Gigahertz longitudinal acoustic phonons originating from ultrafast ligand field transitions in hematite thin films

The creation and propagation of longitudinal acoustic phonons(LAPs) in high quality hematite thin films(α-Fe2O3)epitaxially grown on different substrates(BaTiO3, SrTiO3, and LaAlO3) are investigated using the femtosecond pump–probe technique. Transient reflection measurements(?R/R) indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of ～500 ps are attributed to the electron-LO phonon coupling and electron–hole nonradiative recombination, respectively. LAPs in α-Fe2O3 film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400-nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated inα-Fe2O3 film on different substrates.     

河南省清凉寺窑址出土富铁釉瓷的无损分析研究

富铁釉瓷在中国古陶瓷史上具有重要地位，综合利用光学相干层析成像技术(OCT)、光学显微镜(OM)、激光拉曼光谱技术(LRS)和X射线荧光光谱分析技术(XRF) 对河南省宝丰清凉寺窑址出土宋金时期富铁釉瓷样品进行无损分析研究，获取了富铁釉瓷的釉层结构、釉面析晶的物相信息及胎釉化学成分等信息，探讨了釉层断面OCT灰度图像特征与其显微结构之间的关系以及富铁釉瓷的烧制工艺。结果表明，酱釉瓷和柿叶红釉瓷表面及黑釉红斑区存在富铁析晶层，黑釉瓷釉层表面未发现富铁析晶层的存在。酱釉瓷釉层富铁析晶层中铁的氧化物以ε-Fe₂O₃晶体为主，而柿叶红釉层则主要是α-Fe₂O₃晶体，黑釉红斑层同时存在ε-Fe₂O₃，α-Fe₂O₃晶体以及磁铁矿。上述差异形成的原因与釉层化学成和烧制工艺存在联系。釉层中高含量碱土金属氧化物及氧化铁区域更容易析出氧化铁晶体。黑釉瓷烧成气氛以还原气氛为主，而酱釉瓷和柿叶红釉瓷则倾向于氧化烧成气氛，且柿叶红釉瓷的烧成温度高于酱釉瓷。该研究将OCT，OM，LRS与XRF相结合，有助于加强对富铁釉瓷的整体认知，为富铁釉瓷的断源断代提供更为丰富的线索。     

The dc magnetic field,temperature and film thickness dependence of microwave surface resistance of YBCO and DyBCO thin films

We investigated the dc magnetic field and temperature dependences of the microwave surface resistance (R_s) of YBa₂Cu₃O_y (YBCO) and DyBa₂Cu₃O_y (DyBCO) superconducting thin films. The YBCO and DyBCO thin films, each with a thickness of 300, 500, or 700 nm, were deposited on MgO (1 0 0) substrates by the thermal co-evaporation method. The R_s was measured using the dielectric resonator method. A dc magnetic field of up to 5.0 T was applied parallel to the c-axis of the superconducting thin films. The results showed that the R_s value had almost the same temperature dependence at various thicknesses in a zero-external field. The R_s of the YBCO and DyBCO thin films increased with the applied dc magnetic field. The DyBCO thin films showed weaker magnetic field dependence of R_s than the YBCO thin films. The R_s ratio (defined as R_s(5 T)/R_s(0 T)) linearly increased with the film thickness. These results show that pinning strength decreased with an increasing film thickness.