（1）主题：Topologically Protected Unidirectional Edge Spin Waves and Beam Splitter
（2）主讲人： 王宪思 微电子与固体电子学院博士后
Magnetic materials are highly correlated spin systems that do not respect the time-reversal symmetry. The low-energy excitations of magnetic materials are spin waves whose quanta are magnons. Like electronic materials that can be topologically nontrivial, a magnetic material can also be topologically nontrivial with topologically protected unidirectional edge states. These edge states should be superb channels of processing and manipulating spin waves because they are robust against perturbations and geometry changes, unlike the normal spin wave states that are very sensitive to the system changes and geometry. Therefore, the magnetic topological matter is of fundamental interest and technologically useful in magnonics. Here, we show that ferromagnetically interacting spins on a two-dimensional honeycomb lattice with nearest-neighbour interactions and governed by the Landau-Lifshitz-Gilbert equation, can be topologically nontrivial with gapped bulk spin waves and gapless edge spin waves. These edge spin waves are indeed very robust against defects under topological protection. Because of the unidirectional nature of these topologically protected edge spin waves, an interesting functional magnonic device called beam splitter can be made out of a domain wall in a strip. It is shown that an in-coming spin wave beam propagating along one edge towards a domain wall will be guided along the domain wall and split into two spin wave beams propagating in two opposite directions along the other edge. It is found that there are two branches of bound spin waves in the domain wall and their superpositions can result in different power division ratios depending on the strip width (domain wall length). Various types of devices are designed based on these findings.
Wang Xiansi received his bachelor degree in University of Science and Technology of China in 2010. He got his PhD degree in the Hong Kong University of Science and Technology in 2016. He currently works as a Postdoctoral Fellow in the School of Microelectronics and Solid-state Electronics of UESTC. Dr. Wang’s research interests include magnetism and spintronics. Specifically, he focuses on numerical and analytical analyses of Landau-Lifshitz-Gilbert equation, domain wall dynamics, and spin transport in multilayer structures.
（1）主题：Efficient and Photostable Perovskite Solar Cells Employing Rationally Designed Charge Contacts
Organic-inorganic lead halide perovskite solar cells are arising as strong candidates for next generation of renewable energy conversion devices due to their high efficiency, low material and fabrication cost, and scalable manufacture capability. With the continued development of the perovskite compositions and optimization of device interfaces, the maximum power conversion efficiency of single junction perovskite solar cells has been boosted to a certified 22.1% for small area device and close to 20% for 1 cm2 size device, approaching the efficiency of commercialized crystalline silicon and CIGS solar cells. While high efficiency has been demonstrated, stability and material cost are two dominate factors that determine the commercialization of perovskite solar cells. Simultaneously realizing the enhanced photostability and high efficiency, herein we demonstrate the rationally designed iron (III) oxide charge extracting materials and low-cost cross-stacked carbon nanotubes back contacts for efficient and photostable perovskite solar cells. Perovskite solar cells fabricated iron (III) oxide electrodes showed a promising power conversion efficiency of over 18% and excellent photostability. Moreover, we will report an inverted CH3NH3PbI3/NiO perovskite solar cell with cross-stacked carbon nanotube film cathode. Rigid and flexible perovskite devices based on carbon nanotube cathodes deliver promising conversion efficiencices of 14.3% and 10.5% respectively. Moreover, both rigid and flexible devices show excellent stability after long-term thermal stress or continuous light soaking. Our results indicate that the rationally designed iron (III) oxide and cross-stacked carbon nanotube are promising electrode materials for long-term device operation and pave the way toward realistic commercialization of perovskite solar cells.
Luo Qiang received his PhD degree in Tsinghua University in 2016. He currently works as a Postdoctoral research Fellow in the School of Microelectronics and Solid-state Electronics of UESTC. Dr. Luo’s research interests focus on inorganic charge contacts and light absorbers for photovoltaic application, including sensitized solar cells, perovskite solar cells etc.
编辑：林坤 / 审核：罗莎 / 发布者：一戈