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Now showing items 81 - 89 of 89

  • Improving the thermoelectric performance of p-type PbSe via synergistically enhancing the Seebeck coefficient and reducing electronic thermal conductivity

    Huang, Zhiwei   Wang, Dongyang   Li, Caiyun   Wang, Jinfeng   Wang, Guangtao   Zhao, Li-Dong  

    In this work, the thermoelectric performance of p-type PbSe was improved dramatically by enhancing the Seebeck coefficient and reducing electronic thermal conductivity through CdTe alloying. CdTe alloying effectively increased the effective mass of p-type Pb0.98Na0.02Se due to band flattening and promoting the valence band convergence, which contributed to a high Seebeck coefficient of similar to 238 mu V K-1 at 860 K and large power factor of similar to 18.2 mu W cm(-1) K-2 at 860 K for Pb0.98Na0.02Se + 4% CdTe. Furthermore, CdTe alloying reduced the carrier mobility of Pb0.98Na0.02Se due to the enhanced scattering of impurity atoms, which results in a remarkable reduction of electronic thermal conductivity. As a result, due to its higher power factor and lower thermal conductivity, a high thermoelectric figure of merit ZT similar to 1.5 at 860 K was achieved for Pb0.98Na0.02Se + 4% CdTe. This paper provides one novel approach to enhance the thermoelectric performance of p-type PbSe through synergistically manipulating electrical and thermal transport properties.
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  • First-principles study on mechanical and magnetic properties of the perovskite and post-perovskite polymorphs of pure end-member FeSiO3 at the core-mantle boundary RID D-4132-2009 RID A-4982-2009

    Yan, Yumei   Li, Yinwei   Fang, Zhong   Wang, Guangtao   Ma, Yanming  

    We obtained wave velocities of FeSiO3 within perovskite and post-perovskite phases at 130GPa by first-principles calculations in order to understand the abrupt reduction in seismic velocity at the core-mantle boundary. Our results proved that high iron density significantly reduces the seismic velocity. The elastic anisotropy, electronic structure and chemical bonding of FeSiO3 in perovskite and post-perovskite phases are extensively explored to illustrate the variation of mechanical and magnetic properties with pressure. Magnetic collapse was predicted in the perovskite phase, which is attributed to the pressure-induced broadening of 3d valence bands of iron.
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  • Self-hole-doping–induced superconductivity in KCa\r 2\r Fe\r 4\r As\r 4\r F\r 2

    Wang, Guangtao   Wang, Zhenwei   Shi, Xianbiao  

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  • [IEEE 2017 IEEE International Conference on Data Mining Workshops (ICDMW) - New Orleans, LA (2017.11.18-2017.11.21)] 2017 IEEE International Conference on Data Mining Workshops (ICDMW) - Robust Self-Tuning Sparse Subspace Clustering

    Wang, Guangtao   Zhou, Jiayu   Ni, Jingjie   Luo, Tingjin   Long, Wei   Zhen, Hai   Cong, Gao   Ye, Jieping  

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  • [IEEE 2013 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting - Orlando, FL, USA (2013.07.7-2013.07.13)] 2013 IEEE Antennas and Propagation Society International Symposium (APSURSI) - An UWB antenna using modified Sierpinski-carpet Fractal Antenna

    Wang, Guangtao   Shen, Dongya   Zhang, Xiupu  

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  • Origin of the High Activity of the Ceria-Supported Copper Catalyst for H(2)O Dissociation

    Yang, Zongxian   Xie, Luogang   Ma, Dongwei   Wang, Guangtao  

    Ceria-supported copper is a wonderful catalyst for the water-gas shift (WGS) reaction which has been demonstrated experimentally. Using first-principles calculations based on density functional theory (DFT), we identify the mechanisms for the growth of small Cu clusters (Cu(x), x = 1-4) on ceria and the dissociation of H(2)O on the Cu(4)/CeO(2) catalyst. Our calculations indicate that the strong copper oxygen interaction at the Cu4/CeO(2) interface is comparable to the copper copper intracluster interactions, and the competitions between them determine the morphologies of Cu clusters on ceria. H(2)O dissociates with small barriers (0.19-0.31 eV) on the Cu(4)/CeO(2) catalyst, and the highly catalytic activity originates from the enhanced electrostatic interaction between the positively charged Cu sites and the polar H(2)O molecule. The Cu/O interface sites of the ceria-supported copper catalyst are identified as the active sites for H(2)O dissociation. As a buffer to accept/release electrons, the ceria support not only activates the Cu sites but also participates in the H(2)O dissociation reaction at the Cu/O interface.
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  • A compact printed UWB antenna with triple band-notched characteristics

    Xu, Jie   Wang, Guangtao  

    In this letter, a compact printed ultra-wideband antenna with triple band-notched characteristics is presented. The antenna consists of a circular radiating patch and a 50-O coplanar waveguide transmission line. The triple band-notched characteristics are obtained by etching two C-shaped slots on the radiating patch and a pair of symmetric C-shaped slots on the ground. Simulated and measured results show that this antenna could operate from 2.8 to 12.6 GHz with stable radiation patterns, except triple notched bands at 3.433.65, 4.955.25, and 5.365.85 GHz for rejecting the WiMAX and wireless local area network signals. (C) 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2146-2150, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27004
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  • The electronic structure of a weakly correlated antiferromagnetic metal, SrCrO(3): first-principles calculations RID D-4132-2009

    Qian, Yumin   Wang, Guangtao   Li, Zhi   Jin, C. Q.   Fang, Zhong  

    On the basis of our idea of degree modulation, by using systematic first-principles calculations, we study the electronic structure and magnetic properties of SrCrO(3). Our results suggest that SrCrO(3) is a weakly correlated antiferromagnetic (AF) metal, a very rare situation in transition-metal oxides. Among various possible AF states, C-type spin ordering with a small amount of orbital polarization (the d(xy) orbital is more occupied than the d(yz/zx) orbital) is favored. The detailed understanding of the mechanism that stabilizes the C-type AF state is analyzed on the basis of the competition between itinerant Stoner instability and superexchange, and our results suggest that magnetic instability rather than lattice or charge instabilities plays an important role in this system. The experimentally observed c-axis compressed tetragonal distortion can be naturally explained with the C-type AF state. By using the LDA + U method to study this system, we show that the wrong ground state will be obtained if U is large.
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  • Self-hole-doping–induced superconductivity in KCa2Fe4As4F2 Guangtao Wang, Zhenwei Wang and Xianbiao Shi Interaction of superconductivity and magnetism in borocarbide superconductors K-H Müller and V N Narozhnyi Spiral magnetism in the single-band Hubbard model: the Hartree–Fock and slave-boson approaches

    Wen Fong Goh   Warren E. Pickett  

    Unlike the other prominent macroscopic spin-paired quantum state, superconductivity, where the focus is on the strongest coupling within a class, untuned weak magnetism presents an intellectual frontier where electronic structure, magnetic coupling, and (quantum and thermal) fluctuations provide a platform for unconventional phenomena. With a strong van Hove singularity (vHs) at the Fermi energy, TiAu (with no magnetic element) is highly unstable toward ferromagnetism, yet orders antiferromagnetically at a wave vector unrelated to Fermi surface nesting. We show that mirrored vHs provide the long-wavelength, low-frequency fluctuations characteristic of a wFM rather than that of a conventional wAFM, whereby wAFM competes on equal terms with wFM for the broken symmetry ordered state. Peierls-like energy gain from cell doubling can help promote wAFM, which is evident in TiAu. Copyright c © EPLA, 2016 Background. – Weak itinerant ferromagnets (wFMs), conventionally understood as stoichiometric compounds comprised of elements not displaying local moments themselves, are few, and command attention for that reason as well as for their existence at the extreme. Their sister weak anti ferromagnetic systems (wAFM) are practically nonexistent, making the recent identification [1] of the wAFM TiAu a phenomenon in need of understanding. wFM phenomena and materials, reviewed recently [2], provide one of the main platforms to study quantum-critical points, where ordered phases (most often magnetic) appear or vanish at second order, or weakly first-order, phase transitions, accessed by tuning such as by doping or pressure. These long-wavelength (wave vector q → 0) processes are displayed in an assortment of materials [2], with some of the prominent examples being understood in terms of the Stoner instability of the nonmagnetic phase [3,4]. In terms of the magnetic exchange coupling I of states at the Fermi surface (FS), and the Fermi level (EF ) density of states N(EF ), if IN(EF ) > 1 the itinerant system can sacrifice increased kinetic energy by a gain in exchange (magnetic) energy, and ferromagnetism – possibly weak – becomes the stable phase. The theory of such phases including the quantum-critical and thermal fluctuations attending the small order parameter, is well developed by Moriya and collaborators [5–8], Hertz [9], Millis [10], and several others since [2,11]. We address here not critical phenomena but rather the underlying origin of the electronic instability. Moriya’s self-consistent renormalization (SCR) theory of spin fluctuations [7] provides a useful guide for our purposes, as it ties the small ω, small q ≡ | q | (for wave vectors Q; q near the ordering wave vector Q ) behavior to averages of various Fermi surface quantities, thus bringing the focus to the geometry, topology, velocity, and effective mass fields of the FS. wAFMs, and their spin density wave (SDW) cousins, have been discussed theoretically almost exclusively in terms of FS nesting [12,13], and the SDW cases generally support that mechanism. TiAu, however, appears to present its strongest nesting [1] at an incommensurate wave vector that is well separated from what is observed. An added conundrum is that the Fermi energy in TiAu lies almost exactly on a sharp, narrow van Hove singularity (vHs) peak in N(E) that implies a Stoner instability, in which case ferromagnetism or possibly superconductivity, rather than wAFM, is expected to emerge. We have found, however, that a re-analysis of the susceptibility accounting for mirrored vHs and techniques based on first density functional theory can resolve these conundrums, revealing an unanticipated mechanism of AFM ordering in itinerant systems. Properties of TiAu. – TiAu, space group Pmma (#51), is orthorhombic with two formula units (f.u.) per
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