Electronic band reconstruction across the insulator-metal transition in colossal magnetoresistive EuCd2P2

While colossal magnetoresistance (CMR) in Eu-based compounds is often associated with strong spin-carrier interactions, the underlying reconstruction of the electronic bands is much less understood from spectroscopic experiments. Here using angle-resolved photoemission, we directly observe an electronic band reconstruction across the insulator-metal (and magnetic) transition in the recently discovered CMR compound EuCd2P2. This transition is manifested by a large magnetic band splitting associated with the magnetic order, as well as unusual energy shifts of the valence bands: both the large ordered moment of Eu and carrier localization in the paramagnetic phase are crucial. Our results provide spectroscopic evidence for an electronic structure reconstruction underlying the enormous CMR observed in EuCd2P2, which could be important for understanding Eu-based CMR materials, as well as designing CMR materials based on large-moment rare-earth magnets.Comment: 6 pages, 4 figure

Quasi-Two-Dimensional Fermi Surface and Heavy Quasiparticles in CeRh2As2

The recent discovery of multiple superconducting phases in CeRh2As2 has attracted considerable interest. These rich phases are thought to be related to the locally noncentrosymmetric crystal structure, although the possible role of a quadrupole density wave preceding the superconductivity remains an open question. While measurements of physical properties imply that the Ce 4f electrons could play an essential role, the momentum-resolved electronic structure remains hitherto unreported, hindering an in-depth understanding of the underlying physics. Here, we report a high-resolution angle-resolved photoemission study of CeRh2As2. Our results reveal fine splittings of conduction bands, which are directly related to the locally noncentrosymmetric structure, as well as a quasi-two-dimensional Fermi surface, implying weak interlayer hopping and possible nesting instabilities. Our experiments also uncover the fine structures and pronounced temperature evolution of the Kondo peak, demonstrating strong Kondo effect facilitated by excited crystal electric field states. Our results unveil the salient electronic features arising from the interplay between the crystal structure and strong electron correlation, providing spectroscopic insight for understanding the heavy fermion physics and unconventional quadrupole density wave in this enigmatic compound

Spin-triplet superconductivity in Weyl nodal-line semimetals

Topological semimetals are three dimensional materials with symmetry-protected massless bulk excitations. As a special case, Weyl nodal-line semimetals are realized in materials either having no inversion or broken time-reversal symmetry and feature bulk nodal lines. The 111-family of materials, LaNiSi, LaPtSi and LaPtGe (all lacking inversion symmetry), belong to this class. Here, by combining muon-spin rotation and relaxation with thermodynamic measurements, we find that these materials exhibit a fully-gapped superconducting ground state, while spontaneously breaking time-reversal symmetry at the superconducting transition. Since time-reversal symmetry is essential for protecting the normal-state topology, its breaking upon entering the superconducting state should remarkably result in a topological phase transition. By developing a minimal model for the normal-state band structure and assuming a purely spin-triplet pairing, we show that the superconducting properties across the family can be described accurately. Our results demonstrate that the 111-family reported here provides an ideal test-bed for investigating the rich interplay between the exotic properties of Weyl nodal-line fermions and unconventional superconductivity

Effects of Elevated CO2 and N Addition on Growth and N2 Fixation of a Legume Subshrub (Caragana microphylla Lam.) in Temperate Grassland in China

It is well demonstrated that the responses of plants to elevated atmospheric CO2 concentration are species-specific and dependent on environmental conditions. We investigated the responses of a subshrub legume species, Caragana microphylla Lam., to elevated CO2 and nitrogen (N) addition using open-top chambers in a semiarid temperate grassland in northern China for three years. Measured variables include leaf photosynthetic rate, shoot biomass, root biomass, symbiotic nitrogenase activity, and leaf N content. Symbiotic nitrogenase activity was determined by the C2H2 reduction method. Elevated CO2 enhanced photosynthesis and shoot biomass by 83% and 25%, respectively, and the enhancement of shoot biomass was significant only at a high N concentration. In addition, the photosynthetic capacity of C. microphylla did not show down-regulation under elevated CO2. Elevated CO2 had no significant effect on root biomass, symbiotic nitrogenase activity and leaf N content. Under elevated CO2, N addition stimulated photosynthesis and shoot biomass. By contrast, N addition strongly inhibited symbiotic nitrogenase activity and slightly increased leaf N content of C. microphylla under both CO2 levels, and had no significant effect on root biomass. The effect of elevated CO2 and N addition on C. microphylla did not show interannual variation, except for the effect of N addition on leaf N content. These results indicate that shoot growth of C. microphylla is more sensitive to elevated CO2 than is root growth. The stimulation of shoot growth of C. microphylla under elevated CO2 or N addition is not associated with changes in N2-fixation. Additionally, elevated CO2 and N addition interacted to affect shoot growth of C. microphylla with a stimulatory effect occurring only under combination of these two factors

Effects of grazing on CO2, CH4, and N2O fluxes in three temperate steppe ecosystems

Terrestrial ecosystems play a critical role in regulating the emission and uptake of the most important greenhouse gases (GHGs) such as CO2, CH4, and N2O. However, the effects of grazing on these GHG fluxes in different steppe types remain unclear. Here, we compared the effects of grazing on seasonal CO2, CH4, and N2O fluxes in the meadow (MS), typical (TS), and desert (DS) temperate steppe ecosystems in northern China. CO2 emission rates increased from 311.4 +/- 73.2 to 349.6 +/- 55.4 mg.m(-2).h(-1) in MS, but decreased in TS (from 341.3 +/- 93.0 to 239.5 +/- 81.9 mg.m(-2).h(-1)) and DS ( from 212.1 +/- 53.7 to 163.0 +/- 83.4 mg.m(-2).h(-1)) in response to summer grazing (SG). N2O emission rates increased in MS from 4.7 +/- 2.2 to 8.1 +/- 3.4 mu g.m(-2).h(-1), but not significantly changed in TS (9.2 +/- 4.2 vs. 8.4 +/- 2.4 mu g.m(-2).h(-1)) and DS (6.3 +/- 1.5 vs. 5.7 +/- 1.6 mu g.m(-2).h(-1)) by SG. CH4 uptake rates increased in MS from 33.0 +/- 11.7 to 47.1 +/- 10.4 mu g.m(-2).h(-1) and decreased from 64.4 +/- 7.6 to 56.2 +/- 5.9 mu g.m(-2).h(-1) in TS in response to SG. In MS and DS, N2O emissions were positively related to seasonal CO2 emissions and negatively related to CH4 uptakes. No significant relationships were found between GHG fluxes in TS. Summer grazing did not affect the relationship between CO2 and N2O emissions in MS, but reduced the relationship by enhancing the effect of aboveground biomass (AGB) on N2O emission in DS. The significant negative relationship between CH4 uptake and N2O emission in MS and DS could be attributed to the significant relationship between soil temperature (ST) and AGB in MS and to the significant effects of soil moisture on both CH4 uptake and N2O emission in DS. The decrease in the magnitude of the correlation coefficients between CH4 uptake and N2O emission by SG was due to the negative relationship between ST and AGB simultaneously in MS and DS. Our results suggest that effects of SG on GHG fluxes varied in different steppes and the relationship among GHGs was steppe-dependent and SG also changed the relationship by affecting GHG fluxes induced by varied soil and environmental factors

Moderate grazing has little effect on global warming potential in the temperate steppes of northern China

Grazing has been reported to significantly affect the flux of three greenhouse gases (GHGs: CO2, CH4 and N2O) in grasslands, but its effect on total global warming potential (GWP) is still unclear. To assess the effect of grazing on GWP, we simultaneously measured the flux of these three GHGs using static chambers in meadow, typical, and desert steppes under no grazing (NG) and summer grazing (SG) conditions during the 2012-14 growing seasons. We aimed to examine the impact of grazing on total GWP across different steppes and to assess the relative contribution of different environmental factors to changes in GWP. Our results showed that total GWP values were almost entirely negative in all steppe environments and displayed high spatio-temporal variability. Net ecosystem exchange was the most important predictor of total GWP in all three steppes, and the positive GWP induced by N2O emission was approximately equal to the negative GWP induced by CH4 uptake. Steppe type and sampling year-but not grazing treatment-were found to affect GWP. Air temperature and precipitation were the major factors driving total GWP change under the no grazing treatment. In contrast, soil temperature, soil moisture, and precipitation explained a significant percentage of variation in total GWP under the summer grazing treatment. Our study suggests that moderate grazing does not change the role of temperate steppe's function in mitigating climate change; however, multi-year GWP data are necessary for extrapolation to a regional scale

Effects of water and nitrogen addition on ecosystem respiration across three types of steppe: The role of plant and microbial biomass

Evaluating the regional variation of ecosystem respiration (R-eco) in its response to the changes of soil water and nitrogen (N) availability is crucial for fully understanding ecosystem carbon (C) exchange and its feedbacks to global changes. Here, we examined the responses of R-eco, plant community aboveground biomass (AB), microbial biomass carbon (MBC) and soil moisture (SM) to water and N addition, using intact soil monoliths from three different temperate steppes along a precipitation gradient, including meadow steppe, typical steppe, and desert steppe in northern China. We found that the meadow steppe held the highest value of R-eco. Water addition significantly enhanced R-eco while N addition had no effect on R-eco in all three ecosystems. The response of R-eco in the typical steppe was more sensitive than the other two ecosystems. The changes of plant community AB exhibited amuch stronger explanatory power than that of MBC for R-eco in the typical steppe. In contrast, MBC was the dominant factor explaining the variation of R-eco in the desert steppe and the meadow steppe. These findings contribute to our understanding of regional patterns of ecosystem C exchange under scenarios of global changes and highlight the importance of water availability in regulating ecosystem processes in temperate steppe grasslands. (c) 2017 Published by Elsevier B.V

Role of LPSO Phase in Crack Propagation Behavior of an As-Cast Mg-Y-Zn Alloy Subjected to Dynamic Loadings

In this work, the role of long period stacking ordered (LPSO) phase in the crack propagation behavior of an as-cast Mg95.5Y3Zn1.5 alloy was investigated by dynamic four-point bent tests. The as-cast Mg95.5Y3Zn1.5 alloy is mainly composed of Mg matrix, 18R LPSO phase located at the grain boundaries and 14H LPSO phase located within the Mg matrix. The alloy exhibits excellent dynamic mechanical properties; the yield stress, maximum stress and strain to failure are 190.51 ± 3.52 MPa, 378.32 ± 4.26 MPa and 0.168 ± 0.006, respectively, at the strain rate of ~3000 s−1. The LPSO phase effectively hinders dynamic crack propagation in four typical ways, including crack tip blunting, crack opening inhibition, crack deflection and crack bridging, which are beneficial to the mechanical properties of the alloy under dynamic loadings

Response of soil methane uptake to simulated nitrogen deposition and grazing management across three types of steppe in Inner Mongolia, China

The response of soil methane (CH4) uptake to increased nitrogen (N) deposition and grazing management was studied in three types of steppe (i.e., meadow steppe, typical steppe, and desert steppe) in Inner Mongolia, China. The experiment was designed with four simulated N deposition rates such as 0, 50, 100, and 200 kg N ha(-1), respectively, under grazed and fenced management treatments. Results showed that the investigated steppes were significant sinks for CH4, with an uptake flux of 1.12-3.36 kg ha(-1) over the grass growing season and that the magnitude of CH4 uptake significantly (P < 0.05) decreased with increasing N deposition rates. The soil CH4 uptake rates were highest in the desert steppe, moderate in the typical steppe, and lowest in the meadow steppe. Compared with grazed plots, fencing increased the CH4 uptake by 4.7-40.2% with a mean value of 20.2% across the three different steppe types. The responses of soil CH4 uptake to N deposition in the continental steppe varied depending on the N deposition rate, steppe type, and grazing management. A significantly positive correlation between CH4 uptake and soil temperature was found in this study, whereas no significant relationship between soil moisture and CH4 uptake occurred. Our results may contribute to the improvement of model parameterization for simulating biosphere-atmosphere CH4 exchange processes and for evaluating the climate change feedback on CH4 soil uptake. (C) 2017 Elsevier B.V. All rights reserved