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21.
We consider an Ising model with Kac potential dK(¦x¦) which may have arbitrary sign, and show, following Gates and Penrose, that the free energy in the classical limit0+ can be obtained from a variational principle. When the Fourier transform of the potential has its maximum atp=0 one recovers the usual mean-field theory of magnetism. When the maximum occurs forp
00, however, one obtains an oscillatory or helicoidal phase in which the magnetization near the critical point oscillates with period 2/¦p
0¦. An example with a potential possessing parameter-dependent oscillations is shown to exhibit crossover phenomena and a multicritical Lifshitz point in the classical limit. 相似文献
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25.
N. Angelescu M. Bundaru G. Costache C. J. Thompson 《Journal of statistical physics》1986,43(1-2):33-49
A generalizedO(n) matrix version of the classical Heisenberg model, introduced by Fuller and Lenard as a classical limit of a quantum model, is solved exactly in one dimension. The free energy is analytic and the pair correlation functions decay exponentially for all finite temperatures. It is shown, however, that even for a finite number of spins the model has a phase transition in then limit. The transition features a specific heat jump, zero long-range order at all temperatures, and zero correlation length at the critical point. The Curie-Weiss version of the model is also solved exactly and shown to have standard mean-field type behavior for all finiten and to differ from the one-dimensional results in then limit. 相似文献
26.
Svilen Bobev Paul H. Tobash Joe D. Thompson John L. Sarrao 《Journal of solid state chemistry》2005,178(6):2091-2103
A number of rare-earth alumo-silicides (R-Al-Si) have been synthesized from the corresponding elements by high-temperature reactions, carried out in excess of aluminum to serve as a flux. Under these experimental conditions, large single crystals of all R-Al-Si ternary phases were readily produced. The crystal structures these ternaries adopt were studied by means of powder and single-crystal X-ray diffraction and were classified as follows: (1) the early rare-earths (R=La, Ce, Pr, Nd, Sm, Gd) yield RAlxSi2−x, x∼1, non-stoichiometric ternary derivatives of the body-centered α-ThSi2-type; (2) the late rare-earths (R=Tb, Dy, Ho, Er, Tm) form stoichiometric R2Al3Si2 compounds that crystallize in the C-centered monoclinic Y2Al3Si2-type; (3) the divalent Eu and Yb produce EuAl2Si2 and YbAl2Si2 with the trigonal CaAl2Si2-type, whereas the last lanthanide element, Lu, forms LuAlSi with C-centered orthorhombic YAlGe-type. These structural trends are reviewed, and the evolution of the basic physical properties such as magnetism, heat capacity and electrical resistivity when moving across the series is described in detail. 相似文献
27.
Illumination of a p-type semiconductor—NH3 interface (or heterojunction) with photons of energy greater than the semiconductor band gap causes electrons to be injected into the NH3. There is no hole injection at a n-type electrode. Suggestions are made for the required energy levels in the liquid. 相似文献
28.
Razak J. Al-Essa Richard J. Puddephatt Charles F.H. Tipper Peter J. Thompson 《Journal of organometallic chemistry》1978,157(2):C40-C42
The platinacyclobutane complexes PtCl2L2(C3H5Me)], L pyridine, CD3CN, or tetrahydrofuran, exist as mixtures of isomers containing PH2 or P groups in rapid equilibrium. Decomposition occurs in some cases to give [PtCl2L(CH3CH2CHCH2)]. Stereospecific skeletal isomerisation also occurs in metallocyclobutanes containing the groups PH2 PHR, when R aryl further decomposition gives ν-allylplatinum complexes. 相似文献
29.
Damien Thompson 《Chemphyschem》2007,8(11):1684-1693
Molecular recognition between guest ink molecules and beta-cyclodextrin (beta-CD) cavities at self-assembled monolayers provides a molecular printboard for nanopatterning applications. We recently used molecular dynamics (MD) simulations to describe the specificity of ink-printboard binding and here extend the simulations to include charged cyclodextrin hosts, necessary to broaden the chemistry of molecular printboards and bind charged inks such as the ferrocenium cation. Shifting to high pH, or alternatively grafting a charged sidearm onto beta-CD, created three distinct types of anionic beta-CD cavity and we used electronic structure calculations and MD simulations to measure host-guest charge transfer and binding strengths. We find that steric recognition of uncharged organic molecules is retained at the charged printboards, and that improved guest-host electrostatic contacts can strengthen binding of larger inks while penalising small inks, enhancing the level of discrimination. A prudent choice of complementary host-guest shape and charge states thus provides a means of tuning both ink binding strength and specificity at molecular printboards. 相似文献
30.
The photoelectron spectra of the six propellanes 1 to 6 have been recorded. Those bands which correspond to ejection of an electron from a π-orbital have been assigned, using the concept of through bond and through space interaction. Homoconjugative interaction between the π-systems of the two butadiene moieties of 6 is of the same order as that of the π-orbitals in norbornadiene. 相似文献