Coulomb repulsion versus Hubbard repulsion in a disordered chain

We study the difference between on site Hubbard and long range Coulomb repulsions for two interacting particles in a disordered chain. The system size L (in units of the lattice spacing) is of the order of the one particle localization length and the energies are taken near the band center. In the two cases, the limits of weak and strong interactions are characterized by uncorrelated energy levels and are separated by a crossover regime where the states are more extended and the spectra more rigid. U denoting the interaction strength and t the kinetic energy scale, the crossovers take place for interaction energy to kinetic energy ratios U/t and U/(2tL) of order one, for Hubbard and Coulomb repulsions respectively. While Hubbard repulsion can only yield weak critical chaos with intermediate spectral statistics, Coulomb repulsion can drive the two particle system to quantum chaos with Wigner-Dyson spectral statistics. The interaction matrix elements are studied to explain this difference. Received 21 March 2000 and Received in final form 5 February 2001     

Stability of bipolarons in the presence of a magnetic field

The stability of large Fröhlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω _c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α _c (above which the bipolaron is stable) decreases with increasing ω _c and (ii) the critical Coulomb repulsion strength U _c (below which the bipolaron is stable) increases with increasing ω _c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω _c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ? 1) we obtain for ω _c ? 1 that E _estim ? E _estim(ω _c = 0) + c(u)ω _c/α ⁴ with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω _c ? α ²? 1 we find in 3D E _estim ? ω _c - α ² A(u) ln²(ω _c/α ²), (also with an explicit analytical expression for A(u)) whereas in 2D E _estim ^2D ? ω _c - α√ω _cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.     

Coulomb repulsion effects in driven electron transport

We study numerically the influence of strong Coulomb repulsion on the current through molecular wires that are driven by external electromagnetic fields. The molecule is described by a tight-binding model whose first and last site is coupled to a respective lead. The leads are eliminated within a perturbation theory yielding a master equation for the wire. The decomposition into a Floquet basis enables an efficient treatment of the driving field. For the electronic excitations in bridged molecular wires, we find that strong Coulomb repulsion significantly sharpens resonance peaks which broaden again with increasing temperature. By contrast, it has only a small influence on effects like non-adiabatic electron pumping and coherent current suppression.     

Spin-singlet small bipolarons in Nb-doped BaTiO3

The magnetic susceptibility and the electrical resistivity of n-type BaTi1-xNbxO3 have been measured over a wide temperature range. It is found that, for 0相似文献   

Splitting of a high-energy photon in a strong Coulomb field

The helical splitting amplitudes of a high-energy photon in an external Coulomb field are found exactly with respect to Zα. Both screened and unscreened potentials are investigated. The treatment is carried out within the semiclassical approach, which is valid for small angles between the momenta of all photons. A new representation is used for the semiclassical electronic Green’s function. The resulting expressions are analyzed in detail for the case of transverse components of the momenta of the final photons that are large compared with the electron mass. Zh. éksp. Teor. Fiz. 112, 1921–1940 (December 1997)     

Scattering of a low-energy electron in a strong Coulomb field

The low-energy electron scattering cross section in a strong Coulomb field is analyzed theoretically. It is shown that the exact cross section in a wide energy range significantly differs from the results obtained in the first Born approximation and in the nonrelativistic approximation.     

Density-functional calculation of the Coulomb repulsion and correlation strength in superconducting LaFeAsO

Constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund’s exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3–4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F ⁰ = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure. The article is published in the original.     

Reduced Coulomb repulsion in NMP-TCNQ through intra- and interchain polarization

The “bare” Coulomb repulsion between two delocalized electrons in a one-dimensional poly (TCNQ^?) chain of the organic charge transfer crystal NMP-TCNQ is shown to be substantially reduced by attractive terms arising (1) from the polarization of the localized electronic system of the TCNQ chain itself and (2) from the polarization of the delocalized electrons in neighbouring NMP chains. The resulting effective repulsion is 0.33 eV at the Fermi level.     

Two-loop self-energy correction in a strong Coulomb nuclear field

The two-loop self-energy correction to the ground-state energy levels of hydrogen-like ions with nuclear charges Z≥10 is calculated without the Zα expansion, where α is the fine-structure constant. The data obtained are compared with the results of analytical calculations within the Zα expansion; significant disagreement with the analytical results of order α²(Zα)⁶ has been found. Extrapolation is used to obtain the most accurate value for the two-loop self-energy correction for the 1s state in hydrogen.     

Coulomb collisions in strong magnetic fields

Summary Scattering cross-sections, reflection and transmission coefficients are derived for charged-particle potential scattering in the presence of a quantizing constant magnetic field within the Green's function approach. The optical theorem and the limit of the cross-section for vanishing values of the magnetic field have also been obtained. A numerical analysis of the total cross-section for different magnetic-field intensities and values of the screening constant has been performed. The total cross-sections are found to differ significantly from the field-free ones only for magnetic-field intensities and incident particle energies such that only few Landau channels are open. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Excitations in a quantum Hall ferromagnet with strong Coulomb interaction

A quantum Hall ferromagnet is considered at integer fillings ν, provided typical Coulomb interaction energy E _c is large compared to the cyclotron energy ω_H. Low-energy collective modes consist of a magnetoplasmon exciton and a gapless spin exciton. All charged excitations have a gap. The activation energy gap for a pair of charged topological excitations—skyrmion and antiskyrmion—is small, i.e., Δ< vω_H. The electric charge of a skyrmion is the multiple q=eνQ, where Q is the integer topological charge.     

Vibration isolation in the presence of Coulomb friction

Transmissibility curves for a Coulomb-damped flexible mounting are presented for both rigidly and elastically coupled damping. The closed solutions easily obtainable for the slipping and sticking phases of the motion are used in the method, and no conceptual approximation is involved. The transmissibility curves are compared with previously published approximations based on linearization of the damping. It is shown that the approximate method seriously underestimates the heights of significant resonant peaks and gives optimum friction values somewhat in error, but gives reasonable approximations to the transmissibilities at the high frequencies corresponding to effective isolation.