时 间： 2017 年5月22日 15 : 00 - 17 : 00
地 点： 南五楼 613 学术报告厅 报告人： Prof. Junqiao Wu, University of California, Berkeley
邀请人： 游 龙 教授
Vanadium dioxide (VO2) has been a model quantum material system to study strongly correlation electron physics due to its metal-insulator phase transition coupled with a structural transition that occurs at 341K. In this talk, I will discuss our recent efforts in exploring exotic physics and novel applications of VO2: 1) violation of the Wiedemann-Franz where the electronic contribution to thermal conductivity is much less than what is expected from a Fermi liquid behavior; and 2) a reconfigurable meta-canvas on which arbitrary meta-photonic devices can be written, erased and re-written. Specifically, in electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. We discover an order of magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 341 K in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently. For the novel application of developing a meta-canvas, we present an all-solid, rewritable meta-canvas on which nearly arbitrary photonic devices can be rapidly and repeatedly written and erased. Using the meta-canvas, we demonstrate dynamic manipulation of optical waves for light propagation, polarization and reconstruction. The meta-canvas supports physical (re)compilation of photonic operators akin to that of field-programmable gate arrays, opening up possibilities where photonic elements can be field-programmed to deliver complex, system-level functionalities.
Prof. Junqiao Wu received a BS from Fudan University and a MS from Peking University, China, both in physics. He obtained a PhD degree from the University of California, Berkeley for work on nitride semiconductors and highly mismatched semiconductor alloys. He did postdoctoral research in the Department of Chemistry at Harvard University on phase transitions in transition metal oxides. In 2006, he started his faculty appointment at UC Berkeley where he is now a full professor, as well as a faculty scientist at Lawrence Berkeley National Laboratory. He is also a Professor in the Tsinghua-Berkeley Shenzhen Institute, and a Changjiang Adjunct Professor at Peking University. His honors include the Berkeley Fellowship, the 29th Ross N. Tucker Memorial Award, the UC Regents’ Junior Faculty Fellowship, the Berkeley Presidential Chair Fellowship, the US-NSF Career Award, the US-DOE Early Career Award, the Presidential Early Career Award for Scientists and Engineers (PECASE) from the White House, the Outstanding Alumni Award from Peking University China, and the Bakar Faculty Fellowship. The Wu group explores novel properties and applications of strongly correlated electron materials with reduced dimensions, phase transitions at the nanoscale, and optoelectronic, thermal and thermoelectric properties of semiconductor alloys and interfaces. He has made ground-breaking contributions to development and understanding of narrow-bandgap nitride semiconductors, highly mismatched semiconductor alloys, phase transitions in oxide nanomaterials, defect physics of low-dimensional semiconductors, and thermal/thermoelectric physics of novel materials. He has published over 200 papers that have received over 12,000 citations. More information can be found at http://mse.berkeley.edu/~jwu .