排序方式: 共有41条查询结果,搜索用时 15 毫秒
21.
Lammert PE 《The Journal of chemical physics》2006,125(7):074114
The unsolved problem of determining which densities are ground state densities of an interacting electron system in some external potential is important to the foundations of density functional theory. A coarse-grained version of this ensemble V-representability problem is shown to be thoroughly tractable. Averaging the density of an interacting electron system over the cells of a regular partition of space produces a coarse-grained density. It is proved that every strictly positive coarse-grained density is coarse-grained ensemble V representable: there is a unique potential, constant over each cell of the partition, which has a ground state with the prescribed coarse-grained density. For a system confined to a box, the (coarse-grained) Lieb [Int. J. Quantum Chem. 24, 243 (1983)] functional is also shown to be Gateaux differentiable. All results extend to open systems. 相似文献
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A two-field model provides a unifying framework for elasticity, lattice dynamics and electromechanical coupling in graphene and carbon nanotubes, describes optical phonons, nontrivial acoustic branches, strain-induced gap opening, gap-induced phonon softening, doping-induced deformations, and even the hexagonal graphenic Brillouin zone, and thus explains and extends a previously disparate accumulation of analytical and computational results. 相似文献
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Nisoli C Wang R Li J McConville WF Lammert PE Schiffer P Crespi VH 《Physical review letters》2007,98(21):217203
We analyze the rotational demagnetization of artificial spin ice, a recently realized array of nanoscale single-domain ferromagnetic islands. Demagnetization does not anneal this model system into its antiferromagnetic ground state: the moments have a static disordered configuration similar to the frozen state of the spin ice materials. We demonstrate that this athermal system has an effective extensive degeneracy and we introduce a formalism that can predict the populations of local states in this icelike system with no adjustable parameters. 相似文献
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Conversion of two diametrically opposed atomic rows on a carbon nanotube to sp(3) hybridization produces two identical weakly coupled one-dimensional electronic systems within a single robust covalently bonded package: a biribbon. Arm-chair tubes, when so divided, acquire a pair of narrow spin-polarized bands at the Fermi energy; interaction across the sp(3) dividers produces a tunable band splitting in the THz range. For semiconducting tubes, the eigenvalues of the low-energy electronic states are surprisingly unaffected by the bifurcation; however, the tubes' response functions to external electric fields are dramatically altered. These modified tubes could be produced by uniaxial compression transverse to the tube axis followed by site-selective chemisorption. 相似文献
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Marco H.P. Rietveld Lammert Nagelholt David M. Grove Nora Veldman Anthony L. Spek Monika U. Rauch Wolfgang A. Herrmann Gerard van Koten 《Journal of organometallic chemistry》1997,530(1-2):159-167
From the reaction of MeReO3 with the neutral arylamine C6H5CH2NMe2 and the aryldiamine C6H4(CH2NMe2)2−1,3, have been isolated in good yields the 1/1 adduct complex [MeReO3 · C6H5CH2NMe2], 1, and the 2/1 adduct complex [(MeReO3)2 · C6H4(CH2NMe2)2− 1,3], 2, respectively. The X-ray molecular structure of 2 shows that both rhenium centres have a trigonal bipyramidal geometry and in the axial positions of each rhenium centre are one of the NMe2 units of the aryldiamine ligand and a methyl group. The mono(ortho)-chelated arylaminorhenium trioxide complex [ReO3(C6H4CH2NMe2−2], 3, can be synthesized by a transmetallation reaction of ClReO3 with [ZnC6H4CH2NMe2−22] in a 2:1 molar ratio. In a similar way the bis(ortho)-chelated arylaminorhenium trioxide complex [ReO3C6H3(CH2NMe2)2−2,6], 4, can be synthesized by addition of a mixture of [Li2C6H3(CH2NMe2)2−2,62] and ZnCl2 to ClReO3. Complexes 3 and 4 have been isolated as white solids in 66% and 81% yields respectively. The rhenium centre in complex 4 has a bicapped tetrahedral geometry in which the monoanionic C6H3(CH2NMe2)2−2,6− ligand is pseudo-facially bonded with a characteristic N1-Re-N2 angle of 107.7(3)°, a Re-Cipso bond length of 2.112(11) Å and Re-N1 and Re-N2 bond lengths of 2.518(9) Å and 2.480(8) Å respectively. 相似文献
29.
Paxton WF Kistler KC Olmeda CC Sen A St Angelo SK Cao Y Mallouk TE Lammert PE Crespi VH 《Journal of the American Chemical Society》2004,126(41):13424-13431
Rod-shaped particles, 370 nm in diameter and consisting of 1 microm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2-3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10(-14) N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR(2)gamma/muDL, where S is the area-normalized oxygen evolution rate, gamma is the liquid-vapor interfacial tension, R is the rod radius, mu is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol-water solutions confirmed that the velocity depends linearly with the product Sgamma, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion. 相似文献
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Contreras JA Murray JA Tolley SE Oliphant JL Tolley HD Lammert SA Lee ED Later DW Lee ML 《Journal of the American Society for Mass Spectrometry》2008,19(10):1425-1434
A novel gas chromatograph-mass spectrometer (GC-MS) based on a miniature toroidal ion trap mass analyzer (TMS) and a low thermal
mass GC is described. The TMS system has an effective mass/charge (m/z) range of 50–442 with mass resolution at full-width half-maximum (FWHM) of 0.55 at m/z 91 and 0.80 at m/z 222. A solid-phase microextraction (SPME) fiber mounted in a simple syringe-style holder is used for sample collection and
introduction into a specially designed low thermal mass GC injection port. This portable GC-TMS system weighs <13 kg (28 lb),
including batteries and helium carrier gas cartridge, and is totally self-contained within dimensions of 47×36×18 cm (18.5×14×7in.).
System start-up takes about 3 min and sample analysis with library matching typically takes about 5 min, including time for
column cool-down. Peak power consumption during sample analysis is about 80 W. Battery power and helium supply cartridges
allow 50 and 100 consecutive analyses, respectively. Both can be easily replaced. An on-board library of target analytes is
used to provide detection and identification of chemical compounds based on their characteristic retention times and mass
spectra. The GC-TMS can detect 200 pg of methyl salicylate on-column. n-Butylbenzene and naphthalene can be detected at a concentration of 100 ppt in water from solid-phase microextraction (SPME)
analysis of the headspace. The GC-TMS system has been designed to easily make measurements in a variety of complex and harsh
environments. 相似文献