Research Topics
A cartoon illustration of Zhao group research topics: quantum materials with strong interactions among 4 degrees of freedom (DoFs) from three-dimensional (3D) bulk crystals down to two-dimensional (2D) atomic layers/structures.
Overview Our research focuses on the experimental investigation of emergent behaviors in quantum materials. Emergence describes the collective behavior of the whole that differs from that of its individual parts because of interactions. Emergence is widely present everywhere around us, manifested, for example, as the formation of cultures in human society, the flocking of birds, and the schooling of fish, as well as the occurrence of varied phases of matter in materials – my research are. In quantum materials, emergence corresponds to the spontaneous organization of 10^23 electrons and atoms in static and dynamic ways into ordered states that differ in the orchestration of their four individual quantum DoFs: lattice, charge, spin, and orbital. Discovering new emergent phases and understanding their origins advance the knowledge of the organizing principles in many-body quantum materials and can give insight into the guided search for yet more new quantum phases. Furthermore, establishing controls over them could stimulate revolutionary leaps in future quantum materials-based technologies.
By developing and utilizing optical techniques, we aim at discovering, understanding, and controlling novel phases of matter emergent from two distinct interaction regimes, as well as the interplay between these two: (1) the 2D limit where the interactions are confined within two dimensions; and (2) the strongly correlated regime where especially strong interactions between electrons induce both competition and cooperation among the 4 DoFs.
If you are interested, please see more details about the four major research topics in our group listed below.
Area 1: Strongly correlated and strong SOC physics in 5d transition metal oxides
Of the various interactions among the four DoFs, strong electron correlations and strong SOC have led separately to the two major threads in quantum materials research, strongly correlated electron physics and topological electron physics, respectively. The combination of these two interactions holds prospects of realizing fundamentally new quantum phases of matter quite beyond a simple sum of those in the two individual research threads. Luckily, the experimental realization of coexisting strong electron correlations and strong SOC within a single material is achieved in high-quality single crystals of 4d and 5d transition metal oxides (TMOs). One main research topic in my group is to experimentally investigate novel electronic and magnetic phases in these materials.
"Polarized Raman spectroscopy study of metallic (Sr1−xLax)3Ir2O7: a consistent picture of disorder-interrupted unidirectional charge order" Physical Review B (Rapid Communications) 99, 041109 (2019)
"Symmetry-resolved two-magnon excitations in a strong spin-orbit-coupled bilayer antiferromagnet" Physical Review Letters 125, 087202 (2020)
Area 2: High-rank multipolar orders with tensor order parameters
Multipolar orders describe the phenomena of long-range ordering of multipolar moments that develop from the complex electric or mag- netic dipole distributions within one primitive cell of solids. Although much less studied than dipolar orders (e.g., ferroelectricity (FE), ferromagnetism (FM)), multipolar orders are widely present in a broad class of quantum materials including f -electron systems, 5d TMOs, multiferroics, chiral magnets and so on, and play essential roles in determining such materials’ physical properties. The key challenge in investigating multipolar orders is the lack of proper experimental techniques to efficiently couple with their tensor order parameters, because they call for tensor fields of the right symmetries, which unfortunately are not readily available. Another focused effort in my group is to experimentally study multipolar orders in quantum materials.
"Observation of a ferro-rotational order coupled with second-order nonlinear optical fields" Nature Physics 16, 42 (2020)
"Ultrafast modulations and detection of a ferro-rotational charge density wave using time-resolved electric quadrupole second harmonic generation" Physical Review Letters 127, 126401 (2021)
"Electric quadrupole second harmonic generation revealing dual magnetic orders in a magnetic Weyl semimetal" Nature Photonics (in press) (2023)
Area 3: Two-dimensional magnetism in atomically thin magnetic crystals
Spontaneous symmetry breaking orders are expected to exhibit distinct behaviors in reduced dimensionalities. On the one hand, the reduced dimensionality promotes electronic and magnetic instabilities, stimulating the development of new electronic and magnetic phases. On the other hand, the lower dimensionality enhances thermal and quantum fluctuations, prohibiting the formation of long-range orders. The outcome of this competition re- mains elusive, which has stimulated enormous interests in exploring phases and phase transitions in reduced dimensionalities. Thanks to the discovery of 2D atomic crystals, we now have the ideal platform to study electronic and magnetic ordering in 2D. One major research interest of my group is to experimentally explore 2D magnetic and electronic phases in atomic crystals.
"Magnetic-field-induced quantum phase transitions in a van Der Waals magnet" Physical Review X 10, 011075 (2020)
"Tunable layered-magnetism-assisted magneto-Raman effect in a two-dimensional magnet CrI3" Proceeding of National Academy of Sciences 117, 24664 (2020)
"Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3" Nature Communications 11, 4780 (2020)
Area 4: Moire engineering of two-dimensional magnetism
A moire superlattice results from the interference between two atomic lattices due to lattice mismatch or angular misalignment and emerges as one fruitful venue to design the physical properties of 2D materials. While a great number of discoveries have been made in moire electronics, the power of moire superlattice in designing the magnetic properties is less explored, partially because 2D magnetism itself is a much newer topic than 2D electronic materials. Thanks to the recent studies on natural 2D magnets, we now know that the interlayer stacking symmetry determines the interlayer magnetic exchange coupling (including both signs and magnitudes). Based on this knowledge, a moire superlattice contains a full parameter space of shift vector between adjacent layers, and therefore, a full range of periodic modulation of magnetic exchange coupling. One recent focus of my group is to investigate moire magnetism in twisted 2D magnets.
"Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures" Nature Physics 18, 30 (2022)
"Evidence of Noncollinear Spin Texture in Magnetic Moiré Superlattices" Nature Physics 10.1038/s41567-023-02061-z (2023) with a "news and views" highlight at Nature Physics (Noncollinear spin textures with a twist)