1,721,239 research outputs found
Chemical and Hydrostatic Pressure Effect on Charge Density Waves of SmNiC2
No full textUsing a first-principles density functional theory method, we have investigated the chemical and the hydrostatic pressure effects on the charge density wave (CDW) properties of the quasi-one-dimensional (1D) compound SmNiC2. With increasing pressure, the relative 1D anisotropy of the electronic structure along a direction is enhanced because of its Ni chain structures. From the analysis of the Fermi surface and the generalized susceptibility, we also find that the Fermi surface nesting is enhanced along the modulation vector q(1) = (0.5, 0.52, 0) but is suppressed along q(R) = (0.5, 0.5, 0.5) under pressure. The enhancement of 1D anisotropy of SmNiC2 under pressure is responsible for increasing CDW strength along q(1). We suggest that this quantitative analysis could be used for analysis of the pressure effect on CDW materials.open1176sciescopu
Enhancing the Thermoelectric Properties of Layered Transition-Metal Dichalcogenides 2H-MQ(2) (M = Mo, W; Q = S, Se, Te) by Layer Mixing: Density Functional Investigation
No full textWe explored how to improve the thermoelectric properties of the layered transition-metal dichalcogenides 2H-MQ(2) (M = Mo, W; Q = S, Se, Te) by comparing the thermoelectric properties of hypothetical mixed-layer systems 2H-MQ(2)/2H-MQ(2)', in which two different layers 2H-MQ(2) and 2H-MQ(2)' (Q, Q' = S, Se, Te) alternate, with those of their pure components on the basis of density functional calculations. Our study predicts that the mixed-layer compounds MS2/MTe2 (M = Mo, W) strongly enhance the thermoelectric properties as a consequence of reducing the band gap and the interlayer van der Waals interactions. The layer-mixing is predicted to be a promising way of improving the thermoelectric properties of 2H-MQ(2.)114641sciescopu
Control of valley degeneracy in MoS 2 by layer thickness and electric field and its effect on thermoelectric properties
No full textWe have investigated the valley degeneracy of MoS2 multilayers and its effect on thermoelectric power factors. By modulating the layer thickness and external electric field strength, the hole valleys in the highest energy valence band at Gamma and K points and the electron valleys in the lowest energy conduction band at K and Sigma(min) points are shifted differently. The hole valley degeneracy is observed in MoS2 monolayer, while that of electron valley is in MoS2 bilayer and monolayer under the external electric field. By tuning the valley degeneracy, the Seebeck coefficient and electrical conductivity can be separately controlled, and the maximum power factor can be obtained in n-type (p-type) MoS2 monolayer with (without) the external electric field. We suggest that the transition metal dichalcogenides are good examples to investigate the role of valley degeneracy in the thermoelectric and optical properties with the control of interlayer interaction and external electric field strength.111711Nsciescopu
Magnetic structure of (C5H12N)CuBr3: origin of the uniform Heisenberg chain behavior and the magnetic anisotropy of the Cu2+ (S = 1/2) ions
No full textThe magnetic properties and electric polarization of the organic/inorganic hybrid system (C5H12N)CuBr3 (C5H12N = piperidinium) were examined on the basis of density functional theory calculations. The spin exchanges of (C5H12N)CuBr3 evaluated by energy-mapping analysis show that its uniform Heisenberg antiferromagnetic chain behavior is not caused by the CuBr3 chains made up of edge-sharing CuBr5 square pyramids, but by the two-leg spin ladders resulting from interchain interactions. The magnetic anisotropy of the Cu2+ ions in (C5H12N)CuBr3 originates largely from the Br- ligands rather than the Cu2+ ions. The electric polarization of (C5H12N)CuBr3 arises from the absence of inversion symmetry in the crystal structure, and is weakly affected by the magnetic structure.110sciescopu
Two-dimensional organometallic porous sheets with possible high-temperature ferromagnetism
No full textWith the rapid development of modern nanotechnology, molecular self-assembly has become an important method to fabricate new functional devices, and to provide an arena for theoretical material designs. In this paper, we propose that freestanding two-dimensional organometallic porous sheets (PSs), which can be formed by molecular self-assembly on metal surfaces, are ideal low-dimensional magnetic materials with room-temperature ferromagnetism. Through comprehensive first-principles calculations, we show that the freestanding organometallic sheets, which are assembled by transition metals (TMs) (Mn and V) and benzene molecules, favor ferromagnetic coupling with strong exchange interactions. More importantly, we predict that the Curie-temperature of V-PS is close to room temperature using a simplified mean-field expression, compared to any organometallic sheets discovered previously. In terms of the recent progress in the molecular self-assembly approach, our results indicate great potential for building room-temperature magnetic organometallic sheets with small magnetic molecules.open111615sciescopu
Organic-inorganic hybrid perovskites ABI(3) (A = CH3NH3, NH2CHNH2; B = Sn, Pb) as potential thermoelectric materials: a density functional evaluationt
No full textTo assess the feasibility of the organic-inorganic perovskite iodides ABI(3) (A = CH(3)NH3, NH2CHNH2; B = Sn, Pb; X =1) for thermoelectric applications, we estimated their figures of merit (ZTs) as well as that of Bi2Te3, which is optimized for temperatures around 300 K, as a function of chemical potential on the basis of density functional theory calculations. Our analysis employed the tetragonal structures (P4mm) of (CH3NH3)PbI3 and (CH3NH3)(5)nI(3), the trigonal (P3m1) structure of (NH2CHNH2)PbI3, and the orthorhombic (Amm2) structure of (NH2CHNH2)SnI3 to examine their thermoelectric properties around room temperature. Our work reveals that the ZTs of electron-doped ABI(3) perovskites can be as large as that of hole-doped Bi2Te3 whereas those of hole-doped ABI(3) are rather smaller so that, in thermoelectric performance, electron-doped perovskites ABI(3) can be as good as hole-doped Bi2Te3.112117sciescopu
Thermoelectric transport properties of tetradymite-type Pb1-xSnxBi2Te4 compounds
No full textWe report thermoelectric transport properties of Pb1-xSnxBi2Te4 (x = 0, 0.25, 0.5, 0.75 and 1) compounds. They exhibited n-type conduction behavior up to x = 0.5, while it changed to p-type due to the explosive hole generation at relatively high Sn contents. Their complicated charge transport behavior was interpreted by high-temperature Hall measurements and also by the first-principles calculation based on Boltzmann transport equation. Effects of the Sn content on the density of state (DOS) effective mass was investigated, and its relation to the observed Seebeck coefficients and power factors of the compounds has been explained. (C) 2016 Elsevier B.V. All rights reserved.112Nsciescopu
Seebeck coefficients of layered BiCuSeO phases: Analysis of their hole-density dependence and quantum confinement effect
No full text1162Nsciescopu
Analysis of the Difference between the Pyroxenes LiFeSi2O6 and LiFeGe2O6 in Their Spin Order, Spin Orientation, and Ferrotoroidal Order
No full textThe pyroxenes LiFeSi2O6 and LiFeGe2O6 are isostructural and isoelectronic, but they differ in their spin order, spin orientation and ferrotoroidal order. The reasons for these differences were probed by density functional theory electronic structure calculations and magnetic dipole-dipole (MDD) interaction energy calculations. The ferromagnetic Fe-zigzag chains of LiFeSi2O6 arise from the antiferromagnetic interchain spin exchanges and the antiferromagnetic Fe-zigzag chains of LiFeGe2O6 from the antiferromagnetic intrachain spin exchange. The preferred spin orientations of LiFeGe2O6 and LiFeSi2O6 are not caused by spin-orbit coupling but by MDD interactions. LiFeSi2O6 undergoes a ferrotoroidal order because it has spin exchange rings made up of an even number of antiferromagnetic spin exchanges paths with comparable strengths. A ferrotoroidal order is not expected for LiFeGe2O6 because it has no such spin exchange rings.X1165sciescopu
Density functional theory investigation of the electronic structure and thermoelectric properties of layered MoS2, MoSe2 and their mixed-layer compound
No full textFirst principles density functional theory calculations were carried out for the 2H-MoQ(2) (Q=S and Se) and their hypothetical mixed-layer compound. Due to the different electronegativities of S and Se atoms on MoQ(2), the band gap size could be adjusted in mixed-layer compound MoS2/MoSe2. Also, the indirect band gap in pure MoQ(2) compounds is changed to the pseudo direct band gap in mixed-layer MoS2/MoSe2 which is similar to the monolayer compound. The layer mixing enhances the thermoelectric properties because of the increased density of states around the Fermi level and the decreased band gap size. Therefore, we suggest that this layer mixing approach should be regarded as a useful way to modulate their electronic structures and to improve their thermoelectric properties. (C) 2014 Published by Elsevier Inc.X111410sciescopu
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