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Solid-state nuclear magnetic resonance can be enhanced using unpaired electron spins with a method known as dynamic nuclear polarization (DNP). Fundamentally, DNP involves ensembles of thousands of spins, a scale that is difficult to match computationally. This scale prevents us from gaining a complete understanding of the spin dynamics and applying simulations to design sample formulations. We recently developed an ab initio model capable of calculating DNP enhancements in systems of up to ∼1000 nuclei; however, this scale is insufficient to accurately simulate the dependence of DNP enhancements on radical concentration or magic angle spinning (MAS) frequency. We build on this work by using ab initio simulations to train a hybrid model that makes use of a rate matrix to treat nuclear spin diffusion. We show that this model can reproduce the MAS rate and concentration dependence of DNP enhancements and build-up time constants. We then apply it to predict the DNP enhancements in core–shell metal-organic-framework nanoparticles and reveal new insights into the composition of the particles' shells. [ABSTRACT FROM AUTHOR]

Solar-driven ejection–compression refrigeration is helpful for building space cooling but is based on the precondition of solar heat being infinite and free. This is not valid in many cases, such as multistorey buildings in metropolises with limited space. Even worse, in the carbon-neutral future, space surrounding buildings will be occupied by PV (photovoltaic) panels in priority. Therefore, a potential ejector-partially coupled enhanced compression refrigeration cycle consuming less heat is presented. Models of the new cycle for a multistorey building are established. Then, energy comparison and advanced exergy analysis are conducted. The analysis results first show that the new cycle needs no auxiliary system. Powered by limited low-grade heat, its C O P e increased by 20.66% and total annual cost reduced by 7.8%, compared with the traditional ejector–compressor cycle. Advanced exergy analysis also finds that the order of improvement potential is compressor (4739.45 W), evaporator (4329.85 W), and condenser (2843.51 W), according to the sum of avoidable exergy destruction. This is different from the results of conventional exergy analysis. The sensitivity analysis of the ejector components' efficiencies is investigated, which has seldom been reported for refrigeration system with ejector. The results reveal that once the efficiencies of ejector components reach a certain point (0.94), the performance no longer improves. Generally, this work provides a potential ejection–compression cycle, demonstrates system improvement direction through advanced exergy analysis, and reveals good understanding of the system. [ABSTRACT FROM AUTHOR]

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices. [ABSTRACT FROM AUTHOR]

We investigated the ultrafast proton migration and the Coulomb explosion (CE) dynamics of methyl chloride (CH 3 Cl) in intense femtosecond laser fields at the wavelengths of 800 nm (5.5 × 10 14 W/cm 2 ) and 400 nm (4 × 10 14 W/cm 2 ), respectively. Various fragment channels from molecular dication and trication were observed by coincidence momentum imaging through the measurement of their kinetic energy releases (KERs). The proton migration from different charged parent ions was analyzed from the obtained KER distributions. For the direct CE channel of CH 3 + + Cl + and CH 3 + + Cl 2+ , the contribution of multiply excited electronic states and multicharged states is identified. In addition, the measurements of relative yields of the fragmentation channel at different laser wavelengths provide a selective control of proton migration for CH 3 Cl molecules in intense laser fields.