Low-frequency localized oscillations of the DNA double strand

The results of the molecular dynamic study on the low-energy elementary linear and nonlinear excitations of DNA macromolecule obtained within the framework of the coarse-grain model of the DNA double strand are presented. The characteristics of the basic states of the model agree well with the experimental parameters of both A and B conformations of the DNA double strand. The correlation between the directly calculated dispersion curves and density distribution of the frequency spectrum obtained in the simulation experiments is found. Special attention is focused on the soliton type of nonlinear localized excitations (breathers). These excitations are shown to exist in several frequency gaps in which the propagation of harmonic linear waves is prohibited. The types of motions corresponding to all calculated breathers are identified. The correlation between the two types of breathers and their analogs studied in terms of unidimensional models and treated as elementary excitations responsible for the initial stage of the opening of the DNA double strand is established.     

Exciton binding energy in semiconducting single-walled carbon nanotubes

The exciton binding energy serves as a critical criterion for identification of the nature of elementary excitations (neutral excitons versus a pair of charged carriers) in semiconductor materials. An exciton binding energy of 0.41 eV is determined experimentally for a selected nanotube type, the (8,3) tube, confirming the excitonic nature of the elementary excitations. This determination is made from the energy difference between an electron-hole continuum and its precursor exciton. The electron-hole continuum results from dissociation of excitons following extremely rapid exciton-exciton annihilation and possibly also ultrafast relaxation from the second to the first exciton states and is characterized by distinct spectroscopic and dynamic signatures.     

Entanglements in glass

Ground state and elementary excitations (tunnelling modes) in glass are obtained from an analysis of its symmetry, a local gauge invariance. The configuration of glass is represented as a discrete fiber bundle. The base space is a continuous random network, standard model of the structure of covalent glasses. The connection is determined naturally by the elasticity of the network. The bundle is non-trivial, the elastic connection is entangled in one of two ways. Sources of non-triviality are closed loops, threading through odd rings in the network. To restore gauge invariance, tunnelling must occur between the two possible configurations about an odd loop. Entanglement and elementary excitations are labelled by permutations of the covalent bonds incident on an atom.Work supported by the Herbette Foundation.     

Electronic excitations in narrow quantum wells via intersubband Raman scattering: Theoretical considerations

In this work a generalized self-consistent field theory was applied to investigate the elementary excitations of two-dimensional electron gas formed from narrow quantum wells via resonant intersubband Raman scattering. The developed model considers the existence of equally coupled and degenerated excitations of the electron gas and allows to observe that in extreme resonance regime the plasma oscillations splits into two contributions: a set of renormalized collective excitations (plasmons) and unrenormalized electronic transitions (single-particle excitations). Our results show that the asymmetries which appear in the Raman profile of doped narrow quantum wells can be interpreted as the entrance or exit of resonance of collective modes overlapped with single-particle transitions.     

Two- and four-component relativistic generalized-active-space coupled cluster method: implementation and application to BiH

A string-based coupled-cluster method of general excitation rank and with optimal scaling which accounts for special relativity within the four-component framework is presented. The method opens the way for the treatment of multi-reference problems through an active-space inspired single-reference based state-selective expansion of the model space. The evaluation of the coupled-cluster vector function is implemented by considering contractions of elementary second-quantized operators without setting up the amplitude equations explicitly. The capabilities of the new method are demonstrated in application to the electronic ground state of the bismuth monohydride molecule. In these calculations simulated multi-reference expansions with both doubles and triples excitations into the external space as well as the regular coupled-cluster hierarchy up to full quadruples excitations are compared. The importance of atomic outer core-correlation for obtaining accurate results is shown. Comparison to the non-relativistic framework is performed throughout to illustrate the additional work of the transition to the four-component relativistic framework both in implementation and application. Furthermore, an evaluation of the highest order scaling for general-order expansions is presented.     

Roton excitations in Bose–Einstein condensates and a fluid–solid transition

Several authors have recently discussed the existence of roton excitations in Bose–Einstein condensates (BECs) and considered how a roton dip in the dispersion curve of elementary excitations may be related to the formation of a spatially modulated ground state. Here attention is drawn to a theoretical study of Minguzzi et al. on interatomic correlations in a BEC from dipole–dipole interactions induced by laser light of increasing intensity. Attractive interactions in superfluid ⁴He and repulsive interactions in ‘untuned’ BECs are then compared and contrasted, the experiments of Woods and Cowley and of Greiner et al. providing the focus respectively. It is stressed that, contrary to a very recent assertion by Nazario and Santiago, ⁴He is crucially different from BECs at the lowest temperatures.     

Analytical Study of Discrete Optical Breathers in Spiral Polymer Chain

Physico‐mechanical properties of polymers in solid state, in particular conditions of their structural transformations, are substantially defined by existence and mobility of elementary nonlinear excitations. The localized oscillatory excitations (breathers), which have been revealed earlier in crystalline polyethylene alongside with topological solitons, turn out to be significant for thermodynamic studies as well as for investigation of energy transfer. We consider analytically the breathers in more complex crystalline polytetrafluoroethylene. It is shown that introduction of adequate variables allows to construct the continuum model describing spatially localized excitations with several internal degrees of freedom. Characteristic parameters of breathers and conditions of their mobility are defined.

     

Free energy in relation to order parameter in magnets and pyroelectrics

Theoretical models will be presented in which the internal energy of (a) a ferromagnet and (b) a pyroelectric material is expressed in terms of magnetization and electric polarization, respectively. For the ferromagnet, simple models of elementary excitations (e.g., spin wave theory in an insulator, to which Stoner excitations must be added in a metal) lead to formulas for the internal energy at low temperature as power laws in the change of magnetization from its saturation value. An unconventional use of two order parameters, the sublattice magnetization plus the metallic discontinuity in momentum distribution at the Fermi surface, allows the phase transition between metallic and insulating states of antiferromagnets to be treated at T = 0, the three-dimensional transition-metal dichalcogenides being an example here. The treatment of the internal energy of the ferromagnet is then extended to include the entropy also, using specifically the Ising model in nonzero external magnetic field. Its relevance to the Landau theory of phase transitions will be emphasized. Some comments will finally be made about the analogue for pyroelectrics. © 1996 John Wiley & Sons, Inc.     

ESR and optical measurement on poly(3-methylthiophene)

The elementary excitations of poly(3-methyl-thiophene) in a doping range up to 7.5% have been found to be polarons as evidenced by ESR-measurements. Above 7.5% doping Pauli susceptibility indicates metallic behaviour. In contrast, at 5% doping the optical absorption spectra show a transition from the three peak structure due to polarons to one with two peaks and commonly assigned to the formation of bipolarons. It is argued that a widening of the polaron levels gives rise to this phenomenon and not the induction of bipolarons. Above 20% doping the formation of a new phase of the material is observed, which is characterized by the disappearance of the peak at 1.6 eV and a well defined isosbestic point at 1.4 eV.     

Quantum Chemistry Close to the Fermi Level: Reducing Clusters to Few Active Hole and/or Electron Systems

Various approximate models to describe the electronic properties of some families of clusters are reviewed. They correspond to specific elementary situations close to the Fermi level where one or few electrons are either removed from (or added to) a closed shell wavefunction. Simple hole-particule excitations are also considered. The models discussed involve diatomics-in-molecules schemes, use of pseudopotential framework extended to replace inert atoms, and finally combinations of both techniques for excited states. Applications to electronic structure of alkaline earth clusters, rare-gas systems or chromophores interacting with rare-gas systems are given also prospects for more complex molecular nanosystems or assemblies.     

Nonconservation of excitons as a mechanism of light-energy capturing

The analysis of nonstationary processes, taking place in exciton systems in consequence of the nonconservation of the number of elementary excitations, is presented and it is pointed out to a new possibility of light-energy capturing which exists in ecological systems due to the nonconservation of excitons. It is shown that, due to the nonconservation, the one-dimensional molecular chain is able to capture permanently the light energy of the order of 50 keV to 5 MeV. A part of the captured energy is expected to have a role in internal bioprocesses. It is also shown that the system is a seat where the processes of continuous creations and annihilations of pairs of excitons having opposite momenta take place with a probability of the order of 10^?4 to 10^?2. Further investigations would allow to identify biological phenomena caused by those processes.     

Density oscillations in a model of water and other similar liquids

It is suggested that the dynamics of liquid water have a component consisting of O^?2z (oxygen) anions and H^+z (hydrogen) cations, where z is a (small) reduced effective electron charge. Such a model may apply to other similar liquids. The eigenmodes of density oscillations are derived for such a two-species ionic plasma, included the sound waves, and the dielectric function is calculated. The plasmons may contribute to the elementary excitations in a model introduced recently for the thermodynamics of liquids. It is shown that the sound anomaly in water can be understood on the basis of this model. The results are generalised to an asymmetric short-range interaction between the ionic species as well as to a multi-component plasma, and the structure factor is calculated.     

Calculation of Sound Velocities of Liquid Metals at their Melting Point via the Percus-Yevick Theory of Melting

Abstract

In this brief note, we utilize the 3N collective coordinate theory of liquids of Percus and Yevick¹ as applied to the phenomenon of melting by Omini² to calculate the sound Velocities in various simple liquid metals at their melting temperatures. We perform this calculation by taking derivatives of Omini's² calculated Percus-Yevick dispersion relations for their “liquid phonon” collective coordinate elementary excitations of the liquid. We compare these calculated sound velocities with experimental data, where possible, to ascertain the validity of the liquid phonon dispersion relation as a source of sound velocities.     

Interplay between collective and single electron excitations in large metal clusters

Collective and single electron excitations of large metal clusters supported on transparent substrates are investigated. The applied experimental techniques include extinction spectroscopy and laser induced dissociation accompanied by the ejection of individual atoms. The optical spectra depend on the electromagnetic far field and reflectcollective electron excitations of the conduction electrons, i.e. surface plasmons. Dissociation, however, is correlated to repulsivesingle electron energy levels. The characteristics of these localized excitations indicate a strong influence of collective excitations. In particular, it is found that nonlocal optical effects are important. In this picture surface plasmons catalytically enhance the number of single electron excitations and therefore of the metal atoms ejected as a result of the absorption of visible light. Results will be presented, which illustrate this interplay between collective and single electron excitations.     

Approximate treatment of higher excitations in coupled-cluster theory

The possibilities for the approximate treatment of higher excitations in coupled-cluster (CC) theory are discussed. Potential routes for the generalization of corresponding approximations to lower-level CC methods are analyzed for higher excitations. A general string-based algorithm is presented for the evaluation of the special contractions appearing in the equations specific to those approximate CC models. It is demonstrated that several iterative and noniterative approximations to higher excitations can be efficiently implemented with the aid of our algorithm and that the coding effort is mostly reduced to the generation of the corresponding formulas. The performance of the proposed and implemented methods for total energies is assessed with special regard to quadruple and pentuple excitations. The applicability of our approach is illustrated by benchmark calculations for the butadiene molecule. Our results demonstrate that the proposed algorithm enables us to consider the effect of quadruple excitations for molecular systems consisting of up to 10-12 atoms.     

Dipole excitation of Na clusters with a non-local energy density functional

The elementary dipole excitations of the ionized clusters Na ₉ ⁺ , Na ₂₁ ⁺ and Na ₄₁ ⁺ are investigated by solving the equations of the Random-Phase Approximation. The ground and excited states are described using the jellium model for the ionic background and a non-local energy density functional for the valence electrons. Non-local effects are specifically analyzed. The excitation energies thus obtained approach better than those of the Local Density Approximation both the full Hartree-Fock and the experimental results.     

Studies of EPR parameters for Mn5+ -doped Li3PO4 and Li3VO4 crystals

The EPR parameters (zero-field splitting D and g factors g parallel, g perpendicular) of Mn5+ -doped Li3PO4 and Li3VO4 crystals are calculated from the complete high-order perturbation formulas based on a molecular orbital scheme for a 3d2 ion in tetragonal MX4 clusters. These formulas include not only the contribution coming from crystal-field excitations, but also that arising from charge-transfer excitations (which is discarded in crystal field theory). The calculated results are in reasonable agreement with the observed values. The contributions to EPR parameters coming from the charge-transfer excitations are comparable with those arising from the crystal-field excitations. It appears that for a high valence state 3dn ion in crystals, the reasonable explanations of EPR parameters should take the contributions due to both crystal-field and charge-transfer excitations into account.