Presence Of Coulomb Interaction Research Articles

Phason dynamics in charge and spin density waves

Phason propagators in charge and spin density waves in presence of long range Coulomb interaction are studied within mean field theory. In charge density wave the longitudinal phason splits into an acoustic mode and an optical mode in the presence of Coulomb interaction. Except at low temperatures (T≤0.2 T c ) the acoustic mode exhausts most of the optical weight. In spin density wave there is no longitudinal acoustic mode; the phason becomes the plasmon. On the other hand in the transverse limit, the plasmon decouples completely from the phason both in charge and spin density waves

Coulomb interactions and stability of the spiral states in the t-J model.

A Schwinger-boson mean-field theory of the t-J model in the presence of Coulomb interactions is presented. It is shown that incommensurate spiral metallic states are favored over other important homogeneous states, and are stable against hole condensation for concentration \ensuremath{\delta}>${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$. For nearest-neighbor and screened-Coulomb potentials of reasonable strength the region of stability may coincide with the metallic phase of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$. However, the instability always exists if \ensuremath{\delta} is small enough, a property which provides a mechanism for the occurrence of metal-insulator transition and antiferro- magetism in the low-\ensuremath{\delta} regime.

On the turbidity of silica gels: Aggregation in the presence of coulomb interactions

Silica gels are readily formed by the hydrolysis and condensation of a dilute solution of a silicon alcoxide under acidic or basic conditions. Sols formed from basic solutions of tetramethoxysilicon, where the hydrolysis products (silicic acids) are negatively charged, condense very slowly and yield rigid, transparent gels. However, we have found that the addition of small amounts of salt catalyzes the reaction by several orders of magnitude, and leads to opaque gels. We present a quasielastic and elastic light scattering study of the growth and structure of silica clusters that indicates that the increase in the turbidity is due to an increase in the fractal dimension of the dilute sol, due in turn to the ionic screening of Coulomb interactions.

Bogoliubov quasiparticles, spinons, and spin-charge decoupling in superconductors.

We show that, in the presence of Coulomb interactions, the charge of a quasiparticle in a fully gapped superconductor is a sharp quantum observable. It follows that the familiar Bogoliubov quasiparticles are neutral, spin-1/2 fermions. Since the current-carrying collective modes are spinless, charge and spin are carried by separate excitations in a superconductor. The quantum numbers of these quasiparticles are identical to those of a spinon in a ``resonating-valence-bond'' (RVB) superconductor. This supports the notion that a RVB superconductor is simply a strong-coupling version of a BCS superconductor. We describe an experiment that would directly verify the separation of charge and spin in a bulk superconductor.

Elastic properties of phosphotungstate glasses

Elastic properties of potassium and lead phosphotungstate glasses have been investigated using ultrasonic velocity measurements. The composition dependence of elastic moduli in WO3-K2O-P2O5 glasses suggests that at low alkali oxide concentrations the atomic ring size increases by network modification, which results in the decrease of elastic moduli. In the highly modified regime, due to the presence of coulombic interaction, the rate of decrease of elastic moduli is reduced. In the WO3-PbO-P2O5 glasses the behaviour of elastic moduli suggests that PbO behaves both as a network former and network modifier. The incorporation of PbO into the network is quantitatively determined by the concentration of P2O5 in the system. The results are consistent with the structural model proposed earlier, based on characterization studies.

Hard gaps in localized interacting systems

We study the one-particle density of excitations and the density of states of a strongly Anderson-localized system in the presence of Coulomb interactions. Using a pair correlation function obtained from computer simulation, we obtained, analytically, a density of excitations, including the effects of electron-hole correlations. These results are used to calculate hard gaps in the density of states by the techniques of Efros and Davies, and also in a new calculations where stabilization of the ground state is aided by polaron transitions.

Calculations of the mean width of the Coulomb gap in disordered systems by two simple methods

Abstract Two simple models, a dielectric model and a Thomas-Fermi model, are used to calculate the Coulomb gap E g in disordered systems, and hopping excitation energies in the presence of Coulomb interactions. An important approximation in both methods is that the mean width of the Coulomb gap is obtained by assuming that the response to the presence of an electron at any location is the average response. A very important element in the determination of E g is the inability of the electronic system to sustain short-range screening. This is quantified in dielectric theory by a cut-off wave-vector for the dielectric function, and in the Thomas-Fermi theory by a saturation charge. The latter is easier to relate to known properties of the system; this is one reason why the Thomas-Fermi method is deemed more reliable. Although more sophisticated methods for calculating Eg exist in the literature, the advantages of the present method are its simplicity and ready applicability to complex situations. One such ap...

Nonlinear Absorption in III—V Semiconductors

AbstractA coherent radiation‐exciton interaction model is employed to study analytically the phenomenon of nonlinear absorption in important III—V semiconductors (viz., GaAs, GaSb, InSb, InAs), the crystals being irradiated by suitable lasers with energies (hω) nearly equal to the band gap energies (hωg). The nonoptical processesare accounted for phenomenologically by introducing a damping parameter into the equation of motion of the probability amplitude of the excited electron—hole pair state. The Wannier‐Mott type of exciton wave function is found to be responsible for the giant nonlinearity of the crystal absorption coefficient in the near resonant optical interband transition regime with h | ω ‐ ωg | of the order of the crystal exciton binding energy. The effect of renormalization of the crystal band gap in the presence of Coulomb interaction and Starkbroadening is also discussed.

Vibronic States of Polyacetylene, (CH)x

Electronic states of π-electrons coupled with lattice vibrations in a linear conjugated carbon chain, are calculated with a particular purpose of clarifiing the vibronic structure of bond alternation defect. First the vibronic Hamiltonian is derived from a general Hamiltonian. It is shown in the adiabatic approximation that the ground state of an odd number of carbon atoms corresponds to the bond alternation defect with an unpaired electron. Then taking account of the kinetic energy of lattice vibrations, the vibronic states of the defect is obtained. It is shown that the unpaired electron moves relatively freely. Finally a method to calculate self-consistently the electronic states and the lattice deformation in the presence of Coulomb interaction is presented. It is shown within the Hartree-Fock approximation that the ground state changes from CDW state to SDWstate with increase of on-site Coulomb interaction.

Resonant states in the presence of Coulomb interactions

continuum. However, as is always the case, the derivation relied on the strict finite range of the potential. In such a situation, the analytic structure of the Green's function in the complex-energy plane can readily be established. When a long-range force is present, such as the Coulomb potential, the Green's function no longer satisfies the usual outgoingwave boundary conditions and the arguments leading to the eigenfunction expansion become invalid. In this note we impose a suitable modification of the boundary conditions for the radial wave functions in order to include a Coulomb potential in the Schrgdinger equation. In addition, we show that the arguments leading to the eigenfunction expansion of the Green's function in terms of its poles can be recovered, once the essential singularity at the origin of the complex k-plane is properly taken into account. For simplicity, we restrict ourselves to a particle of zero angular momentum and choose units in which h = 2m= 1. The extension to higher angular momentum is straighforward. The Schr5dinger equation that' we consider is then

Forward- and backward-angle regge-pole contribution in the presence of coulomb interaction

By assuming that the total scattering function is the product of the usual Coulomb scattering function and a model nuclear-scattering function, which is an even function ofλ=l+1/2 having a simple pole atλ=λ0 in the first quadrant of theλ-plane, it is shown that, for large values of |λ0| and of the Sommerfeld parameter, the scattering amplitude can be expressed as the sum of a saddle-point contribution and a pole contribution. Owing to the presence of the Coulomb term in the scattering function, an angleθc exists such that the pole contribution changes its form in going fromθ θc. The two forms of the pole contribution are in agreement with the contribution to the scattering amplitude, calculated according to the Keller rules, of generalized Coulomb trajectories diffracted at the surface of a sphere whose radius is connected with Reλ0. By using the fact that our amplitude in a certain limit becomes a representation of the Rutherford amplitude, it is shown that this quantity is completely «near-side».

New methods for the solution of the coupled equations describing inelastic scattering in the presence of Coulomb interaction

Inelastic heavy-ion scattering is described by a set of coupled homogeneous second-order differential equations. In the canonical method the boundary value problem corresponding to the scattering wave solutions had to be solved by calculating a complete set of linear independent solutions and choosing the appropriate linear combination. We now reformulate the Schrödinger equations for the radial wave functions, obtaining first-order differential equations which can be solved by iteration. Only few iterations of the coupled equations are then needed. A discussion of the accuracy and feasibility of two different iteration methods is given.

Enhancement of the magnetic susceptibility of TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) by Coulomb correlations

In order to gain some insight into the nature of the interactions in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), we compare the magnitude of the observed magnetic susceptibility $\ensuremath{\chi}$ with the predictions of two simple models. First we consider a tight-binding band of noninteracting electrons and find that the magnitude of the observed high-temperature $\ensuremath{\chi}$ is large compared to that calculated using the expected bandwidth. The same conclusion is also reached by a more general consideration of the magnitude of the product $T\ensuremath{\chi}$. It is concluded that at high temperatures $\ensuremath{\chi}$ is enhanced. We then compare the observed $\ensuremath{\chi}$ to calculations using a Hubbard model with on-site Coulomb repulsion energy $U$. $\ensuremath{\chi}$ is calculated in both the low-$U$ and large-$U$ limits and, in either limit, the large magnitude of $\ensuremath{\chi}$ can be readily accounted for. Using these two results, a relationship between the two parameters $U$ and the bandwidth $4t$ is obtained, knowing the magnitude of $\ensuremath{\chi}$ at 300 K. From independent estimates of $4t$, it is concluded that $U$ is comparable or larger than $4t$, at least on one stack. Additional evidence for and against the presence of Coulomb interactions in TTF-TCNQ is summarized.

Separable expansions for local potentials with Coulomb interactions

If two particles are interacting via a short range potential and a repulsive Coulomb potential the t matrix can be written as a sum of the Coulomb and the ''nuclear'' t matrices. In order to solve the three-nucleon problem with Coulomb interactions usually we need a separable representation of this ''nuclear'' t matrix. A recently proposed method for finding a separable expansion for local potentials is here extended to find a rapidly convergent separable expansion, with analytic form factors, for the ''nuclear'' part of the t matrix of a local potential, in the presence of Coulomb interactions. The method is illustrated for a two-term Malfliet-Tjon potential. In each rank the ''nuclear'' phase shift is close to the corresponding phase shift when the Coulomb interaction is switched off. (AIP)

Coulomb effects on the Peierls transition

The influence of the intrachain Coulomb interactions on the Peierls transition is examined. The change of the form of the electronic polarizability at twice the Fermi wavevector, to a power law divergence in the presence of Coulomb interactions, is found to enhance the mean field transition temperature for the Peierls transition.

On the collective excitations of super-conductors with the P-pairing

In this paper the theory of collective excitations of superconductors with the P-pairing is developed. We start from the ground state of the Balian and Wethamer type, with the isotropic energy gap. The collective excitations we get by use of the self-consistent field method, applied by Bogolubov to S-superconductors. In contradistinction to the S-superconductors in our case we have only one spin branch of excitations. We obtain two-quasiparticle excitations with the continuous energy spectrum, separated by a gap. If Coulomb interaction disappears we get the quasi-acoustic excitations with discrete energy spectrum. If the interaction strength multiplied by leveldensity at Fermi surface tends to zero, the dispersion law is the same as for S-superconductors. Applying the same limit in presence of Coulomb interaction we obtain for sufficiently small pairs momenta the Langmuir frequency. In the last section we obtain the electrodynamics of a state with P-pairing. Among other results the gauge-invariance of the Meissner-effect is proved. All results are obtained without any special choice of the form of interaction. Some formulae in the paper are satisfied also for superposition of S- and P-pairing.