Stability and correlations in dilute two-dimensional boson systems

The hyperspherical adiabatic expansion method is used to describe correlations in a symmetric boson system rigorously confined to two spatial dimensions. The hyperangular eigenvalue equation turns out to be almost independent of the hyperradius, whereas the solutions are strongly varying with the strength of the attractive two-body potentials. Instability is encountered in hyperangular, hyperradial, and mean-field equations for almost identical strengths inversely proportional to the particle number. The derived conditions for stability are similar to mean-field conditions and closely related to the possible occurrence of the Thomas and Efimov effects. Renormalization in mean-field calculations for two spatial dimensions is probably not needed.Comment: 5 pages, two figures, submitted to Phys. Rev. A, second version contains added discussion, especially of renormalizatio

Direct and indirect effects of climate on richness drive the latitudinal diversity gradient in forest trees

Data accessibility statement: Full census data are available upon reasonable request from the ForestGEO data portal, http://ctfs.si.edu/datarequest/ We thank Margie Mayfield, three anonymous reviewers and Jacob Weiner for constructive comments on the manuscript. This study was financially supported by the National Key R&D Program of China (2017YFC0506100), the National Natural Science Foundation of China (31622014 and 31570426), and the Fundamental Research Funds for the Central Universities (17lgzd24) to CC. XW was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB3103). DS was supported by the Czech Science Foundation (grant no. 16-26369S). Yves Rosseel provided us valuable suggestions on using the lavaan package conducting SEM analyses. Funding and citation information for each forest plot is available in the Supplementary Information Text 1.Peer reviewedPostprin

Effects of Habitat Filtering on Tree Growth and Mortality across Life Stages in an Old-Growth Temperate Forest

A demographic (growth and mortality) trade-off plays a central role in the assembly and dynamics of ecological communities and contributes to tree species’ coexistence. On the basis of field investigation data from the 2010 and 2015 censuses, we evaluated the degrees to which the relative growth rate (RGR) and mortality rate (MR) of saplings and large trees were related to habitat filtering for temperate tree species from a 9 ha forest dynamics plot. The results showed that the relationship between RGR and MR was stronger in saplings than that in large trees. In saplings, the total P (TP) and organic C (OC) of the soil had a significantly positive correlation with RGR. In large trees, volumetric water content had a significantly negative correlation with RGR. In saplings, the bulk density and available P had a significantly positive correlation with MR. In large trees, MR showed a significantly negative correlation with aspect and a significantly positive correlation with TP and OC. Principal component analysis showed that species–habitat association status significantly affected the demographic parameters. A linear regression analysis revealed that the process of habitat filtering contributed to the ontogenetic variation that controlled RGR and MR as the community transitioned from saplings to large trees. Moreover, water availability for large trees played a key role in this process in an old-growth temperate forest

Calculation of microwave peak absorbing frequency of polar liquids including dielectric friction

Reorientation time is the basis of the calculation of microwave peak absorbing frequency (MPAF) of liquids. In the Stokes–Einstein–Debye theory, the reorientation time of polar molecules is considered mainly due to mechanical friction, and an obvious deviation exists between the predictions of this theory and the experimental observations. In this paper, with the theory of Onsager’s effective field, the phase relation between reaction field and rotating polar molecule is derived, and this reaction field hinders the rotation of molecules due to the hysteretic phase. Hence, the reorientation time is extended to the sum of mechanical reorientation time and dielectric reorientation time. With the relation of reorientation time of molecule and Debye relaxation time of dielectric, Debye relaxation time of dielectric is modified, and then an improved formula of MPAF is derived. The MPAFs of coumarin C522B, C307, C138 in alcohol solvents are calculated with this formula, and the changes in MPAFs of water, methanol and ethanol with temperature are also numerically studied. These calculated results are consistent with the experimental data

FeSiCr-Based Soft Magnetic Composites with SiO₂ Insulation Coating Prepared Using the Elemental Silicon Powder Hydrolysis Method

In this work, FeSiCr powders were coated with a SiO2 insulation layer for soft magnetic composites (SMCs) through elemental silicon powder hydrolysis, without using any expensive precursors. The effects of the reaction temperature and ammonia concentration on the structure and performance of SMCs were investigated. Through the elemental silicon powder hydrolysis process, the formation of an FeSiCr–SiO2 core-shell structure effectively reduced the core loss, increased resistivity, and improved the quality factor of SMCs. SMCs prepared with 0.10 mL/g ammonia concentration at 50 °C exhibited the best combination of properties, with saturation magnetization Ms = 169.40 emu/g, effective permeability μe = 40.46, resistivity ρ = 7.1 × 106 Ω·cm, quality factor Q = 57.07 at 1 MHz, and core loss Ps = 493.3 kW/m3 at 50 mT/100 kHz. Compared to the uncoated sample, SMCs with a SiO2 coating exhibit 23% reduction in Ps, with only 6.6% reduction in μe. Compared to SMCs fabricated using the traditional sol-gel method, the sample prepared through hydrolysis of elemental silicon powder has higher permeability and lower core loss. In particular, this new approach gives an effective coat solution for the mass production of high-temperature-resistant SMCs

Enhancing the Properties of FeSiBCr Amorphous Soft Magnetic Composites by Annealing Treatments

Fe-based amorphous powder cores (AMPCs) were prepared from FeSiBCr amorphous powders with phosphate–resin hybrid coating. The high-frequency magnetic properties of AMPCs annealed at different temperatures were systematically studied. After annealing at low temperatures, the effective permeability and core loss improved due to the internal stress of the powder cores being released. The sample annealed at 480 °C exhibits the lowest hysteresis loss of about 29.6 mW/cm3 at 800 kHz as well as a maximum effective permeability of 36.4, remaining stable until 3 MHz, which could be useful for high-frequency applications