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Grassland degradation has become a worldwide ecological problem. Although soil microorganisms, as the main participants in the process of grassland degradation, play a key role in maintaining ecosystem function and improving soil productivity, little is known about the changes in microbial communities caused by grassland degradation and their relationship with soil properties and plant communities. In this study, we used Illumina MiSeq sequencing to analyze the soil fungal communities of subalpine meadow soil at four different degradation stages[i.e., non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and heavily degraded (HD)] on Mount Wutai. The results showed that Ascomycota, Basidiomycota, and Zygomycota were the dominant phyla of soil fungi in the subalpine meadow, regardless of degradation stage. LEfSe showed that the subalpine meadows with different degradation degrees were enriched with different biomarkers. Compared with ND, MD and HD were enriched with more pathogenic fungi. Moreover, HD apparently decreased the richness and Shannon indexes of soil fungal communities compared with those of ND. Non-metric multidimensional scaling (NMDS) and similarity analysis (ANOSIM) indicated that the compositions and structures of fungal communities were significantly different among meadows with different degradation degrees (

Hybrid organic-inorganic perovskite solar cells (HOIPs), especially CH3NH3PbI3 (MAPbI3), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A- and B-sites and anions at the X-site in the general form ABX3. In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca2+ and Sr2+ were mixed with the less stable Ge2+ and Sn2+ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the lead-containing perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI3. All Pb-free perovskites have suitable and direct band gaps (1.42-1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (105 cm-1) and strong absorption efficiencies (above 90%) in a wide spectral range (300-1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

Organic–inorganic halide perovskites are quite promising in applications of large scale photovoltaic technology. However, toxicity is one of the crucial issues in these materials, and searching for environmentally friendly perovskite materials for green energy applications is in high demand. Here we present a systematic ab initio study on the replacement of toxic Pb in the perovskite CH3NH3PbI3 (MAPbI3) with possible mono- and a few binary replacements. In the mono-replacements study, Ge and Sn are the best alternatives to Pb. In the binary replacements, we replace Pb by mixing Ca/Si and Zn/Si. In case of Ca/Si, a monotonic decrease in band gaps with a monotonic increase in the optical absorption was observed with increasing the Ca concentration. It is observed for the first time that the substitution of Ca/Si (or Zn/Si) at the B-site with various ratios would lead to remarkably high device absorption efficiencies. The band gaps of the studied mixed replacements are in the ideal ranges for single-junction...

In just a few years the efficiency of organic–inorganic perovskites has reached 22.1%. Long term stability and lead toxicity are their two main hurdles to overcome for commercialization as solar cells. Keeping these significant challenges in mind, we use ab initio density functional theory (DFT) and incorporate mixed Ca/(Ge, Sn) and Sr/(Ge, Sn) at the B-site of CH3NH3PbI3 (MAPbI3) to get Pb-free perovskites with excellent properties. Tunability of the band gaps is observed while decreasing the ratios of Sn and Ge. The predicted band gaps (

The stability, geometric structure and electronic properties of a novel body-centered tetragonal carbon allotrope Bct C 8 are systematically investigated. Bct C 8 are formed by sp 3 -bonded carbon atoms and can be regarded as a compressed bundle of carbon nanotubes (CNT). The transition path from CNT to Bct C 8 has been simulated, indicating a possible approach to synthesize Bct C 8 . The X-Ray diffraction, phonon and Raman spectroscopies are presented as reference for experimental studies. Besides, Bct C 8 is a semiconductor with an indirect gap of 1.66 eV and will transform to semimetal when doping with boron and nitride atoms. Furthermore, considering its porous structural character, the lithium storage capability of Bct C 8 are also discussed, a directional fast lithium-ion mobility with a rather low 0.04 eV barrier is revealed. Therefore, the Bct C 8 , once synthesized experimentally, would have fruitful applications in carbon-based electronics and energy storage.

Mn-PBP is discovered to be the first ferromagnetic 2D MOF with the Shastry-Sutherland lattice and the predicted Curie temperature is 105 K., Inspired by the successful synthesis of Fe/Cu-5,5′-bis(4-pyridyl)(2,2′-bipirimidine) (PBP), a family of two-dimensional (2D) metal–organic frameworks (MOFs) with the Shastry-Sutherland lattice, i.e., transition metal (TM)-PBP (TM = Cr, Mn, Fe, Co, Ni, Cu, Zn) has been systematically investigated by means of first-principles density functional theory calculations and Monte Carlo simulations. Mn-PBP is discovered to be the first ferromagnetic 2D MOF with the Shastry-Sutherland lattice and the Curie temperature is predicted to be about 105 K, while Fe-PBP, TM-PBP (TM = Cr, Co, Ni) and TM-PBP (TM = Cu, Zn) are found to be stripe-order antiferromagnetic, magnetic-dimerized and nonmagnetic, respectively. The electronic structure calculations reveal that TM-PBP MOFs are semiconductors with band gaps ranging from 0.12 eV to 0.85 eV, which could be easily modulated by various methods. Particularly, Mn-PBP would exhibit half-metallic behavior under compressive strain or appropriate electron/hole doping and a Mn-PBP based spintronic device has been proposed. This study not only improves the understanding of the geometric, electronic and magnetic properties of the 2D TM-PBP MOF family, but also provides a novel spin lattice playground for the research of 2D magnetic systems, which has diverse modulating possibilities and rich potential applications.

Based on the spinor Boltzmann equation, we obtain the continuity equations of charge and spin accumulation in two-dimensional ferromagnets with Rashba or Dresselhaus spin-orbit coupling, in which the spin diffusion equation (the continuity equation of spin accumulation) gives the spin-orbit torques that have an extra term of charge density gradient that is paid little attention in previous studies. Numerical results indicate this term plays an important role in inhomogeneous systems. In order to study the magnetization dynamics of ferromagnets, we solve the Landau-Lifshitz-Gilbert equation together with the spin diffusion equation. Our results indicate that the magnetization in systems with Rashba SOI is easier to be switched by an external electric field than the system with Dresselhaus SOI, and for anisotropic ferromagnets, we study the critical external electric field that manipulate the reversal of magnetization.

The current-induced magnetic switching of a single-molecule magnet (SMM) attached on the central region of a spin valve is explored, and the condition for the switching current is derived. Electrons flowing through the spin valve will interact with the SMM via the s–d exchange interaction, producing the spin accumulation that satisfies the spin diffusion equation. We further describe the spin motion of the SMM by a Heisenberg-like equation. Based on the linear stability analysis, we obtain the critical current from two coupled equations. The results of the critical current versus the external magnetic field indicate that one can manipulate the magnetic state of the SMM by an external magnetic field.

The properties of diamond polytypes under high pressure have been investigated by means of the first-principles calculations. They are unveiled to be thermodynamically and kinetically more stable than hexagonal diamond (h-diamond) even under pressure. The electronic and elastic properties of the diamond polytypes are also explored, revealing very close cohesive energy, band gap and equilibrium density to those of cubic diamond (c-diamond). The Vickers hardness is uncovered to be comparable or even larger than those of the c- and h-diamond. Furthermore, they experience lower energy barriers than the h-diamond during the transforming process from graphite under pressure. Especially, we explored the characterized Raman modes under pressure from 0 to 40 GPa for each allotrope, which can be used to identify the diamond polytypes from the c-diamond T 2 g background mode, and the result may serve as a useful guide for future experimental studies. In addition, the simulated X-ray diffraction spectra under pressure is also presented.

The geometric, electronic, and magnetic properties of twisted zigzag-edged graphene nanoribbons (ZGNRs) and novel graphene Mobius strips (GMS) are systematically investigated with the first-principles calculations based on the density functional theory. All the structures of ZGNRs and GMS are optimized, and their structural stabilities are examined. The molecular energy levels and the spin polarized density of states of ZGNRs are also calculated. It is found that the atomic bonding energies of the twisted ZGNRs decrease quadratically with the increase of the twisted angle, and the gaps between the lowest unoccupied molecular orbital and the highest occupied molecular orbital are varied with the twisted angle. The spin densities of ZGNRs and GMS reveal that the ground states with antiferromagnetic edges persist during the twisting, and the spin flip at some positions of the zigzag edges of GMS can be observed.

In this paper we present an implementation of the modified Becke–Johnson meta-GGA functional (TB09) in the PWSCF program of Quantum ESPRESSO package via the Libxc library. Using the functionals of TB09 we calculated the band gaps of some semiconductors and compared the results with previous calculations and experiments. We showed that combining GGA pseudo-potential with TB09 functionals would improve greatly the band gaps compared with the GGA calculations. The details of our implementation and code samples are also given.

We systematically investigated the geometric, electronic and thermoelectric (TE) properties of bulk black phosphorus (BP) under strain. The hinge-like structure of BP brings unusual mechanical responses such as anisotropic Young's modulus and negative Poisson's ratio. A sensitive electronic structure of BP makes it transform among metal, direct and indirect semiconductors under strain. The maximal figure of merit ZT of BP is found to be 0.72 at 800 K that could be enhanced to 0.87 by exerting an appropriate strain, revealing BP could be a potential medium-high temperature TE material. Such strain-induced enhancements of TE performance are often observed to occur at the boundary of the direct-indirect band gap transition, which can be attributed to the increase of degeneracy of energy valleys at the transition point. By comparing the structure of BP with SnSe, a family of potential TE materials with hinge-like structure are suggested. This study not only exposes various novel properties of BP under strain, but also proposes effective strategies to seek for better TE materials.

The anisotropic antiferromagnetic Ising model on the fractal Sierpi\'{n}ski gasket is intensively studied, and a number of exotic properties are disclosed. The ground state phase diagram in the plane of magnetic field-interaction of the system is obtained. The thermodynamic properties of the three plateau phases are probed by exploring the temperature-dependence of magnetization, specific heat, susceptibility and spin-spin correlations. No phase transitions are observed in this model. In the absence of a magnetic field, the unusual temperature dependence of the spin correlation length is obtained with $0 \leq$J$_b/$J$_a, Comment: 9 pages, 12 figures

By means of extensive ab initio calculations, a new two-dimensional (2D) atomic material tin selenide monolayer (coined as tinselenidene) is predicted to be a semiconductor with an indirect gap (1.45 eV) and a high hole mobility (of order 10000 cm2V-1S-1), and will bear an indirect-direct gap transition under a rather low strain (, 25 pages, 4 figures, 1 table

The isosteric heat of adsorption was used to study the interaction between hydrogen molecules and the Multi-Walled Carbon Nanotubes (MWCNTs). Characterizations of the MWCNTs sample were carried out based on the N2 adsorption isotherm at 77 K and the images from TEM and HRTEM. Step by step method was used to volumetrically measure hydrogen adsorption isotherms at equilibrium temperature-pressures from 123-310 K and 0-12.3 MPa. Isosteric heats of adsorption at seven excess adsorption amounts and that at zero surface loading were respectively determined by the slopes of the adsorption isosteres and the plot of the temperature dependence of the Henry’s constants. Results show that the limit of the isosteric heat of adsorption at zero surface loading is about and the mean under the experimental condition is about . The values are in the same grade as those of hydrogen on the activated coconut charcoal but smaller than those of hydrogen on the graphitized carbon black P33, the activated carbon AX-21 and the Single-Walled Carbon Nanotubes (SWCNTs). Conclusions are drawn that relatively lower adsorption amounts and the isosteric heat of hydrogen adsorption on the MWCNTs could be ascribed to the small specific surface area and the large mesopores.

For developing carbon based adsorbents for hydrogen storage, Staram PCTPro E&E was employed and activated carbon SAC-02 having specific surface area about 2074m2/g was selected to measure isotherms of hydrogen adsorption respectively from temperature range 77.15K-110.15K and 253.15K-293.15K. Isosteres of hydrogen adsorption on the activated carbon was then plotted to determine the isosteric heat of hydrogen adsorption, and temperature dependent of Henry law constants were further applied to set the limit isosteric heat of adsorption at zero surface coverage. Results show that the isosteric heat of adsorption is about 3.6-5.4kJ/mol with a mean value 4.38kJ/mol at zero surface coverage. Results also reveal that adsorption isosteres of adsorption data at different temperature regions bring about a different isosteric heat of adsorption due to the variation in contributions from thermal motion of adsorbate molecules. Conclusions are drawn that adsorption isosteres on the adsorption data in correspondence with the lowest and highest temperatures of the hydrogen storage system should be carried out to determine the isosteric heat of adsorption for effectively managing the thermal effect.

Five representative coal samples used for coking were carbonized in a thermogravimetric analyzer to simulate an industrial coking process. The gaseous organic compounds generated were analyzed with a coupled mass spectrometer. During coal carbonization, thermal detachment of aliphatic side groups causes disintegration of the coal structure. Methyl groups detach at higher temperatures than methylene and methine groups, and the temperatures corresponding to peak generation of these groups increase with the metamorphic grade of the coal. Methane is generated by three mechanisms: below 370 °C, methane adsorbed in coal is thermally released; at about 510 °C, methyl groups are thermally detached from the coal to form CH3+ ions, which further combine with hydrogen to form methane; finally, at about 720 °C, methane is produced as a result of the condensation of aromatic rings to form larger fused rings. Benzene is also generated by three mechanisms: at 400–500 °C, aromatic structures in coal lose side groups (e.g. methylene or methine) to form benzene ions, which subsequently react with hydrogen to form benzene; at 500–700 °C, benzyl structures in coal lose methyl groups to form benzene ions, which then combine with hydrogen to form benzene; finally, at about 800 °C, condensation of fragments in coal also forms benzene.

The stable boron carbon nanotube superlattices (BCNTSLs) that are constructed by periodically connecting carbon nanotube (CNT) and boron nanotube (BNT) with different lengths and diameters are predicted by employing the density functional first-principles calculations. The geometrical and electronic structures as well as quantum conductance of BCNTSLs are studied. It is found that the superlattices can be metallic or semiconducting depending on tube diameters and the ratio of BNT to CNT segments in a periodic unit. The confined states in the superlattice are observed. The present study could offer a useful way for designing some functional nanodevices.

The geometrical structures, electronic properties, and spectroscopies of non-IPR (isolated pentagon rule) fullerene C 66 and its derivatives C 66 X 4 (X = H, F, Cl) have been studied by the first-principle calculations based on the density functional theory. By searching through all 4478 isomers of C 66 , the ground state of C 66 is observed to bear C s symmetry and have two pairs of fusion pentagons. It is found that the addition of H, F, Cl atoms to the pentagon–pentagon fusion vertex of C 66 cages can form new non-IPR fullerenes such as C 66 X 4 , where the molecule C 66 X 4 with C 2 v symmetry are uncovered to be the most stable among others. The Mulliken charge populations, gap energies between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO), and density of states of these unconventional fullerenes are calculated, showing that different atoms added to non-IPR C 66 cages will alter the charge populations remarkably; the chemical deriving could affect the electronic structures distinctly, and improve the stability of the fullerenes. The calculated results of IR, Raman, NMR spectra of C 66 X 4 are also presented.

A systematic first-principles study within density functional theory on the geometrical structures and electronic properties of unconventional fullerene C64 and its derivatives C64X4 (X = H, F, Cl, Br) has been performed. By searching through all 3465 isomers of C64, the ground state of C64 is found to be spherical shape with D2 symmetry, which differs from the parent cage of the recently synthesized C64H4 that is pear-shaped with C3v symmetry. We found that the addition of the halogen atoms like F, Cl, and Br to the pentagon−pentagon fusion vertex of C64 cage could enhance the stability, forming the unconventional fullerenes C64X4. The Mulliken charge populations, LUMO−HOMO gap energies, and density of states are calculated, showing that different halogen atoms added to C64 will cause remarkably different charge populations of the C64X4 molecule; the chemical deriving could enlarge the energy gaps and affect the electronic structures distinctly. It is unveiled that C64F4 is even more stable than C64H4, a...

The effect of the spin accumulation on the spin-dependent transport properties in a ferromagnet–quantum dot (QD)–ferromagnet tunnel junction is reexamined in a self-consistent manner by means of Keldysh nonequilibrium Green function method. It has been found that when the spin and charge densities accumulated in the central QD are self-consistently treated, the electrical current and spin current differ from the previous results where the chemical potential shifts induced by the spin imbalance are not included, and the tunnel magnetoresistance is enhanced remarkably. The spin and charge accumulations in the QD are also discussed.

A recent elegant experimental realization [S. Yuasa et al., Science 297 (2002) 234] of the spin-polarized resonant tunneling in magnetic tunnel junctions is interpreted in terms of a two-band model. It is shown that the tunnel magnetoresistance (TMR) decays oscillatorily with the thickness of the normal metal (NM) layer, being fairly in agreement with the experimental observation. The tunnel conductance is found to decay with slight oscillations with the increase of the NM layer thickness, which is also well consistent with the experiment. In addition, when the magnetizations of both ferromagnet electrodes are not collinearly aligned, TMR is found to exhibit sharp resonant peaks at some particular thickness of the NM layer. The peaked TMR obeys nicely a Gaussian distribution against the relative orientation of the magnetizations.

The electronic structures of haloform intercalated molecular solids C60·2CHX3 (X=Cl,Br) at low temperature are investigated by a first-principle calculation. It is found that the density of states of these solids at the Fermi level are quite low, and a phenomenon of hole doping induced insulator–metal transition in these systems is observed at low temperature.

We have investigated the current-induced spin transfer torque of a ferromagnet-insulator-ferromagnet tunnel junction by taking the spin-flip scatterings into account. It is found that the spin-flip scattering can induce an additional spin torque, enhancing the maximum of the spin torque and giving rise to an angular shift compared to the case when the spin-flip scatterings are neglected. The effects of the molecular fields of the left and right ferromagnets on the spin torque are also studied. It is found that $\tau ^{Rx}/I_{e}$ ($\tau ^{Rx}$ is the spin-transfer torque acting on the right ferromagnet and $I_{e}$ is the tunneling elcetrical current) does vary with the molecular fields. At two certain angles, $\tau ^{Rx}/I_{e}$ is independent of the molecular field of the right ferromagnet, resulting in two crossing points in the curve of $\tau ^{Rx}/I_{e}$ versus the relevant orientation for different molecular fields., Comment: Revtex, 11 papes, 4 figures, appendix

By means of the nonequilibrium Green function technique, the effect of spin-flip scatterings on the spin-dependent electrical transport in ferromagnet-insulator-ferromagnet (FM-I-FM) tunnel junctions is investigated. It is shown that Julliere's formula for the tunnel conductance must be modified when including the contribution from the spin-flip scatterings. It is found that the spin-flip scatterings could lead to an angular shift of the tunnel conductance, giving rise to the junction resistance not being the largest when the orientations of magnetizations in the two FM electrodes are antiparallel, which may offer an alternative explanation for such a phenomenon observed previously in experiments in some FM-I-FM junctions. The spin-flip assisted tunneling is also observed., Comment: Revtex, 4 figures

The effect of nonmagnetic impurities and the effective masses on the spin-dependent transport in a ferromagnet-normal metal-ferromagnet junction is investigated on the basis of a two-band model. Our results show that impurities and the effective masses of electrons in two ferromagnetic electrodes have remarkable effects on the behaviors of the conductance, namely, both affect the oscillating amplitudes, periods, as well as the positions of the resonant peaks of the conductance considerably. The impurity tends to suppress the amplitudes of the conductance, and makes the spin-valve effect less obvious, but under certain conditions the phenomenon of the so-called impurity-induced resonant tunneling is clearly observed. The impurity and the effective mass both can lead to nonmonotonous oscillation of the junction magnetoresistance (JMR) with the incident energy and the thickness of the normal metal. It is also observed that a smaller difference of the effective masses of electrons in two ferromagnetic electrodes would give rise to a larger amplitude of the JMR., Comment: Revtex, 10 figures

We systematically investigated the geometric, electronic and thermoelectric (TE) properties of bulk black phosphorus (BP) under strain. The hinge-like structure of BP brings unusual mechanical responses such as anisotropic Young's modulus and negative Poisson's ratio. A sensitive electronic structure of BP makes it transform among metal, direct and indirect semiconductors under strain. The maximal figure of merit $ZT$ of BP is found to be 0.72 at $800\,\mathrm{K}$ that could be enhanced to 0.87 by exerting an appropriate strain, revealing BP could be a potential medium-high temperature TE material. Such strain-induced enhancements of TE performance are often observed to occur at the boundary of the direct-indirect band gap transition, which can be attributed to the increase of degeneracy of energy valleys at the transition point. By comparing the structure of BP with SnSe, a family of potential TE materials with hinge-like structure are suggested. This study not only exposes various novel properties of BP under strain, but also proposes effective strategies to seek for better TE materials., Comment: 6 figures. Supplemental Information is available in Scientific Reports and also involved in this source package

In this paper, we apply Buttiker's gauge invariant, charge conservation, nonlinear transport theory to explore the spin-polarized tunneling of ferromagnet/insulator (semiconductor) single and double junctions. The Green function of spin-polarized tunneling is calculated by the tight-binding approximation method. The energy and the angle (between the molecular field and the vertical axis) dependences of the weakly nonlinear dc transport coefficient and the linear low frequency ac transport coefficient are investigated. The ac tunneling magnetoresistance is also discussed.

Based on the double exchange model, together with superexchange interaction and Coulomb interaction, the ground state of a lightly doped manganese system is studied in the semi-classical approximation of spins. It is shown that for a lightly doped system, the ground state energy of a canted state is lower than that of a spiral state; a canted state is therefore deduced to be more stable than a spiral state. This result is in agreement with experiments.

The present investigation searched for new boron nitride (BN) polymorphs by means of first-principles simulations. The ab initio random structure searching strategy was implemented. The electronic and mechanical properties and equation of states of three new metastable B4N4 crystal forms with equilibrium energies close to the most stable BN form, c-BN, are presented. Results show either dynamically stable semiconductors or insulators, one of which is even slightly harder than c-BN. The equation of states is also presented and a phase transition is predicted. (Some figures may appear in colour only in the online journal)

We investigate the electronic transport properties of a ferromagnet/normal/ferromagnet junction on the surface of a topological insulator with a gate voltage exerted on the normal segment. It is found that the conductance oscillates with the width of normal segment and gate voltage, and the maximum of conductance gradually decreases while the minimum of conductance approaches zero as the width increases. The conductance can be controlled by tuning the gate voltage like a spin field-effect transistor. It is found that the magnetoresistance ratio can be very large, and can also be negative owing to the anomalous transport. In addition, when there exists a magnetization component in the surface plane, it is shown that only the component parallel to the junction interface has an influence on the conductance., 7 pages,8 figures

By means of the first-principles calculations combined with the tight-binding approximation, the strain-induced semiconductor-semimetal transition in graphdiyne is discovered. It is shown that the band gap of graphdiyne increases from 0.47 eV to 1.39 eV with increasing the biaxial tensile strain, while the band gap decreases from 0.47 eV to nearly zero with increasing the uniaxial tensile strain, and Dirac cone-like electronic structures are observed. The uniaxial strain-induced changes of the electronic structures of graphdiyne come from the breaking of geometrical symmetry that lifts the degeneracy of energy bands. The properties of graphdiyne under strains are disclosed different remarkably from that of graphene., 7 pages, 7 figures

The thermoelectric properties of cubic zincblend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large ZT, we study possible energetically stable configurations, and disclose that, when N dopants locate at the center, a small number of Si vacancies at corners are most favored for n-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for p-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the ZT value for the n-type is shown capable of reaching 1.78 at 900K. The conditions to get higher ZT values for longer SiCNWs are also addressed., 9 pages, 10 figures

By means of first-principles calculations within the density functional theory, we study the structural and optical properties of codoped ZnO nanowires and compare them with those of the bulk and film. It is disclosed that the low negatively charged ground states of nitrogen-related defects play a key role in the optical absorption spectrum tail that narrows the band gap and enhances the photoelectrochemical response significantly. A strategy of uncompensated N, P, and Ga codoping in ZnO nanowires is proposed to produce a redshift of the optical absorption spectra further than the exclusive N doping and to get a proper formation energy with a high defect concentration and a suppressed recombination rate. In this way, the absorption of the visible light can be improved and the photocurrent can be raised. These observations are consistent with the existing experiments, which will be helpful to improve the photoelectrochemical responses for the wide-band-gap semiconductors, especially in water splitting applications.

A complete procedure is given by which one can construct q-deformed KdV hierarchies and obtain their exact solutions with arbitrarily many q-deformed solitons. As an elementary example, we obtain the exact solution of the q-deformed single soliton of the simplest (3,2) system, i.e. the q-deformed KdV equations, and investigate this solution in the case of small deformation. It is interesting that this method can supply many exact solutions for systems of new conventional nonlinear partial differential equations.

A structurally stable crystalline carbon allotrope is predicted by means of the first-principles calculations. This allotrope can be derived by substituting each atom in diamond with a carbon tetrahedron, and possesses the same space group $Fd\overline{3}m$ as diamond, which is thus coined as T-carbon. The calculations on geometrical, vibrational, and electronic properties reveal that T-carbon, with a considerable structural stability and a much lower density $1.50\text{ }\text{ }\mathrm{g}/{\mathrm{cm}}^{3}$, is a semiconductor with a direct band gap about 3.0 eV, and has a Vickers hardness 61.1 GPa lower than diamond but comparable with cubic boron nitride. Such a form of carbon, once obtained, would have wide applications in photocatalysis, adsoption, hydrogen storage, and aerospace materials.

The superspace formulation of the N = 1 superconformal field theory and superconformal Ward identities are used to give a precise definition of fusion. Using the fusion procedure, superconformally covariant differential equations are derived and consequently a complete and straightforward algorithm for finding null vectors in Verma modules of the Neveu-Schwarz algebra is given.

Based on ab initio calculations, we have studied the geometrical, electronic properties and chemical bonding of boron fullerenes B(32+8k) (0 < or = k < or = 7) with four-membered rings and B(32) solid phases. The relative energies and the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been calculated, showing that the stabilities grow with the increase of fullerene size, where the smallest cage B(32) bears the largest HOMO-LUMO gap. The frontier orbitals of B(32+8k) show some similarities with those of the corresponding carbon fullerenes C(24+6k), implying that they may have similar chemical properties. It is found that B(32) cages can condense to form solid phases of simple cubic (sc), face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, where the bct phase is observed to be the most stable. Electronic structure calculations reveal that the sc, fcc and bcc phases of B(32) solids are metallic, but the bct phase is a semimetal.

The spin transfer effect in ferromagnet-quantum dot (insulator)-ferromagnet Aharonov-Bohm (AB) ring system with Rashba spin-orbit (SO) interactions is investigated by means of Keldysh nonequilibrium Green function method. It is found that both the magnitude and direction of the spin transfer torque (STT) acting on the right ferromagnet electrode can be effectively controlled by changing the magnetic flux threading the AB ring or the gate voltage on the quantum dot. The STT can be greatly augmented by matching a proper magnetic flux and an SO interaction at a cost of low electrical current. The STT, electrical current, and spin current are uncovered to oscillate with the magnetic flux. The present results are expected to be useful for information storage in nanospintronics., Comment: 17pages, 7figures

Using the fusion procedure, a complete and straightforward algorithm for finding null vectors in Verma modules of the {ital N}=1 superconformal algebra is given.

Based on ab initio calculations, we have studied the geometrical, electronic properties and chemical bonding of boron fullerenes B(32+8k) (0 < or = k < or = 7) with four-membered rings and B(32) solid phases. The relative energies and the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been calculated, showing that the stabilities grow with the increase of fullerene size, where the smallest cage B(32) bears the largest HOMO-LUMO gap. The frontier orbitals of B(32+8k) show some similarities with those of the corresponding carbon fullerenes C(24+6k), implying that they may have similar chemical properties. It is found that B(32) cages can condense to form solid phases of simple cubic (sc), face-centered cubic (fcc), body-centered cubic (bcc), and body-centered tetragonal (bct) structures, where the bct phase is observed to be the most stable. Electronic structure calculations reveal that the sc, fcc and bcc phases of B(32) solids are metallic, but the bct phase is a semimetal.

By means of first-principles calculations within the density-functional theory, we find that stable face-centered-cubic (fcc) ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids can be formed. For both solids, two possibly stable geometrical phases (identified as phase $A$ and phase $B$) with different lattice parameters can exist, where phase $A$ has a lattice parameter smaller than phase $B$. In phase $A$, ${\text{B}}_{80}$ clusters are significantly distorted and two or four intercluster covalent bonds are formed for ${\text{K}}_{3}{\text{B}}_{80}$ or ${\text{Mg}}_{3}{\text{B}}_{80}$, respectively. In phase $B$, ${\text{B}}_{80}$ units are slightly distorted and no intercluster covalent bonds exist. The phase $A$ of ${\text{Mg}}_{3}{\text{B}}_{80}$ bears the largest cohesive energy among them and is more stable than the fcc ${\text{B}}_{80}$ solid. The charge population analysis shows that K and Mg are ionized and donate electrons to the other boron atoms of ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids. The different ionic radii of K and Mg lead to major geometrical differences between ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids and the competition of the covalent and ionic bondings could explain the emergence of two different geometrical phases for both. The electronic structural calculations reveal that both fcc ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids are metals.