The Atomic Rearrangement of GaN-Based Multiple Quantum Wells in H2/NH3 Mixed Gas for Improving Structural and Optical Properties

In this work, three GaN-based multiple quantum well (MQW) samples are grown to investigate the growth techniques of high-quality MQWs at low temperature (750 °C). Instead of conventional temperature ramp-up process, H2/NH3 gas mixture was introduced during the interruption after the growth of InGaN well layers. The influence of hydrogen flux was investigated. The cross-sectional images of MQW via transmission electron microscope show that a significant atomic rearrangement process happens during the hydrogen treatment. Both sharp interfaces of MQW and homogeneous indium distribution are achieved when a proper proportion of hydrogen was used. Moreover, the luminescence efficiency is improved strongly due to suppressed non-radiative recombination process and a better homogeneity of MQWs. Such kind of atomic rearrangement process is mainly caused by the larger diffusion rate of gallium and indium adatoms in H2/NH3 mixed gas, which leads to a lower potential barrier energy to achieve thermodynamic steady state. However, when excessive hydrogen flux is introduced, the MQW will be partly damaged, and the luminescence performance will deteriorate.

Structural evolution and synthesis mechanism of ytterbium disilicate powders prepared by cocurrent chemical coprecipitation method

Ytterbium disilicate powders were synthesized by cocurrent chemical coprecipitation method. The influence of Si/Yb molar ratio and calcination temperature on compositions and structures of Yb2Si2O7 products were investigated. The formation mechanism and thermal behavior of precursor as well as the phase evolution of Yb2Si2O7 were also discussed in depth. Results show that pure β-Yb2Si2O7 powders with nanoscale size can be obtained from the precursor with Si/Yb molar ratio of 1.1 after being calcinated at temperatures above 1200 ℃. The Yb2Si2O7 precursor is an amorphous polymer cross-linked with -[Si-O-Yb]- chain segments which are formed though Yb atoms embedding in the -[Si-O-Si]- network. After a continuous dihydroxylation and structural ordering, the amorphous precursor transformed to α-Yb2Si2O7 crystals by atomic rearrangement. Elevated calcination temperature can induce to the coordination structures and environment evolutions of structural units and then converted to stable (Si2O7) groups and (YbO6) polyhedrons, which results in the formation of β-Yb2Si2O7.

DRAWINGS TO LEARN SCIENCE: SOME REFLECTIONS

Teacher needs to reinvent himself or herself all the time, proposing activities in the classroom that enable students to build and reconstruct knowledge. Particularly in science, this knowledge is mediated through scientific models, often inaccessible to students’ understanding. For the comprehension of a chemical reaction, for example, the student is invited to imagine how the interactions between the particles would be, the atomic rearrangement, totally abstract thought and based on models, often expressed by visualizations, that need to be constantly constructed and reviewed by students. However, how to revise these abstract concepts? What strategies could be used? The answer to these questions is complex, but we would like to propose a reflection on the use of drawings, as a possibility, among so many existing ones.

Detecting dark photons from atomic rearrangement in the galaxy

We study a new dark sector signature for an atomic process of “rearrangement” in the galaxy. In this process, a hydrogen-like atomic dark matter state together with its anti-particle can rearrange to form a highly-excited bound state. This bound state will then de-excite into the ground state emitting a large number of dark photons that can be measured in experiments on Earth through their kinetic mixing with the photon. We find that for DM masses in the GeV range, the dark photons have enough energy to pass the thresholds of neutrino observatories such as Borexino and Super-Kamiokande that can probe for our scenario even when our atomic states constitute a small fraction of the total DM abundance. We study the corresponding bounds on the parameters of our model from current data as well as the prospects for future detectors.