Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2.

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant µ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Theoretical Determination of the Electronic Structures and Energy-Level Splitting for Pr3+-Doped Y2SiO5 Crystals.

Trivalent praseodymium (Pr3+)-doped yttrium silicate (Y2SiO5) crystals have been widely used in various phosphors owing to their excellent luminescence characteristics. Although a series of studies have been carried out on its application prospects, the electronic structures and energy-transfer mechanisms of Pr3+-doped Y2SiO5 (Y2SiO5:Pr) remain an exploratory topic. Herein, the crystal structure analysis by the particle swarm optimization structure search method is used to study the structural evolution of Y2SiO5:Pr. Two novel structures with local [PrO7]-11 and [PrO6]-9 [Y2SiO5:Pr (I) and Y2SiO5:Pr (II)] are successfully identified. The impurity Pr3+ ions occupy the Y3+ sites and successfully integrate into the Y2SiO5 host crystal with a Pr3+ concentration of 6.25%. The calculated electronic band structures show that the doping of Pr3+ induces a reduction in band gaps for the host Y2SiO5 crystal. The conduction bands near the Fermi level are completely composed of f states. For the atomic energies of Pr3+ in Y2SiO5, the Stark levels and transitions are properly simulated based on a new set of crystal field parameters (CFPs) at the C1 site symmetry. A satisfactory r.m.s. dev. of 15.57 cm-1 with 9 free ion parameters (plus 27 fixed CFPs as obtained from ab initio calculation) fitted to the 33 observed levels is obtained for the first time. The plentiful energy-level transition lines, from the visible light to the near-infrared region, are deciphered for Pr3+ in Y2SiO5. Blue 3P0 → 3H4 at 465 nm is calculated to be a strong emission line, and it might be an ideal channel for laser actions. These results could not only provide important insights into the rare-earth-doped crystals but also lay the foundation for future research studies of designing the new laser materials.

Himalayan mountains imposing a barrier on gene flow of wheat yellow rust pathogen in the bordering regions of Pakistan and China.

The wheat yellow rust pathogen has been shown to be diverse and potentially originated in the Himalayan region. Although Himalayan populations of Pakistan, Nepal and Bhutan have been previously compared, little is known about the relative divergence and diversity in Puccinia striiformis populations in the bordering regions of Pakistan and China. To assess the relative diversity and divergence in these regions of Pakistan (Gilgit-Baltistan, Hazara and Azad Jammu Kashmir) and China (Xinjiang, Qinghai, Tibet, Sichuan, Guizhou and Yunnan), a total of 1245 samples were genotyped using 17 microsatellite SSR markers. A clear divergence was observed between the bordering regions of Pakistan and China (FST = 0.28) without any resampling of genetic groups and multilocus genotypes across two sides of the Himalayan mountains. The closest subpopulations across the two countries were Xinjiang and Gilgit-Baltistan (Nei's distance = 0.147), which were close geographically. A very high diversity and recombinant population structure was observed in both populations, though slightly higher in China (Genotypic diversity = 0.970; r¯d = 0.000) than in Pakistan (Genotypic diversity = 0.902; r¯d = 0.065). The distribution of genetic groups and resampling of MLGs revealed more gene flow across Yunnan, Guizhou and Sichuan regions in China, while between Hazara and Azad-Jammu Kashmir in Pakistan. The lack of gene flow between Pakistan and China populations is due to geographical barriers and a large patch of land without wheat. The information on the relative diversity and divergence in different geographical zones of the pathogen center of diversity and neighboring region should be considered in resistant wheat deployment while considering the invasion potential of the pathogen at regional and global contexts.

Photoluminescence and energy transfer mechanisms of Tm3+ doped Y2O3 laser crystals: experimental and theoretical insights.

Rare-earth thulium (Tm3+) doped yttrium oxide (Y2O3) host single crystals are promising "eye-safe" laser materials. However, the mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals are not yet understood at the fundamental level. Here, we synthetize a series of Y2O3:Tm3+ samples by the sol-gel method. Our experimental results show that the most intensive absorption line of the 3H6 → 1D2 transition occurs at 358 nm, and the strongest emission line of the 1D2 → 3F4 transition is located at 453 nm, which are in good agreement with the calculations of 363 nm and 458 nm, respectively. By using the CALYPSO structural search method, the ground state structure of Y2O3:Tm3+ with P2 space group symmetry is uncovered. The complete energy levels, including free-ion LS terms and crystal-field LSJ multiplet manifolds, of Y2O3:Tm3+ are obtained based on our developed WEPMD method. The present findings show that our WEPMD method can be used in experiments to elucidate the underlying mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals, which offer insights for further understanding of other rare-earth doped laser materials.

Genetic Characteristics and Linkage of Virulence Genes of the Puccinia striiformis f. sp. tritici TSA-6 Isolate to Yr5 Host Resistance.

To understand the inheritance of the TSA-6 Puccinia striiformis f. sp. tritici (Pst) isolate that is virulent to Yr5 and was recently detected in China, we analyzed avirulence and virulence of 120 selfed progeny lines from Berberis shensiana. The results showed that the TSA-6 isolate is virulent against the Yr5 resistance gene, and overall progeny lines were categorized into 73 virulence phenotypes (VPs); of these, 72 VPs differed from the isolate TSA-6, and only one VP, including three progeny, was identical to the parental isolate. The analyses indicated that the TSA-6 isolate is homozygous for avirulence at the Yr10, Yr15, and Yr26 resistance loci and virulence at the YrA resistance locus. The TSA-6 isolate is heterozygous for avirulence at the Yr2, Yr3, Yr5, Yr7, and Yr8 resistance loci, which are controlled by a dominant/recessive relationship. The Yr1, Yr6, Yr9, Yr17, Yr27, Yr25, Yr28, Yr29, Yr32, YrTr1, and YrSP resistance loci are governed by two complementary dominant/recessive genes. Avirulence against heterozygous Yr4, Yr43, Yr44, Yr76, and YrExp2 resistance loci is regulated by a dominant and recessive or a dominant and suppressor gene pair. In total, 117 multilocus genotypes were detected at 24 KASP-SNP marker loci among the 120 progenies. Using these marker loci, we constructed a linkage map with a genetic distance interval spanning 624.5 cM. Quantitative trait loci corresponding to phenotypic segregation for virulence at 20 Yr resistance loci in addition to the Yr1 resistance locus were identified. These results facilitate our understanding of Pst virulence evolution and simplify breeding of wheat cultivars with effective resistance to wheat stripe rust.

Self-Assembled Nanodot Actuator with Changeable Fluorescence by π-π Stacking Force Based on a Four-Armed Foldable Phthalocyanine Molecule and Its Supersensitive Molecular Recognition.

Designing intelligent molecules and smart nanomaterials as molecular machines is becoming increasingly important in the nanoscience fields. Herein, we report a nanodot actuator with changeable fluorescence by π-π stacking force based on a four-armed foldable phthalocyanine molecule. The assembled nanodot possessed a three-dimensional molecular space structure and multiple supramolecular interactions. The arms of the nanodot could fold and open intelligently in response to environmental molecular stimuli such as natural plant mimosa, which could lead to multiple variable fluorescence emissions. The nanodot was highly sensitive to the biomolecule thyroxine at the molecular level. The accurate molecular recognition and the changeable fluorescence conversion of the nanodot were attributed to multiple supramolecular interactions, including photoinduced electron transfer (PET), intramolecular fluorescence resonance energy transfer (FRET), and π-π stacking of the nanodots, resulting in an intelligent "nanodot machine with folding arms". The self-assembled nanodot actuators with changeable fluorescence have potential applications in advanced intelligent material fields.

Countrywide inter-epidemic region migration pattern suggests the role of southwestern population in wheat stripe rust epidemics in China.

Understanding countrywide pathogen population structure and inter-epidemic region spread is crucial for deciphering crop potential losses. Wheat stripe rust caused by Puccinia striiformis f. sp. tritici is a destructive disease that affects worldwide wheat production, widespread in China, representing largest epidemic region globally. This study aimed to understand the population structure and migration route of P. striiformis f. sp. tritici across China based on sampling from 15 provinces representing six epidemic zones, viz., over-summering, over-wintering, eastern, Yun-Gui, Xinjiang and Tibet epidemic regions. High genotypic diversity was recorded in over-summering, Tibet and over-wintering epidemic regions. Epidemic regions partly explain population subdivision with variable divergence (FST  = 0.005-0.344). Xinjiang and Tibet epidemic regions were independent epidemic zones with least sharing of genotypes. Among other epidemic zones, i.e. over-summering, over-wintering, eastern and Yun-Gui epidemic zones, re-sampling MLGs, clustering-based structure, DAPC analyses, relative migration and low divergence (FST from 0.006 to 0.073) revealed frequent geneflow. Yun-Gui epidemic regions, with a potential for both over-summering and over-wintering, could play an important role in causing epidemics in main wheat-cultivating areas of China. High diversity, recombination signatures and inter-epidemic region migration patterns need to be considered in host-resistant cultivar development in China and neighbouring countries, considering risk of long-distance migration capacity of pathogen.

Two Main Routes of Spore Migration Contributing to the Occurrence of Wheat Stripe Rust in the Jiangsu and Zhejiang Coastal Sporadic Epidemiological Region in 2019, Based on Phenotyping and Genotyping Analyses.

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is a destructive disease in many countries. In China, wheat stripe rust generally occurs in northwestern and southwestern China and sporadically in the Jiangsu and Zhejiang coastal epidemiological region (JZER), where an outbreak of the disease occurred in 2019. To understand the population structure and potential inoculum sources of the pathogen in this region, 171 isolates collected from 93 wheat fields of 53 counties in 10 provinces were phenotyped with two sets of wheat differentials and genotyped with 20 pairs of single-nucleotide polymorphism primers. Phenotype tests indicated that identical races (CYR34, CYR33, Su11-139, and Su11-14-1) detected in Jiangsu and Zhejiang were shared with the oversummering regions (Gansu), overwintering regions (Hubei, Henan, and Shaanxi), and Yun-Gui epidemiological regions (Yunnan and Guizhou). In JZER, races CYR32, G22-14, and G22-68 were detected in Jiangsu, but not in Zhejiang, and Su11-208 was identified in Zhejiang, but not in Jiangsu. Genotypic analysis revealed remarkable gene flows among the Jiangsu, Yunnan, Henan, and Anhui populations, as well as those of Zhejiang, Guizhou, and Sichuan, showing that wheat stripe rust in Zhejiang and Jiangsu was from spores that migrated from different routes. Major gene flows were detected between the Jiangsu and Zhejiang populations. P. striiformis f. sp. tritici from both overwintering regions (Yunnan, Sichuan, Guizhou, Henan, Hubei, and Shaanxi) and oversummering regions (Gansu) contributed to the wheat stripe rust epidemic in the JZER region in 2019.

Insights into the Microstructures and Energy Levels of Pr3+-Doped YAlO3 Scintillating Crystals.

Trivalent praseodymium (Pr3+)-doped materials have been extensively used in high-resolution laser spectroscopy, owing to their outstanding conversion efficiencies of plentiful transitions in the visible laser region. However, to clarify the microstructure and energy transfer mechanism of Pr3+-doped host crystals is a challenging topic. In this work, the stable structures of Pr3+-doped yttrium orthoaluminate (YAlO3) have been widely searched based on the CALYPSO method. A novel monoclinic structure with the Pm group symmetry is successfully identified. The Pr3+ impurity can precisely occupy the Y3+ position and get incorporated into the YAlO3 (YAP) host crystal with a Pr3+ concentration of 6.25%. The result of the electronic band structure reveals a 3.62 eV band gap, which suggests a semiconductor character of YAP:Pr. Using our developed well-established parametrization matrix diagonalization (WEPMD) method, we have systematically analyzed the energy level scheme and proposed a set of newly improved parameters. Additionally, the energy transfer mechanism of YAP:Pr is clarified by deciphering the numerical electric dipole and magnetic dipole transitions. The popular red emission at 653 nm is assigned to the transition 3P0 → 3F2, while the transition 3P0 → 3H4 with a large branching ratio is predicted to be a good laser channel. Many promising emission lines for laser actions are also obtained in the visible light region. Our results not only provide important insights into the energy transfer mechanisms of rare-earth ion-doped materials but also pave the way for the implementation of new types of laser devices.

Investigation of the Structure and Luminescence Mechanism of Tm3+-Doped LiYF4: New Theoretical Perspectives.

The absorption and emission transitions of Tm3+-doped LiYF4 have been extensively investigated due to the excellent properties and enormous applications of these materials as laser materials. However, the challenging issues regarding the local structure and luminescence mechanism have not been conclusively established to date. To address these challenges, the CALYPSO structure search method is employed, and the results first reveal the ground-state structure of Tm3+-doped LiYF4, which crystallizes in the space group P4̅ (No. 81) of the tetragonal system. The Y3+ ions are replaced by Tm3+ ions, forming a local configuration of [TmF8]5-. Furthermore, the complete Stark energy levels of Tm3+-doped LiYF4 are predicted by using our newly developed WEPMD method, which provides preliminary preparation for further spectral exploration. Judd-Ofelt analysis is performed to evaluate the electric dipole transition intensities. Two prominent transitions, 3H5 → 3H6 (1223 nm) and 3H4 → 3H6 (801 nm), are predicted to be good candidates for near-infrared lasers. This study not only is useful for determining the luminescence properties of Tm3+-doped LiYF4 but also offers an effective way to search for other rare-earth-doped lasing crystals for the future design of lasing materials.

Unraveling the local structure and luminescence evolution in Nd3+-doped LiYF4: a new theoretical approach.

Neodymium ion (Nd3+)-doped yttrium lithium fluoride (LiYF4, YLF) laser crystals have shown significant prospects as excellent laser materials in many kinds of solid-state laser systems. However, the origins of the detailed information of their local structure and luminescence evolution are still poorly understood. Herein, we use an unbiased CALYPSO structure searching technique and density functional theory to study the local structure of Nd3+-doped YLF. Our results reveal a new stable phase with the P4[combining macron] (No. 81) space group for Nd3+-doped YLF, indicating that the host Y3+ ion site was naturally occupied by the Nd3+ ion impurity. On the basis of our newly developed WEPMD method, we adopt a specific type of orthogonal correlation crystal field to obtain a new set of crystal-field parameters as well as 182 complete Stark energy levels. Many absorption and emission lines for Nd3+-doped YLF are calculated and discussed based on Judd-Ofelt theory, and our results indicate that some of the observed absorption and emission lines are perfectly reproduced by our theoretical calculations. Additionally, we predict several promising transition lines in the visible and near-infrared spectral regions, including the electronic dipole emission lines 4F5/2 → 4I9/2 at 808 nm and 2H9/2 → 4I9/2 at 799 nm, as well as the magnetic dipole emission lines 4F3/2(27) → 4I11/2(6) at 1047 nm and 4F3/2(27) → 4I11/2(8) at 1052 nm. These transition channels indicate that Nd3+-doped YLF laser crystals have greatly promising laser actions for serving as a solid-state laser material.

Deciphering the Microstructure and Energy-Level Splitting of Tm3+-Doped Yttrium Aluminum Garnet.

Thulium-doped yttrium aluminum garnet (Tm:YAG) is an important solid-state laser crystal. The energy-level splitting within it is still an unresolved problem. Here, we perform a theoretical study on the microstructure of Tm3+-doped YAG using the CALYPSO structure search method in conjunction with first-principles calculations. The calculated results show that the 4.16% doping concentration of Tm3+ impurity causes an obvious structural distortion of YAG crystal, forming an orthorhombic phase in C222 symmetry. On the basis of our developed WEPMD method, we obtain a new and complete set of free-ion and crystal field parameters by a good fit (with proper irreducible representations) to 69 observed energy levels and determine the exact energy-level splitting of Tm3+ in YAG. The calculated Stark levels and electric dipole transitions are in excellent agreement with the measured data and similar theoretical calculations. Some promising emission lines between 3F3, 3F2, 1D2, and 1I6 states are presented. These findings offer fundamental insights and practical tools for further exploration of the structural and electronic properties of other transition-metal-doped YAG crystal.

Insights into the Microstructure and Transition Mechanism for Nd3+-Doped Bi4Si3O12: A Promising Near-Infrared Laser Material.

Due to its unusual optical properties, neodymium ion (Nd3+)-doped bismuth silicate (Bi4Si3O12, BSO) is widely used for its excellent medium laser amplification in physics, chemistry, biomedicine, and other research fields. Although the spectral transitions and luminescent mechanisms of Nd3+-doped BSO have been investigated experimentally, theoretical research is severely limited due to the lack of detailed information about the microstructure and the doping site of Nd3+-doped BSO, as well as the electric and magnetic dipole transition mechanisms. Herein, we systematically study the microstructure and doping site of Nd3+-doped BSO using an unbiased CALYPSO structure search method in conjunction with first-principles calculations. The result indicates that the Nd3+ ion impurity occupies the host Bi3+ ion site with trigonal symmetry, forming a unique semiconducting phase. Based on our newly developed WEPMD method, the electric dipole and magnetic dipole transition lines, including a large number of absorption and emission lines, in the region of visible and near-infrared spectra of Nd3+-doped BSO are calculated. It is found that the 4G5/2 → 4I9/2, 2H9/2 → 4I9/2, and 4F3/2 → 4I11/2 channels are promising laser actions of Nd3+-doped BSO. These findings indicate that Nd3+-doped BSO crystals can serve as a promising multifunctional material for optical laser devices.

Geometries, stabilities and fragmental channels of neutral and charged sulfur clusters: Sn(Q) (n = 3-20, Q = 0, ±1).

We have performed unbiased searches for the global minimum structures of neutral and charged sulfur clusters Sn(Q) (n = 3-20, Q = 0, ±1) relying on the CALYPSO structure searching method combined with density functional theory geometric optimization. Very accurate ab initio calculations are used to determine relative stabilities and energy ranking among competing low-lying isomers of the neutral and charged sulfur clusters obtained from the structure search. The harmonic vibrational analysis is also undertaken to assure that the optimized geometries are the true minima. It is shown that the most equilibrium geometries of sulfur clusters are closed three-dimensional (3D) helical rings, which is in agreement with the experimental observations. The binding energies, second-order energy differences, and highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of the considered species are calculated and analyzed systematically. Additionally, the fragmentation channels are determined and the results indicate that the Sn(Q) → S2 + Sn-2(Q) channel is a route that the small clusters (n = 3-10) favor, while the larger species (n = 13-20) prefer the Sn(Q) → S8 + Sn-8(Q) channel.

Ab Initio Search for Global Minimum Structures of Pure and Boron Doped Silver Clusters.

The global minimum structures of pure and boron doped silver clusters up to 16 atoms are determined through ab initio calculations and unbiased structure searching methods. The structural and electronic properties of neutral, anionic, and cationic Ag(n)B (n ≤ 15) and Ag(n)B2 (n ≤ 14) clusters are much distinct from those of the corresponding pure silver. Considering that Ag and B possess one and three valence electrons, respectively, both the single and the double boron-atom doped silver clusters with even number of valence electrons are more stable than those with odd number of electrons, a feature also observed in the pure silver clusters. We demonstrate that the species with a valence count of 8 and 14 appear to be magic numbers with enhanced stability irrespective of component or the charged state. A new putative global minimum structure of Ag13(-) cluster, with high symmetry of C(2v), is unexpectedly observed as the ground state, which is lower in energy than the previous suggested bilayer structure.

Concentration effects on the local structures and electronic properties of ErxBaY2-xF8: a first-principles study.

Er3+doped barium yttrium fluoride (BaY2F8) crystal has gained long-term attention due to its great potential in laser and medical device applications. However, the local structures of Er3+doped BaY2F8system (Er:BYF) remain uncertain, and the effect of doping concentration on structures and properties is unknown. Therefore, in this study, the first-principles study of the structural evolution of ErxBaY2-xF8(x= 0.125, 0.25) crystals was carried out. By means of density functional theory and particle swarm optimization algorithm, the stable structures of Er:BYF crystals with two different concentrations are shown as standard monoclinic structures withP2 symmetry for the first time. The impurity Er3+ions successfully enter the main lattice, replacing the Y3+ions, and forming a [ErF8]5-polyhedron withC2point group symmetry. By calculating the electronic properties, the band gap values of the two structures are significantly reduced compared with that of pure BaY2F8crystal. However, the conduction band does not break through the Fermi level, and the crystals still maintain the insulation characteristic. According to the calculation of the electron local density function, we conclude that Er-F and Y-F in Er:BYF are connected by ionic bonds. These results fill a theoretical gap in the study of Er:BYF crystals and provide inspiration for structural evolution and material design at different doping concentrations.

Prediction of potential high-temperature superconductivity in ternary Y-Hf-H compounds under high pressure.

Compressed ternary alloy superhydrides are currently considered to be the most promising competitors for high-temperature superconducting materials. Here, the stable stoichiometries in the Y-Hf-H ternary system under pressure are comprehensively explored in theory and four fresh phases are predicted: Pmna-YHfH6 and P4/mmm-YHfH7 at 200 GPa, P4/mmm-YHfH8 at 300 GPa and P-6m2-YHfH18 at 400 GPa. The four Y-Hf-H ternary phases are thermodynamically and dynamically stable at corresponding pressure. In addition, structural features, bonding characteristics, electronic properties, and superconductivity of the four ternary Y-Hf-H phases are systematically calculated and discussed. As the hydrogen content and the density of states of H atoms at the Fermi level increase, the superconducting transition temperatures (Tc) of Y-Hf-H system are significantly enhanced. The P-6m2-YHfH18 with high hydrogen content exhibits a high calculated Tc value of 130 K at 400 GPa.

Electric Field-Controlled Magneto-Optical Kerr Effect in A-Type Antiferromagnetic Fe2CX2 (X = F, Cl) and Its Janus Monolayer.

The magneto-optical Kerr effect (MOKE) is a powerful probe of magnetism and has recently gained new attention in antiferromagnetic (AFM) materials. Through extensive first-principles calculations and group theory analysis, we have identified Fe2CX2 (X = F, Cl) and Janus Fe2CFCl monolayers as ideal A-type collinear AFM materials with high magnetic anisotropy and Néel temperatures. By applying a vertical external electrical field (Ef) of 0.2 V/Å, the MOKE is activated for Fe2CF2 and Fe2CCl2 monolayers without changing their magnetic ground state, and the maximum Kerr rotation angles are 0.13 and 0.08°, respectively. Due to the out-of-plane spontaneous polarization, the intrinsic and nonvolatile MOKE is found in the Janus Fe2CFCl monolayer and the maximal Kerr rotation angle without external electronic field is 0.25°. Moreover, the intrinsic built-in electronic field also gives origin to more robust A-type AFM ordering and reversible Kerr angle against external Ef. Our study suggests that Ef is an effective tool for controlling MOKE in two-dimensional (2D) AFM materials. This research opens the possibility of related studies and applications in AFM spintronics.

The crystal structures, phase stabilities, electronic structures and bonding features of iridium borides from first-principles calculations.

We present results of an unbiased structure search for the lowest energy crystalline structures of various stoichiometric iridium borides, using first-principles calculations combined with particle swarm optimization algorithms. As a result, besides three stable phases of C2/m-Ir3B2, Fmm2-Ir4B3, and Cm-Ir4B5, three promising metastable phases, namely, P21/m-Ir2B, P21/m-IrB, and Pnma-Ir3B4, whose energies are within 20 meV per atom above the convex hull curve, are also identified at ambient pressure. The high bulk modulus of 301 GPa, highest shear modulus of 148 GPa, and smallest Poisson's ratio of 0.29 for C2/m-Ir3B2 make it a promising low compressible material. C2/m-Ir3B2 is predicted to possess the highest Vickers hardnesses, with a Vickers hardness of 13.1 GPa and 19.4 GPa based on Chen's model and Mazhnik-Oganov's model respectively, and a high fracture toughness of 5.17 MPa m0.5. The anisotropic indexes and the three-dimensional surface constructions of Young's modulus indicate that Ir-B compounds are anisotropic with the sequence of the elastic anisotropy of Ir2B > IrB > Ir4B5 > Ir3B4 > Ir4B3 > Ir3B2. Remarkably, these iridium borides are all ductile. We further find that the four Ir-B phases of P21/m-Ir2B, C2/m-Ir3B2, P21/m-IrB, and Fmm2-Ir4B3 possess dominant Ir-B covalent bonding character, while strong B-B and Ir-B covalent bonds are present in Cm-Ir4B5 and Pnma-Ir3B4, which are responsible for their excellent mechanical properties.

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As an important soil cover in deserts, biological crusts play a central role in ecosystem function such as nutrient cycling, nitrogen fixation, and carbon sequestration. Many biological crust organisms could fix CO2 through photosynthesis to improve soil organic carbon content. There is a knowledge gap in the origin of soil organic carbon (SOC) from biological crusts on a regional level, which restricts the prediction of soil carbon pool. Based on 45 plots in the Mu Us sandy land (42200 km2), we measured the SOC content and soil organic carbon density (SOCD) of two types of typical biological crusts (moss crusts, algal crusts) and their underlying soils, and analyzed together with the climate data, soil and vegetation factors to investigate the spatial distribution characteristics and controlling factors of organic carbon of biological crusts at the regional scale. The results showed that: 1) biological crusts significantly increased SOC and SOCD compared with bare ground. Moss crusts and the underlying SOC (4.93 g·kg-1) and SOCD (0.41 kg·m-2) were higher than those of algal crusts (1.89 g·kg-1, 0.18 kg·m-2). 2) On the regional scale, the SOC and SOCD of biological crusts had clear spatial distribution characteristics, demonstrating a banded distribution and block mosaic from northeast to central and west to southeast. 3) The SOC and SOCD of biological crusts and their underlying soils were mainly affected by climate, soil and vegetation conditions, while the main controlling factors depended on the types of biological crusts. The SOC and SOCD of moss crust were controlled by annual maximum temperature and potential evapotranspiration, whereas those of algal crusts were controlled by water vapor pressure.