Our primary research goal is to explore the intertwined interactions of multiple degrees of freedom (i.e. charge, orbital, spin and lattice) in strongly correlated materials and advanced functional materials and to establish the relationship between structure and physical properties in order to achieve a deeper understanding of the underlying physics in those materials. The two main research fields are as follows: 1. Transmission Electron Microscopy (TEM) and its application on strongly correlated materials and functional materials; 2. Ultrafast electron diffraction (UED) and its exploration of correlated materials and functional materials.

First, we plan to enhance structural characterizations of strongly correlated materials and functional materials using transmission electron microscopy with technical developments. As one of the three main scattering methods for studying material structures (the other two are x-ray and neutron scattering), electron scattering and diffraction techniques have distinctive characteristics and can be well integrated with other condensed matter physics techniques and methods. TEM is not only used as a super magnifying scope for imaging atomic structures, but also can be deeply explored in electron diffraction and electron spectrum images, etc., showing data in real space, momentum-transfer space and energy space, thereby expanding the capabilities of the TEM research in condensed matter physics and materials science. The team plans to advance TEM characterization methods, such as the development of analysis of electron diffraction and electron spectroscopy, as well as the processing and analysis of big data under various in situ conditions. We will particularly focus on the ability of in situ observations, especially the observation of low-temperature states for the study of electron-lattice interactions in correlated materials and functional materials. The fine electron probe in TEM enables the observations of interfaces, defects and various inhomogeneous physical states: in addition to illustrating the distribution of atoms in real space, it is also possible to directly probe electronic structures with atomic resolution for the material. All those observations will demonstrate the structural origin of the material’s functionality at a new level.

Second, we will perform UED observations and characterizations of non-equilibrium states of strongly correlated materials and functional materials. In a large group of strongly correlated materials including high-temperature superconducting materials, several degrees of freedom (i.e. charge, orbital, spin and lattice) are intertwined with each other, and it is often difficult to disentangle the interactions between those degrees of freedom by the study of the equilibrium states of the materials. Since the characteristic interaction time between charge and spin, charge and lattice, and spin and lattice is known to be quite different, observations on ultrafast time scales can directly reveal different dynamical processes. Hence, it is possible to unfold those interactions and reveal the physical mechanisms. Taking advantage of the structural sensitivity and quantitative analysis of electron diffraction, combined with pump-probe technique, UED can trace the dynamic processes of crystal structures and electronic structures in the process of photoexcitation and relaxation on the time scales ranging from femtoseconds to nanoseconds. Collaborations from research groups at multiple directions, such as dynamic modeling and calculation for excited states and ultrafast observations using other techniques, are necessary and critical for the interpretation of the results in this research direction.

At the same time, using TEM and UED, we are interested in the manipulation of the structure and properties of materials by means of electron and photon excitations. Several recent works have shown that the electron illumination effect in specific materials can be large enough to vary crystal structures in a controllable manner, and the materials’ properties are also changed accordingly. Alternatively, it is possible to have transient new structures and properties through optical excitation, using instantaneous high-energy laser pulses at selective energy bands. The excitation of photons and electrons can be quantitatively regulated. Such the manipulation of the structures and properties of materials not only provide evidence of excitation mechanisms for fundamental materials research, but also creates "new" materials from another perspective.