Dr. Yuelin Li [Argonne National Laboratory, USA]

Title:   Time Resolved X-ray Science: Revealing Hidden Physics in Complex Oxides

Time:  11:20 - 12:00 PM, July 13, 2015, Monday

Place: Conference room 410, Build 6, HPSTAR (Shanghai)

Host:  Dr. Mao

Abstract:

After a brief introduction of the basic methods in time resolved X-ray science, we report two examples studying the structure dynamics of hetero thin films of complex oxide revealing the physics that is hidden under static conditions. By analyzing the Bragg diffraction peak profile as a function of time after laser excitation, we infer the photo carrier dynamics that is strongly coupled to the sample structure. The first example is the spin-orbit coupling 5-d transition metal oxide Sr2IrO4, where for the first time we identified the lattice correlation of the so called Hubbard exciton due tophoto excitation. Furthermore, we reconstructed the spatiotemporal hopping map of the Hubbard exciton with a nanometer spatial resolution of and subnanosecond temporal resolution. In the second example, we study the multiferroic BiFeO3 thin film. Fitting the coherent Bragg rod around the (0 0 2) diffraction peak allows us to reconstruct the strain profile as a function of time after photon excitation. Together with thickness dependence data, we uncovered the excitonic nature of the photo-carrier, thus a unifiedexciton mediated photovoltaic mechanism for the photostriction and photovoltaic effects. The finding may lead to new strategy for photovoltaic application of ferroelectric material for solar energy. With further refinement, the method we developed can be extended to give atom by atom dynamics in oxide heterostructures, thus the potential of revealing the effect of complex orbit reconstruction at the surface and the interface for oxide heterostructures. The implication of these finding for high pressure physics is briefly discussed.

Biography of the Speaker:

Dr. Yuelin Li is a physicist at Argonne National Laboratory.  He graduated from Peking University and obtained his Ph. D from Shanghai Institute of Optics and Fine Mechanics. He was a Royal Society Research Fellow and Alexander von Humboldt Fellow. Before joining Argonne, he was a visiting scientist at Lawrence Livermore National Laboratory, University of Maryland, Max-Planck Institute for Quantum Optics, and Rutherford Appleton Laboratory. He has published widely on applications of lasers and laser techniques for generation of novel light sources ranging from high power THz to ultrafast X-ray, high brightness beam via photoinjector or laser plasma accelerators, and full time and frequency domain characterization of free electron laser output. His current research interests include applying laser pump-x-ray probe techniques and study of dynamics in strongly correlated materials.