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Now showing items 17 - 32 of 89

  • Electronic Structures and Magnetism of CaFeAsH and CaFeAsF

    Wang, Guangtao   Shi, Xianbiao   Liu, Haipeng   Liu, Qingbo  

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  • Electronic Structures and Magnetism of CaFeAsH and CaFeAsF

    Wang, Guangtao   Shi, Xianbiao   Liu, Haipeng   Liu, Qingbo  

    We studied the electronic structures, magnetism, and Fermi surface (FS) nesting of CaFeAsH and CaFeAsF by first-principles calculations. In the nonmagnetic (NM) states, we found strong FS nesting, which induces magnetic instability and a spin density wave (SDW). Our calculations indicate that the ground state of CaFeAsH and CaFeAsF is the stripe antiferromagnetic state. The calculated bare susceptibility chi 0(q) peaked at the M-point and was clearly suppressed and became slightly incommensurate with both electron doping and hole doping for both materials.
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  • Topological phase transition in half-Heusler compounds HfIrX (X=As, Sb, Bi)

    Wang, Guangtao   Wei, JunHong  

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  • Impedance matching study of surface dielectric barrier discharge plasma generator

    Xu, Guangke   Zhang, Weiwei   Wang, Nan   Wang, Guangtao   Yuan, Shaopu  

    The plasma produced by Dielectric Barrier Discharge (DBD) has a very important application in the field of aerodynamics. The plasma generator needs high-voltage and high-frequency power supply. Due to the existence of internal resistance of the power supply, the impedance characteristics of the generator do not match the power supply. The voltage at both ends of the generator is extremely low. Besides, due to the existence of reactive power, most of the energy transported by the power source is reflected, resulting in extremely low energy conversion efficiency of the power source. In the paper, the mechanism of DBD plasma generator is analysed and on the equivalent circuit is received. And then different frequency impedance characteristics of generator and impedance matching circuit by using LR and the LCR respectively is studied. The results show that the equivalent resistance of plasma generator on frequency change is not obvious. The generator is capacitive and tends to 0 with increasing frequency. When LR is matched, resistance increases with the frequency change is not big, and reactance increases gradually, but its value is small, in the range of n Omega. in general the plasma generator impedance is virtually pure resistive. When LCR is matched, the impedance characteristic gradually changes from the inductive to capacitive with the increase of frequency. (C) 2019 Published by Elsevier Ltd.
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  • Excitonic effects on layer- and strain-dependent optoelectronic properties of PbI2

    Shen, Chenhai   Wang, Guangtao  

    Exciton states have obvious effects on optical properties of two-dimensional (2D) materials. Here, we investigate excitonic effects on electronic and optical properties of PbI2 by using G(0)W(0 )+ Bethe-Salpeter equation method, considering the layer number and strain effect. These studies show that exciton states obviously modify and dominate the optical absorption of 2D PbI2 nanosheets. Also, with the increasing number of layers, the intensity of main absorption peak increases and the exciton binding energy decreases. Meanwhile, the tensile strain can induce the threshold energy of optical spectra shift down the low energy, and exciton binding energy has a maximum at the strain of 3%. Therefore, our results indicate that the 2D PbI2 nanomaterials have excellent ultraviolet absorption and corresponding potential for the application of optoelectronic devices.
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  • The electronic structure and magnetism of BaTi2Sb2O

    Wang, Guangtao   Zhang, Huiping   Zhang, Lin   Liu, Chang  

    The electronic structure and magnetic structures of recently discovered BaTi 2Sb 2O were studied from first-principles calculations. In the nonmagnetic state, the density of states at the Fermi level are mostly derived from the d z2,d x2-y2, and dxy orbitals. The Fermi surface (FS) consists of three sheets. The first FS sheet (along X-R line) will nest with the third FS sheet (along M-A line) by the q-vector qoarr 1=(pi/a, 0, 0) or qoarr 2=(0,pi/a, 0) shift. The calculated bare susceptibility chi 0(q) peaked at X-point, rather than at the M-point in the FeAs-based superconductor. Such peaked susceptibility induces the spin density wave, and the magnetic ground state is nearly two degenerate antiferromagnetism the bi-collinear antiferromagnetism (AF3) or the blocked checkerboard antiferromagnetism (AF4). The peak of susceptibility chi 0(q) is obviously suppressed and become slightly in-commensurate with hole doping, but increased with electron doping. When the spin fluctuation is suppressed, the superconductivity appears. This can explain that why the superconductivity appears only in the hole-doped compounds, not in the electron-doped ones.
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  • The electronic structure of novel BiS2-based layered superconductor

    Wang, Guangtao   Liu, Chang   Zhang, Huiping   Zhang, Lin   Chen, Chen  

    By means of the first-principles calculations, we studied the density of states, band structure and Fermi surface nesting of the recently discovered BiS2 layered superconductor LaO1∿em>xFxBiS2. At x = 0, the density of state and band structure of LaOBiS2 indicate the compound is a insulator with a band gap of 0.15 eV. It becomes metal with F-doping. At x = 0.5, there are four bands crossing the Fermi level. These four bands are mostly derived from the Px and Py orbitals of Bi atoms. Compared with the typical Fe-As based superconductor LaOFeAs, the Fermi surface nesting of LaO1∿em>xFxBiS2 is not as strong as that of LaOFeAs.
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  • The electronic structure of novel BiS2-based layered superconductor

    Wang, Guangtao   Liu, Chang   Zhang, Huiping   Zhang, Lin   Chen, Chen  

    By means of the first-principles calculations, we studied the density of states, band structure and Fermi surface nesting of the recently discovered BiS2 layered superconductor LaO1 xFxBiS2. At x = 0, the density of state and band structure of LaOBiS2 indicate the compound is a insulator with a band gap of 0.15 eV. It becomes metal with F-doping. At x = 0.5, there are four bands crossing the Fermi level. These four bands are mostly derived from the P-x and P-y orbitals of Bi atoms. Compared with the typical Fe-As based superconductor LaOFeAs, the Fermi surface nesting of LaO1-xFxBiS2 is not as strong as that of LaOFeAs. (C) 2013 Published by Elsevier B.V.
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  • Excitonic effects on layer- and strain-dependent optoelectronic properties of PbI2

    Shen, Chenhai   Wang, Guangtao  

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  • The electronic structure and magnetism of BaTi2Sb2O

    Wang, Guangtao   Zhang, Huiping   Zhang, Lin   Liu, Chang  

    The electronic structure and magnetic structures of recently discovered BaTi2Sb2O were studied from first-principles calculations. In the nonmagnetic state, the density of states at the Fermi level are mostly derived from the d(z2); d(x2-y2), and d(xy) orbitals. The Fermi surface (FS) consists of three sheets. The first FS sheet (along X-R line) will nest with the third FS sheet (along M-A line) by the q-vector (q) over right arrow (1) = (pi/a, 0, 0) or (q) over right arrow (2) = (0, pi/a, 0) shift. The calculated bare susceptibility chi(0)(q) peaked at X-point, rather than at the M-point in the FeAs-based superconductor. Such peaked susceptibility induces the spin density wave, and the magnetic ground state is nearly two degenerate antiferromagnetism the bi-collinear antiferromagnetism (AF3) or the blocked checkerboard antiferromagnetism (AF4). The peak of susceptibility chi(0)(q) is obviously suppressed and become slightly in-commensurate with hole doping, but increased with electron doping. When the spin fluctuation is suppressed, the superconductivity appears. This can explain that why the superconductivity appears only in the hole-doped compounds, not in the electron-doped ones. (C) 2013 AIP Publishing LLC.
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  • Electronic Structure and Magnetism of the Multiband New Superconductor CaRbFe4As4

    Shi, Xianbiao   Wang, Guangtao  

    By first-principles calculations, we investigated the electronic structure, magnetism, and Fermi surface (FS) nesting of the newly discovered superconductor CaRbFe4As4. In the nonmagnetic (NM) state, there are ten bands crossing the Fermi level, which is more complicated than other FeAs-based superconductors, showing a multiband character. The FS consists of six holelike sheets around the Gamma-point and four electronlike sheets near the Brillouin zone corner M-point. The holelike FSs will overlap with the electronlike FS sheets, if they are shifted by the vector q =3D (pi, pi, 0). Such FS nesting induces the bare susceptibility peak chi(0)(q) at the M-point. Fixed spin moment calculations indicate that CaRbFe4As4 has a strong tendency towards magnetism. Total energy calculations predicted that the ground state of CaRbFe4As4 is the stripe antiferromagnetic state. Therefore, CaRbFe4As4 is strongly similar to other FeAs-based superconductors, although it crystallized with a unique crystal structure.
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  • Properties of half-Heusler compounds TaIrGe by using first-principles calculations

    Wei, JunHong   Wang, Guangtao  

    The electronic structures, optical and thermoelectric properties of ternary half-Heusler compound TaIrGe were investigated by using the first-principles and Boltzmann transport theory. Spin-orbit coupling (SOC) removed the degeneracy of VBM, and then decreased the Seebeck coefficients and power factor. From the compressive to tensile strain, the band gap gradually increases from 0.96 to 1.11 eV, accompanied by the absorption coefficient peak red-shift. The effective mass (m*(DOS)) of VBM and CBM gradually increases from the compressive to tensile strain, which enhances the Seebeck coefficient and power factor. Our results indicate that the electronic structures, optical and thermoelectric properties of TaIrGe can be effectively tuned by the strain and TaIrGe can be used as an important photoelectric and thermoelectric material in the future.
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  • A novel feature subset selection algorithm based on association rule mining

    Wang, Guangtao   Song, Qinbao  

    In this paper, a novel feature selection algorithm FEAST is proposed based on association rule mining. The proposed algorithm first mines association rules from a data set; then, it identifies the relevant and interactive feature values with the constraint association rules whose consequent is the target concept, detects and eliminates the redundant feature values with the constraint association rules whose consequent and antecedent are both of single feature value. Finally, it obtains the feature subset by mapping the feature values to the corresponding features. As the support and confidence thresholds are two important parameters in association rule mining and play a vital role in FEAST, a partial least square regression (PLSR) based threshold prediction method is presented as well. The effectiveness of FEAST is tested on both synthetic and real world data sets, and the classification results of five different types of classifiers with seven representative feature selection algorithms are compared. The results on the synthetic data sets show that FEAST can effectively identify irrelevant and redundant features while reserving interactive ones. The results on the real world data sets show that FEAST outperforms other feature selection algorithms in terms of classification accuracies. In addition, the PLSR based threshold prediction method is performed on the real world data sets, and the results show it works well in recommending proper support and confidence thresholds for FEAST.
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  • Thermoelectric and topological properties of half-Heusler compounds ZrIrX(As,Sb,Bi)

    Zhang, Minping   Wei, Junhong   Wang, Guangtao  

    The strain dependent electronic structures, thermoelectric and topological properties of the half-Heusler compounds ZrIrX(X=3DAs, Sb, Bi) are investigated by the first-principle calculations. At the equilibrium lattice constants, all the three compounds are trivial insulators and good thermoelectric materials with the Seebeck coefficient S and the power factor over relaxation time S-2 sigma/tau as large as 1180 (mu V/K) and 4.1 (10(11)Wm(-1)K(-2)s(-1)), respectively. The compressive strain enhances the band gap, while the tensile strain decreases the band gap. At some specific tensile strains, the compounds become Dirac-semimetals, with the s-type band Gamma(6) below p-type band Gamma(8), in the cubic phase. When we compress the a(b)-axis and elongate the c-axis of the compounds, they become the type-I Weyl semimetals. For ZrIrAs, the eight Weyl-Points (WPS) locate at (+/- K-x, 0, +/- K-z), (0, +/- K-y, +/- K-z), K-x =3D K-y =3D 0.008 angstrom(-1), K-z =3D 0.043 angstrom(-1). (C) 2018 Elsevier B.V. All rights reserved.
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  • The electronic structure and magnetism of CaFeAs2: First principles calculations

    Wang, Guangtao   Shi, Xianbiao   Zhang, Lin   Yi, Xia  

    The electronic structure, magnetism and Fermi surface (FS) nesting of the recently discovered superconductive parent material CaFeAs2 are studied by the first-principles, based on the GGA and GGA+U methods. In the nonmagnetic state, the density of states at the Fermi level are mostly derived from the d(xy), d(yz) and d(zx) orbits, just like LaOFeAs. The Fermi surfaces consist of four hole like FS sheets around the Gamma-point, two electron like sheets near the Brillouin zone corner M-point, and small pockets near X-point. The hole like Fermi surfaces will strongly overlap with the electron like FS sheets, if they are shifted by the q-vector q = (pi, pi, 0). Such FS nesting will induce the magnetic instability and spin density wave (SDW), which has been confirmed to be more stable than other states by the calculated total energy. The calculated bare susceptibility chi(0)(q) peaked at M-point, and was obviously suppressed with the electron doping. This explains the emergence of the superconductivity in the electron-doped compound Ca1-xLaxFeAs2, because the electron doping suppressed the SDW and induced the superconductivity. (C) 2014 Elsevier Ltd. All rights reserved.
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  • The electronic structure of ScVO3

    Wang, Guangtao   Yang, Jubao   Liu, Chang   Zhang, Minping  

    The electronic, magnetic and orbital structures of ScVO3 are studied by first-principles calculations. The local density approximation (LDA) results show that the ground state is ferromagnetic (FM) state, with a moment of 1.70 mu(B) on V3+ ion. While LDA+U results indicate that the ground state is G-type spin order (G-SO), with a moment of 1.75 mu(B) on V3+, which agrees with experiment results. The two electrons of V3+ occupy the d(zx) and d(zy), orbitals. The spin order and orbital order structures of ScVO3 are different from those of LaVO3 (C-SO, and G-OO) and YVO3 (G-SO, and C-OO) due to the different Jahn-Teller distortions. (C) 2012 Elsevier Ltd. All rights reserved.
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