. For her work, Professor Zhao is a recipient of the 2018 NSF CAREER Award, 2019 Bryan R. Coles Prize of SCES, 2020 AFOSR YIP Award, 2021 Alfred P. Sloan Research Fellowship, 2022 Henry Russel Award, and 2023 Mildred Dresselhaus Junior Guest Professorship. She also serves on the editorial boards for Physical Review X and Progress in Quantum Electronics.
CrSBr is a layered antiferromagnetic semiconductor that has recently attracted significant attention due to its anisotropic orthorhombic crystal lattice, robust magnetism with a high transition temperature, linearly polarized excitons with large binding energy, and strong coupling among magnetism, magnons, and excitons. Interestingly, upon closely examining the magnetic order of CrSBr, we observed an unexpected and counterintuitive trend: the magnetic onset temperature increases as the thickness of CrSBr decreases. In the antiferromagnetic phase of CrSBr, spins align ferromagnetically along the in-plane b-axis within each layer and antiferromagnetically between adjacent layers. The in-plane spin orientation and the absence of net magnetization make it challenging to directly detect the magnetic order parameter (and magnetic fluctuations), particularly in the thin-layer limit.
In this presentation, we utilize polarization-resolved second- and third-harmonic generation (SHG/THG) to probe the magnetic order in both bulk and bilayer CrSBr. In bulk CrSBr, we identified two magnetic phase transitions: a surface-layered antiferromagnetic transition at 140 K, followed by a bulk transition at 131 K, using a combination of SHG and THG [1, 2]. The enhanced magnetic onset at the surface aligns with the observed increase in transition temperature in thinner layers, suggesting stronger magnetic interactions at the surface and in reduced dimensions. For bilayer CrSBr, a PT-symmetric antiferromagnet, we demonstrated the emergence of an effective electric polarization upon application of an out-of-plane magnetic field, as revealed by magneto-SHG measurements [3]. This electric polarization is consistent with the magnetoelectric effect expected in bilayer CrSBr and can be attributed to a noncollinear spin arrangement induced by the magnetic field.
Our work underscores the power of nonlinear optical techniques in probing van der Waals magnetism and positions van der Waals magnets as a compelling platform for exploring magneto-nonlinear optical phenomena.