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1.
The Hausdorff chirality measure quantifies the chirality of a geometric representation of an object by measuring the degree of coincidence of the object with its mirror image. It can also allow comparison between a chiral dopant and host molecules which may illuminate mechanisms for chirality transfer. It has been applied to real molecules very infrequently in comparison to application of chiral indices as it is complex and time consuming to calculate. In this paper we introduce and verify a simulated annealing algorithm for the Hausdorff chirality measure that has proven rapid, robust and relatively simple to apply. We verify the method, finding good agreement between its results and those of Mislow and co-workers. We introduce a Hausdorff structure measure that does not permit overlap and allows a structure to be built one molecule at a time. We present results for a simple model and real biphenyl molecules and discuss promising building blocks of crystal and incommensurate structures formed in relation to experimental results. 相似文献
2.
We consider the one-dimensional t
-
J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J
=
t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering
matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and
the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations
diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string
hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity
depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron
and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic
susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity.
Received: 31 December 1997 / Accepted: 17 March 1998 相似文献
3.
Mn 2+-doped ZnS nanoparticles of average size 2.5±0.3 nm have been studied and characterized in the dopant concentration range 0.1–0.3% using XRD, EPR, XPS and photoluminescence methods. The experimental results obtained from these studies indicate that the doping of Mn 2+ occurs primarily at the T
d sites at low dopant concentration, causing the 4T 1(G) → 6A 1(S) transition to take place in the host lattice; the observed decrease in the intensity of photoluminescence at high dopant concentration is due to the setting in of the strong Mn 2+–Mn 2+ interaction arising from cluster formation at the highly distorted sites near the particle surface.An erratum to this article can be found at 相似文献
4.
Nanostructures are presently enjoying an increasing interest in the
field of materials science. In particular, importance is given to
ordered monolayers prepared by deposition of atoms on a crystalline
surface. The growth of these superlattices can be controlled so as to
obtain an ordered structure by means of the lateral interaction of
adatoms lying on the metal surface.
The objective of our study is to investigate the structural and electronic
properties using DFT total-energy calculations; we employ a jellium-like
model to describe the substrate but we also take into account the
presence of discrete surface states that are known to affect the lateral
interaction. Our treatment of the substrate is based on the model
proposed by E.V. Chulkov et al. [Surf. Sci. 437, 330 (1999)]; in this model one constructs a mono-dimensional potential so as
to reproduce some important electronic properties of the metal surface,
such as i) the energy gap in the projected bulk band-structure and
ii) the energy position of surface states. We put into practice Chulkov
potential implementing into an existing plane-waves code (ABINIT, URL
http://www.abinit.org) an ionic potential, so as to obtain a self-consistent
Kohn-Sham effective potential which corresponds to the Chulkov one.
Using this effective potential in a fully three-dimensional code we are able
to study the adsorption process and the interaction between adsorbates.
We illustrate some details of our implementation of the Chulkov model
and we present our results about the simple system of Na adatoms on a
Cu(111) surface for different coverages.
In particular, we compare electronic properties and adsorption energies
with those obtained within a standard jellium model substrate and with
those obtained for Na adsorption on a realistic Cu(111) surface. 相似文献
5.
Microtextural polarization, phase transitions, and electro-optical effects are studied in a series of nanocomposites, grown by mixing alkyloxybenzoic acids ( nOBAs), displaying hydrogen-bonded dimeric liquid crystal (LC) state, with non-mesogens (single-walled carbon nanotubes (SWCNTs), perfluorooctanoic acid) or mesogens (bent-core LC compound D14F3). Each of the studied nanocomposites, in which the nOBA serves as a matrix, forms complexes with bent-shaped dimeric, caused by the interaction between the dopant structural units and the dimer rings. This feature, coordinated with the surface anchoring, bulk and electrical effects, leads to drastic reduction of the LC system symmetry. As a result, transitions from achiral (characteristic for the pristine nOBA) to chiral states (including ferroelectric smectic C with C 2 symmetry and ferroelectric smectic C G with the lowest C 1 triclinic one) take place. The functionalization of the SWCNTs causes drastic increase of the ferroelectricity. 相似文献
6.
We calculate the Landau interaction function f ( k, k') for the two-dimensional t- t' Hubbard model on the square lattice using second and higher order perturbation theory. Within the Landau-Fermi liquid framework
we discuss the behavior of spin and charge susceptibilities as function of the onsite interaction and band filling. In particular
we analyze the role of elastic umklapp processes as driving force for the anisotropic reduction of the compressibility on
parts of the Fermi surface.
Received 18 March 2002 Published online 9 July 2002 相似文献
7.
Association energies of nearest-neighbour and next-nearest-neighbour associates between substitutional, trivalent rare-earth ions and interstitial fluoride ions CaF 2, SrF 2 and BaF 2 are obtained by lattice simulation calculations. The dopant ion-fluoride ion interaction is described (i) with a set of potentials obtained with electron gas methods, and (ii) with a set of potentials derived semi-empirically from the host lattice cation-anion interaction potentials. The calculations successfully simulate the experimentally observed variations of the dopant-interstitial binding energies with the radius of the dopant ion, and with the lattice parameter of the host. The better quantitative agreement is obtained with the semi-empirical potentials. The variations are explained by an evaluation of the displacements of the ions constituting the associates. 相似文献
8.
ABSTRACT ( S)-(+)-Dihydro-3-hydroxy-4,4-dimethyl-2(3H)-furanone, ( R)-(-)-dihydro-3-hydroxy-4,4-dimethyl-2(3H)-furanone, ( S)-(-)-dihydro-4-hydroxy-2(3H)-furanone, and ( S)-(-)-5-hydroxymethyl-2(5H)-furanone in the presence of pure enantiomers of 2,2,2-trifluoro-1-(9-anthryl)ethanol were studied by 1H NMR in deuterated chloroform solutions. Experimental Job's plots suggest that the resulting solvates are formed with one molecule of solute and one of the chiral solvating agent. From the magnitude of the association constant determined for ( S)-(+)-dihydro-3-hydroxy-4,4-dimethyl-2(3H)-furanone in the presence of ( R)-(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol (1.26 ± 0.09 M ?1), it is inferred that the solvate is weak and cannot be isolated at 298 K. The correlation between the magnitude of induced chemical shifts, NOESY maps, and the known configuration of solutes and chiral solvating agents suggests that intermolecular hydroxyl-hydroxyl interaction is the primary interaction. Accordingly, the secondary interaction might occur between benzylic-hydrogen of the chiral solvating agent and the carbonyl- or furan ring-oxygen atoms of the solute. 相似文献
9.
We show that three dimensional Chern-Simons gauge theories with a compact gauge group G (not necessarily connected or simply connected) can be classified by the integer cohomology group H
4( BG, Z). In a similar way, possible Wess-Zumino interactions of such a group G are classified by H
3( G, Z). The relation between three dimensional Chern-Simons gauge theory and two dimensional sigma models involves a certain natural map from H
4( BG, Z) to H
3( G, Z). We generalize this correspondence to topological spin theories, which are defined on three manifolds with spin structure, and are related to what might be called Z
2 graded chiral algebras (or chiral superalgebras) in two dimensions. Finally we discuss in some detail the formulation of these topological gauge theories for the special case of a finite group, establishing links with two dimensional (holomorphic) orbifold models. 相似文献
10.
π and η decay modes of light baryon resonances are investigated within a chiral quark model whose hyperfine interaction is
based on Goldstone-boson exchange. For the decay mechanism a modified version of the 3
P
0 model is employed. Our primary aim is to provide a further test of the recently proposed Goldstone-boson exchange constituent
quark model. We compare the predictions for π and η decay widths with experiment and also with results from a traditional
one-gluon exchange constituent quark model. The differences between nonrelativistic and semirelativistic versions of the constituent
quark models are outlined. We also discuss the sensitivity of the results on the parameterization of the meson wave function
entering the 3
P
0 model.
Received: 11 May 2001 / Accepted: 17 September 2001 相似文献
11.
Several lattice collaborations performing simulations with 2+1 light dynamical quarks have experienced difficulties in fitting their data with standard N
f = 3 chiral expansions at next-to-leading order, yielding low values of the quark condensate and/ or the decay constant in the N
f = 3 chiral limit. A reordering of these expansions seems required to analyse these data in a consistent way. We discuss such a reordering, known as Resummed Chiral Perturbation Theory, in the case of pseudoscalar masses and decay constants, pion and kaon electromagnetic form factors and K
ℓ3 form factors. We show that it provides a good fit of the recent results of two lattice collaborations (PACS-CS and RBC/UKQCD). We describe the emerging picture for the pattern of chiral symmetry breaking, marked by a strong dependence of the observables on the strange quark mass and thus a significant difference between chiral symmetry breaking in the N
f = 2 and N
f =3 chiral limits. We discuss the consequences for the ratio of decay constants F
K
/F
π and the K ℓ3 form factor at vanishing momentum transfer. 相似文献
12.
We review the current status of the exotic pentaquark baryons. After a brief look at experiments of both positive and negative
results, we discuss theoretical methods to study the structure and reactions for the pentaquarks. First we introduce the quark
model and the chiral soliton model, where we discuss the relation of mass spectrum and parity with some emphasis on the role
of chiral symmetry. It is always useful to picture the structure of the pentaquarks in terms of quarks. As for other methods,
we discuss a model-independent method, and briefly mention the results from the lattice and QCD sum rule. Decay properties
are then studied in some detail, which is one of the important properties of ⊝ +. We investigate the relation between the decay width and the quark structure having certain spin-parity quantum numbers.
Through these analyses, we consider as plausible quantum numbers of ⊝ +, J
P = 3/2 −. In the last part of this note, we discuss production reactions of ⊝ + which provide links between the theoretical models and experimental information. We discuss photoproductions and hadron-induced
reactions which are useful to explore the nature of ⊝ + 相似文献
13.
We have sought to work with an approach to Noether symmetry analysis which uses the properties of infinitesimal point transformations
in the space-time ( q, t) variable to establish the association between symmetries and conservation laws of a dynamical system. In this approach symmetries
are expressed in the form of generators. We have studied the variational or Noether symmetries of two uncoupled Harmonic oscillators
and two such oscillators coupled by an interaction. Both these systems can have alternative Lagrangian representations. We
have studied in detail how the association between symmetries and conservation laws changes as one alters the analytic or
Lagrangian representation. This analysis is carried out with a view to explicitly demonstrate that the correlation between
symmetry transformation and corresponding invariant quantity depends crucially on the choice of the analytic representation.
PACS 45.20.Jj, 45.20.df, 45.20.dh 相似文献
14.
It was shown experimentally (P.V. Dolganov et al., Europhys. Lett. 76, 250 (2006)) and by numerical calculations (C. Bohley, R. Stannarius, Eur. Phys. J. E 23, 25 (2007)) that the c -director profile of a two-dimensional chiral smectic-C (SmC) film around a circular inclusion adopts dipolar rather than
quadrupolar configuration observed in achiral SmC films. We give an analytical argument on how spontaneous bend inherent in
chiral SmC liquid crystals influences the configuration of a SmC liquid crystal film around a circular inclusion imposing
tangential anchoring. We find how the angle α between two surface defects seen from the center of the inclusion depends on
the radius of the inclusion R and the strength of the spontaneous bend q . We show, however, that the contribution of the spontaneous bend to the free energy suffers from mathematical ambiguity;
it depends on the mathematical treatment of the outer boundary even when it is at infinity. This might indicate that the shape
as well as the treatment of the outer boundary of the film can significantly influence the equilibrium configuration of the
c -director and the position of the surface defects. 相似文献
15.
We discuss the process of resonant subbarrier internal conversion of γ rays, where the converted electron is transferred to one of the atomic orbitals. For the first time we study how this process
is affected by residual interactions: the splitting of the atomic terms in the total angular momentum of the atom, configuration
mixing, and the magnetic interaction between the electrons of the atom. The calculations are done by the relativistic multiconfiguration
Dirac-Fock method with allowance for the Breit interaction for the M1 transition with an energy of 35 492 eV in multiply charged ions of 125Te. We show that allowing for the residual interaction is obligatory if we want to calculate the conversion rate in the vicinity
of a resonance correctly.
Zh. éksp. Teor. Fiz. 116, 1565–1574 (November 1999) 相似文献
16.
We discuss three applications of NJL- and PNJL-like models to assess aspects of the QCD phase diagram: First, we study the effect of mesonic correlations on the pressure below and above the finite temperature phase transition within a non-local PNJL model beyond the mean-field approximation. Second, we reconstruct the phase boundary of an NJL model from a Taylor expansion of the chiral susceptibility about μ=0 and compare the result with the exact phase boundary. Finally, we demonstrate the realization of the “non-standard scenario” for the critical surface in a three-flavor PNJL model with a μ-dependent determinant interaction. 相似文献
17.
In a class of F-theory SU(5) GUTs the low energy chiral mass spectrum is obtained from rank one fermion mass textures with a hierarchical structure
organized by U(1) symmetries embedded in the exceptional E
8 group. In these theories chiral fields reside on matter ‘curves’ and the tree-level masses are computed from integrals of
overlapping wave functions of the particles at the triple intersection points. This calculation requires knowledge of the
exact form of the wave functions. In this work we propose a way to obtain a reliable estimate of the various quantities which
determine the strength of the Yukawa couplings. We use previous analysis of KK-threshold effects to determine the (ratios
of) heavy mass scales of the theory which are involved in the normalization of the wave functions. We consider similar effects
from the chiral spectrum of these models and discuss possible constraints on the emerging matter content. In this approach,
we find that the Yukawa couplings can be determined solely from the U(1) charges of the states in the ‘intersection’ and the torsion which is a topological invariant quantity. We apply the results
to a viable SU(5) model with minimal spectrum which satisfies all the constraints imposed by our analysis. We use renormalization group
analysis to estimate the top and bottom masses and find that they are in agreement with the experimental values. 相似文献
18.
The inclusion of nonadiabatic corrections to the electron-phonon interaction leads to a strong momentum dependence in the
generalized Eliashberg equations beyond Migdal's limit. For a s-wave symmetry of the order parameter, this induced momentum dependence leads to an enhancement of when small momentum transfer is dominant. Here we study how the d-wave symmetry affects the above behavior. We find that the nonadiabatic corrections depend only weakly on the symmetry of
the order parameter provided that only small momentum scatterings are allowed for the electron-phonon interaction. In this
situation, We show that also for a d-wave symmetry of the order parameter, the nonadiabatic corrections enhance . We also discuss the possible interplay and crossover between s- and d-wave depending on the material's parameters.
Received 12 May 2000 相似文献
19.
We show that the antiferromagnetic coupling between Mn atoms in MnO can be tuned to a ferromagnetic one by doping it with a small concentration of 2p elements like boron, carbon and nitrogen. Our ab initio density functional calculations in the Hubbard U framework show that the coupling between Mn atoms turns ferromagnetic for 3 at% B doping. B is found to be the most effective one in stabilizing ferromagnetism. We discuss the role of 2p states of the dopant atoms to explain the trend of ferromagnetism in MnO. 相似文献
20.
ABSTRACTTitanium dioxide (TiO 2) nanocrystals are promising materials for photo-electrochemical water splitting. This study focuses on how surface dopant placement can affect the electronic properties. TiO 2 anatase thin films are doped two ways: a cobalt ion replacing a surface titanium ion (surface ingrained) and a cobalt ion chemisorbed to two surface oxygen ions and two NH 3 ligands. Specifically, when studying the binding pattern, the cobalt ion dopant changes from an electron acceptor for the surface ingrained model to an electron donor for the chemisorbed model. The optical absorption peaks of the surface ingrained model are attributed to p→d transitions and are much stronger when compared to the d→d transitions for the chemisorbed model. It is the conclusion of this computational study that one can alter the cobalt dopant on the anatase thin film to focus a positive or negative charge at the surface by changing the surface dopant location. 相似文献
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