High-pressure magnetic properties and electrical transport behaviors of half-metallic ferromagnet CrO2.

The magnetic properties and electrical transport behaviors of half-metallic ferromagnet chromium dioxide (CrO2) powders under high pressure have been investigated by in situ electrical resistivity, magneto-resistivity, and Hall-effect measurements. Our results reveal that the Hall coefficient, carrier concentration, and mobility all present discontinuous changes from 11.7 GPa to 14.9 GPa which can be attributed to the second-order structural transition from the rutile-type to CaCl2-type. However, the resistivity decreases monotonically from ambient pressure to 16.5 GPa. This is due to, first, the decreased carrier concentration and the increased carrier mobility canceling the effects of each other on the resistivity; second, according to the calculation results, the bandgap of CrO2 decreased gradually with the pressure, and the bandgaps of the rutile-type phase and the CaCl2-type phase are extremely similar. CrO2 exhibits a linear and negative magnetoresistance under the applied magnetic field (0∼ ± 15 kOe). As the pressure increases, the magnetoresistance remains negative, but it becomes nonlinear and less symmetric, suggesting that pressure has an appreciable impact on the double-exchange mechanism leading to ferromagnetism in CrO2.

ReaxFF-MPNN machine learning potential: a combination of reactive force field and message passing neural networks.

Reactive force field (ReaxFF) is a powerful computational tool for exploring material properties. In this work, we proposed an enhanced reactive force field model, which uses message passing neural networks (MPNN) to compute the bond order and bond energies. MPNN are a variation of graph neural networks (GNN), which are derived from graph theory. In MPNN or GNN, molecular structures are treated as a graph and atoms and chemical bonds are represented by nodes and edges. The edge states correspond to the bond order in ReaxFF and are updated by message functions according to the message passing algorithms. The results are very encouraging; the investigation of the potential, such as the potential energy surface, reaction energies and equation of state, are greatly improved by this simple improvement. The new potential model, called reactive force field with message passing neural networks (ReaxFF-MPNN), is provided as an interface in an atomic simulation environment (ASE) with which the original ReaxFF and ReaxFF-MPNN potential models can do MD simulations and geometry optimizations within the ASE. Furthermore, machine learning, based on an active learning algorithm and gradient optimizer, is designed to train the model. We found that the active learning machine not only saves the manual work to collect the training data but is also much more effective than the general optimizer.

A high sensitivity dual-mode optical thermometry based on charge compensation in ZnTiO3:M (M = Eu3+, Mn4+) hexagonal prisms.

Optical thermometer based on dual-mode fluorescence intensity ratiometric thermometry has been attracted more attention due to its higher sensitivity. In order to obtain optical thermal probe with high sensitivity, ZnTiO3 hexagonal prisms with hexagonal perovskite structure were fabricated by using self-assembly method, and Al3+ ions were introduced into the crystal lattices of ZnTiO3 doped with Eu3+/Mn4+ to improve the optical properties. The emission intensity assigned to Eu3+ was enhanced about twice with the charge compensation of Al3+ between Eu3+ and Ti3+. While the luminescence ratio between the thermal coupled level of Eu3+ revealed poor temperature dependence property. The emission assigned to 2Eg→4A2g (713 nm) transition of Mn4+ revealed an huge thermal quenching. Using the luminescence ratio between 5D0→7F2 (5D0→7F1) transition of Eu3+ to 2Eg→4A2g of Mn4+, the higher relative sensitivity of 2.7 %K-1was obtained. The charge compensation of Al3+ improved the coordination and reduced the relative sensitivity, Sr =1.85 %K-1. The results suggested the potential application in optical temperature probes for ZnTiO3: Mn4+,Eu3+ phosphor.

Density functional theory study on the reaction mechanism of synthesizing 1,3-dimethyl-2-imidazolidinone by urea method.

We report a first-principles density functional theory investigation on tailoring the fundamental reaction mechanism of synthesizing 1,3-dimethyl-2-imidazolidinone (DMI) through the urea method with water serving as both solvent and catalyst. The nucleophilic cyclization reaction is implemented by two ammonia removal steps. One -NH group of dimethylethylenediamine (DMEDA) first attacks the carbon atom of urea, eliminating one -NH3 group and forming an intermediate state CH3NHC2H4N(CH3)CONH2 (IMI). IMI subsequently undergoes the cyclization process through a secondary ammonia removal via similar manner. Without water, the two ammonia removal steps are both slightly exothermic with high activation barriers (~50 kcal mol(-1)). As water participated in the reaction, the kinetics of the two steps can be significantly improved, respectively. The role that water plays, beside as solvent, more importantly, is to serve as a proton exchange bridge. Due to the spatial configuration, the direct proton migration from the N atoms of ethylenediamine to urea is difficult to occur. The water bridge facilitates the proton migration by shortening the migration distance. As a consequence, the activation barriers are considerably lowered down to ~30 kcal mol(-1), indicating a strong catalytic effect from water. In contrast, the three possible side reactions of IM(I), even catalyzed by water, have higher activation barriers due to strong steric inhibitive effect and consequently become difficult to occur at the same condition. The current computational understanding on the prototypical reaction to DMI can be extended to guide developing more efficient routes to synthesize imidazolidinone derivatives through the urea method.

Effects of Metal Ions on Conductivity and Structure of Single DNA Molecule in Different Environmental Conditions.

We design a novel nano-gap electrode to measure the current of DNA molecule, by which the current-voltage characteristics of individual native DNA, Ag-DNA and Ni-DNA molecules are obtained, respectively. The results show that the voltage gap of Ag- and Ni-DNA is higher than that of native DNA, and the conductance is lower than native DNA in neutral environment. The structure transition from B- to Z-DNA is observed in the presence of high concentrations of nickel ions and Ag-DNA appears chaos state by STM image and U-V spectra characterization. But in alkaline environment, the conductance of Ni-DNA rises and the voltage gap decreases with the increasing of nickel ion concentration denotes that the conductive ability of Ni-DNA is higher than that of native DNA.

[Treatment of developmental dislocation of the hip with improved Pemberton method].

OBJECTIVE: To find a more perfect method of treating developmental dislocation of the hip(DDH). METHODS: From March 1994 to December 2002, on the basis of the ordinary operative method, Pemberton method was improved for 48 cases of DDH 49 articulatio coxae which had superficial and small acetabula but big heads of femur. Osteotomy points were moved higher to enlarge area of bone flap turned over. A piece of full thickness ilium including periosteum was removed as a free bony graft. Capsula articularis growing thicker was made into 2 layers, of which the inner layer was used to tighten articularis and the outer layer was used to stabilize bone flap. RESULTS: All patients were followed up for 18 months to 5 years, the function of articulatio coxae and degree of bony union, inclusion area to head of femur and ischemic necrosis of femoral head were observed through X-ray. According to Zhou Yongde's criteria for the curative effect of congenital dislocation of the hip, there were 18 cases of 26-30 scores(3 hips), 27 cases of 21-25 scores, 2 cases of 16-20 scores(3 hips), and 1 case under 15 scores, and the total superior rate was 93.8%. CONCLUSION: Improved Pemberton method can enlarge inclusion area to head of femur, avoid decreasing acetabular volume, and stabilize articulatio well, which made operative indication bigger than original Pemberton method.