1. Superconductivity and Magnetism
Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials when cooled below a characteristic critical temperature (Tc). Its microscopic mechanism is thought the electron-phonon coupling, the so-called BCS theory. However, the discovery of cuprate superconductors broke the Tc limit allowed by the BCS theory. The mechanism for the high-temperature superconductivity in the cuprates is a long-standing puzzle for scientists. In 2008 a new type of high-temperature superconductors containing iron were discovered, which quickly attracted great attention and triggered extensive research in the fields of condensed matter physics and materials science. I use X-ray, transport and thermodynamic measurements to research structures and physical properties of the iron-based superconductors for understanding their pairing mechanism. Iron-based superconductivity could arise from the spin fluctuation occurring at the vicinity of the full suppression of spin density wave, so some of magnetic iron-based analogs were also studied for their novel magnetism and potential of becoming superconducting.
2. Solid State Chemistry
Development of condensed matter physics and materials science benefits from the discovery of new materials very much. Solid state chemistry provides a shortcut to the discovery of new solid-state materials like new superconductors. I design and synthesize new materials according to the habits of elements and the predictability of linking different structural units. Even though the predictions are incorrect sometimes, the exploratory synthesis can still yield new compounds. The composition of the new compounds will be determined and their structures can be solved by different crystallographic methods. The physical properties of them will be also studied for potential novelty or application.
Crystal structure is the basis for us to understand a material. Crystallography is the experimental science of determining crystal structures of materials. With crystallography we can solve structures of newly discovered materials, research phase transitions in materials, research structure-property relationships of materials, and even reveal microstructures like charge density wave in materials. My capabilities involve ab initio structure solution from single crystal and powder X-ray diffraction data, magnetic structure solution from powder neutron diffraction data, and local structure analysis from pair distribution function. These techniques can be extended to extreme conditions such as low temperature and high pressure. The facilities I use for crystallography includes a STOE single crystal diffractometer and the Advanced Photon Source at Argonne National Laboratory.
4. High-Pressure Physics and Chemistry
High pressure is an effective method to create new materials and a clean way to tune the electronic structures of materials. The most typical application of high pressure is the synthesis of superhard materials. I am interested in studying novel quantum phenomena under high pressure like pressure-induced superconductivity and quantum critical points. Transport measurement, magnetic susceptibility measurement, and almost all the synchrotron X-ray techniques can be carried out in various diamond anvil cells. In a chemistry aspect, I am trying to create new materials by heating chemicals in diamond anvil cells.