Effective Attractive Interaction Research Articles

Excitonic effects in a time-dependent density functional theory

Excited state properties of one-dimensional molecular materials are dominated by many-body interactions resulting in strongly bound confined excitons. These effects cannot be neglected or treated as a small perturbation and should be appropriately accounted for by electronic structure methodologies. We use adiabatic time-dependent density functional theory to investigate the electronic structure of one-dimensional organic semiconductors, conjugated polymers. Various commonly used functionals are applied to calculate the lowest singlet and triplet state energies and oscillator strengths of the poly(phenylenevinylene) and ladder-type (poly)(para-phenylene) oligomers. Local density approximations and gradient-corrected functionals cannot describe bound excitonic states due to lack of an effective attractive Coulomb interaction between photoexcited electrons and holes. In contrast, hybrid density functionals, which include long-range nonlocal and nonadiabatic corrections in a form of a fraction of Hartree-Fock exchange, are able to reproduce the excitonic effects. The resulting finite exciton sizes are strongly dependent on the amount of the orbital exchange included in the functional.

Many body exchange effects close to the s-wave Feshbach resonance in two-component Fermi systems: is a triplet superfluid possible?

We suggest that the exchange fluctuations close to a Feshbach resonance in a two-component Fermi gas can result in an effective p-wave attractive interaction. On the BCS side of a Feshbach resonance, the magnitude of this effective interaction is comparable to the s-wave interaction, therefore leading to a possible spin-triplet superfluid in the range of temperatures of actual experiments. We also show that the particle--hole exchange fluctuations introduce an effective scattering length which does not diverge, as the standard mean-field one does. Finally, using the effective interaction quantities we are able to model the molecular binding energy on the BEC side of the resonance.

Spinon-holon interactions in an anisotropict−Jchain: A comprehensive study

We consider a generalization of the one-dimensional t-J model with anisotropic spin-spin interactions. We show that the anisotropy leads to an effective attractive interaction between the spinon and holon excitations, resulting in a localized bound state. Detailed quantitative analytic predictions for the dependence of the binding energy on the anisotropy are presented and are verified by precise numerical simulations. The binding energy is found to interpolate smoothly between a finite value in the t- Jz limit and zero in the isotropic limit, going to zero exponentially in the vicinity of the latter. We identify changes in spinon dispersion as the primary factor for this nontrivial behavior. © 2007 The American Physical Society.

SANS Study of Polymer-Linked Droplets

We report experimental results obtained from SANS on microemulsion droplets connected by a telechelic polymer. Thanks to its ability to anchor droplets through its short stickers, the addition of this polymer leads to the formation of transient aggregates. Measurements were performed on samples at low surfactant content in such a way that the droplets appear to be isolated with a separation distance longer than the end-to-end distance of the polymer. The locally spherical structure of the micelles is unchanged in size upon polymer addition whereas the large rise in scattered intensity at low Q is due to the induced effective attractive interaction between droplets. The fitting model that we propose allows a quantitative description of the bridging effect.

Nonlocal transport mediated by nonlocal Hikami boxes: Condensation of evanescent quasiparticles injected into the superconducting gap

Evanescent quasiparticles entering a superconductor and propagating over distances larger than the coherence length give rise to intermediate states induced by double Andreev scattering on disorder. The resulting effective attractive interaction between evanescent quasiparticles is retarded at the extremely slow frequency of the applied bias voltage. The out-of-equilibrium mesoscopic superconductor with a fluctuating phase variable is compatible with a recent experiment [S. Russo et al., Phys. Rev. Lett. 95, 027002 (2005)]. Microscopic theory is discussed in the random-phase approximation.

Strongly Correlated Two-Photon Transport in a One-Dimensional Waveguide Coupled to a Two-Level System

We show that two-photon transport is strongly correlated in one-dimensional waveguide coupled to a two-level system. The exact S matrix is constructed using a generalized Bethe-ansatz technique. We show that the scattering eigenstates of this system include a two-photon bound state that passes through the two-level system as a composite single particle. Also, the two-level system can induce effective attractive or repulsive interactions in space for photons. This general procedure can be applied to the Anderson model as well.

Structure of a New Type of Transient Network: Entangled Wormlike Micelles Bridged by Telechelic Polymers

The thermodynamics and structural behavior of a new type of transient network are reported. The network is obtained by adding in a solution of entangled surfactant wormlike micelles a telechelic triblock copolymer whose hydrophobic ends anchor into the micelles and whose hydrophilic tails are swollen in the aqueous solvent and reversibly link the entangled cylindrical micelles. For comparison, we have also studied the same surfactant system decorated with an amphiphilic diblock copolymer which corresponds exactly to a triblock telechelic copolymer cut into two identical diblock copolymers. We find that the addition of telechelic polymers induces an effective attraction of entropic origin between the surfactant that may result, in equilibrium, in the coexistence of a dilute phase and a connected network, as predicted by theory. Small-angle neutron scattering experiments show first that the locally cylindrical structure of the micelles is maintained upon copolymer addition over a wide range of copolymer-to-surfactant molar ratio. On the other hand, the addition of telechelic polymer correlates with (i) the emergence of a broad peak in the structure factor at a finite scattering vector, q, due to the short-range steric repulsion induced by the polymer between the cylinders, and (ii) the large rise of the scattered intensity at low q's (for small enough surfactant concentration), which indicates an increase of the osmotic compressibility of the solution due to the effective attractive interaction between cylinders provoked by the telechelic polymers linking them.

Hybridized mechanism of pairing of fermions in single-walled carbon nanotubes

A two-band fermion model with boundary fields describing the band structure of a single-walled carbon nanotube (SWNT) is proposed and solved exactly by the nested Bethe ansatz. The fermions, occupying two degenerate subbands that are shifted relative to each other, interact via inner- and interband on-site Coulomb interactions, and one-particle and correlated on-site hybridizations. The critical exponents of the correlation functions are calculated using the Bethe ansatz solution and conformal field theory. It is found that a two-component electron liquid state, one of which is defined by an attractive effective electron-electron interaction, is realized for strong hybridized interaction. The attractive interaction leads to the formation of a spinless bound state of Cooper-type pairs and dominating correlations of singlet pairs for arbitrary band fillings. We suggest that this electron pairing mechanism may be the key to resolving the nature of superconductivity in SWNT's.

Density functional theory study of water adsorption at reduced and stoichiometric ceria (111) surfaces

We study the structure and energetics of water molecules adsorbed at ceria (111) surfaces for 0.5 and 1.0 ML coverages using density functional theory. The results of this study provide a theoretical framework for interpreting recent experimental results on the redox properties of water at ceria (111) surfaces. In particular, we have computed the structure and energetics of various absorption geometries at the stoichiometric ceria (111) surface. We find that single hydrogen bonds between the water and the oxide surface are favored in all cases. At stoichiometric surfaces, the water adsorption energy depends rather weakly on coverage. We predict that the observed coverage dependence of the water adsorption energy at stoichiometric surfaces is likely the result of dipole-dipole interactions between adsorbed water molecules. When oxygen vacancies are introduced in various surface layers, water molecules are attracted more strongly to the surface. We find that it is very slightly energetically favorable for adsorbed water to oxidized the reduced (111) surface with the evolution of H(2). In the event that water does not oxidize the surface, we predict that the effective attractive water-vacancy interaction will result in a significant enhancement of the vacancy concentration at the surface in agreement with experimental observations. Finally, we present our results in the context of recent experimental and theoretical studies of vacancy clustering at the (111) ceria surface.

Bridging interactions due to telechelic linkers balanced by screened Coulombicrepulsions

When telechelic polymers (poly-ethyleneoxide grafted at both ends with an aliphatic chain)are added to a microemulsion of decane droplets in water stabilized by a surfactant filmthey can link the droplets and thus introduce an effective attractive interaction between thedroplets, eventually leading to a phase separation. Starting from a neutral microemulsionwe show that this attractive interaction can be offset by adding minute quantities of anionic surfactant, thus introducing a repulsive Coulombic interaction. This opens up thepossibility of ‘weighting’ an unknown effective attraction against a well known repulsion.The phase behaviour and the small angle neutron scattering spectra of different samplesillustrate this point. The spectra are simulated assuming the interaction potentials to bethe sum of attractive and repulsive contributions and a good agreement is foundbetween the experimental and simulated spectra in the neutral microemulsion, inthe charged microemulsion and in the charged and connected microemulsion.

Electrolyte distribution around two like-charged rods: Their effective attractive interaction and angular dependent charge reversal

A simple model for two like-charged parallel rods immersed in an electrolyte solution is considered. We derived the three point extension (TPE) of the hypernetted chain/mean spherical approximation (TPE-HNC/MSA) and Poisson-Boltzmann (TPE-PB) integral equations. We numerically solve these equations and compare them to our results of Monte Carlo (MC) simulations. The effective interaction force, F(T), the charge distribution profiles, rho(el)(x,y), and the angular dependent integrated charge function, P(theta), are calculated for this system. The analysis of F(T) is carried out in terms of the electrostatic and entropic (depletion) contributions, F(E) and F(C). We studied several cases of monovalent and divalent electrolytes, for which the ionic size and concentration are varied. We find good qualitative agreement between TPE-HNC/MSA and MC in all the cases studied. The rod-rod force is found to be attractive when immersed in large size, monovalent or divalent electrolytes. In general, the TPE-PB has poor agreement with the MC. For large monovalent and divalent electrolytes, we find angular dependent charge reversal charge inversion and polarizability. We discuss the intimate relationship between this angular dependent charge reversal and rod-rod attraction.

Conductivity of charge-ordered layered crystals in quantizing magnetic fields in the inversion region of the nonoscillating magnetoresistance component

The conductivity of charge-ordered layered crystals in electric and quantizing magnetic fields orthogonal to the layers is calculated. Charge ordering is considered as an alternation of layers with different charge-carrier densities. When scattering by acoustic phonons, conductivity is calculated in the approximation of profound quantization, so that the intersubband transitions can be assumed to be suppressed. It is shown that, in the transition of the layered crystal from the disordered to almost completely ordered state, the conductivity oscillations exhibit double periodicity. In this case, the high “carrier” frequency is controlled by the effective attractive electron-electron interaction responsible for charge ordering, whereas the low “modulation” frequency is defined by the width of the narrow miniband orthogonal to the layers. It is also shown that, in the transition from a disordered to an almost completely ordered state, the longitudinal conductivity of layered crystals decreases by two or three orders of magnitude. At the same time, the relative contribution of oscillations sharply increases, with satisfactory agreement to the experimental data.

Bose–Einstein condensates: Analytical methods for the Gross–Pitaevskii equation

We present simple analytical methods for solving the Gross–Pitaevskii equation (GPE) for the Bose–Einstein condensation (BEC) in the dilute atomic alkali gases. Using a soliton variational Ansatz we consider the effects of repulsive and attractive effective nonlinear interactions on the BEC ground state. We perform a comparative analysis of the solutions obtained by the variational Ansatz, the perturbation theory, the Thomas–Fermi approximation, and the Green function method with the numerical solution of the GPE finding universal ranges where these solutions can be used to predict properties of the condensates. Also, a generalization of the soliton variational approach for two-species of alkali atoms of the GPE is performed as a function of the effective interaction λi(i=1,2) and the inter-species λ12 and λ21 constants.

Hybridized mechanism of pairing and the heavy fermion state: Exactly solvable two-band model with strong hybridized interactions

The two-band model describing strongly hybridized degenerated electron states in a chain is proposed and solved exactly by means of the Bethe ansatz. The fermions of the subbands interact with each other via one-particle and one-particle correlated on-site hybridized interactions. The chain of electrons is reduced to the model with two noninteracting subbands with a given total number of electrons, in which fermions interact via different effective constants of interactions. In the case of strong hybridized interaction the on-site repulsive interaction is compensated by hybridized interaction, which leads to an effective attractive interaction between particles and the formation of spinless bound states of Cooper type. This electron liquid component determines both heavy fermion- and superconducting-type behavior of electron properties.

Bright solitons in coupled defocusing NLS equation supported by coupling: Application to Bose–Einstein condensation

We demonstrate the formation of bright solitons in coupled self-defocusing nonlinear Schrödinger (NLS) equation supported by attractive coupling. As an application we use a time-dependent dynamical mean-field model to study the formation of stable bright solitons in two-component repulsive Bose–Einstein condensates (BECs) supported by interspecies attraction in a quasi one-dimensional geometry. When all interactions are repulsive, there cannot be bright solitons. However, bright solitons can be formed in two-component repulsive BECs for a sufficiently attractive interspecies interaction, which induces an attractive effective interaction among bosons of same type.

Quantum Monte Carlo method for models of molecular nanodevices

We introduce a quantum Monte Carlo technique to calculate exactly at finite temperatures the Green function of a fermionic quantum impurity coupled to a bosonic field. While the algorithm is general, we focus on the single impurity Anderson model coupled to a Holstein phonon as a schematic model for a molecular transistor. We compute the density of states at the impurity in a large range of parameters, to demonstrate the accuracy and efficiency of the method. We also obtain the conductance of the impurity model and analyze different regimes. The results show that even in the case when the effective attractive phonon interaction is larger than the Coulomb repulsion, a Kondo-like conductance behavior might be observed.

Development of Superconducting Correlation at Low Temperatures in the Two-dimensionalt–JModel

The equal-time pairing correlation function of the two-dimensional t-J model on a square lattice is studied using a high-temperature expansion method. The sum of the pairing correlation, its spatial dependence, and the correlation length are obtained as functions of temperature down to $T \simeq 0.2 t$. By comparison of single-particle contributions in the correlation functions, we find an effective attractive interaction between quasi-particles in $d_{x^2-y^2}$-wave pairings. It is shown that d-wave correlation grows rapidly at low temperatures for 0.5 < n < 0.9, with n being the electron density. The temperature for this growth is roughly scaled by J/2. This is in sharp contrast to the Hubbard model in a weak or intermediate coupling region, where there is no numerical evidence of superconductivity.

VORTICES IN MULTICOMPONENT BOSE–EINSTEIN CONDENSATES

We review the topic of quantized vortices in multicomponent Bose–Einstein condensates of dilute atomic gases, with an emphasis on the two-component condensates. First, we review the fundamental structure, stability and dynamics of a single vortex state in a slowly rotating two-component condensates. To understand recent experimental results, we use the coupled Gross–Pitaevskii equations and the generalized nonlinear sigma model. An axisymmetric vortex state, which was observed by the JILA group, can be regarded as a topologically trivial skyrmion in the pseudospin representation. The internal, coherent coupling between the two components breaks the axisymmetry of the vortex state, resulting in a stable vortex molecule (a meron pair). We also mention unconventional vortex states and monopole excitations in a spin-1 Bose–Einstein condensate. Next, we discuss a rich variety of vortex states realized in rapidly rotating two-component Bose–Einstein condensates. We introduce a phase diagram with axes of rotation frequency and the intercomponent coupling strength. This phase diagram reveals unconventional vortex states such as a square lattice, a double-core lattice, vortex stripes and vortex sheets, all of which are in an experimentally accessible parameter regime. The coherent coupling leads to an effective attractive interaction between two components, providing not only a promising candidate to tune the intercomponent interaction to study the rich vortex phases but also a new regime to explore vortex states consisting of vortex molecules characterized by anisotropic vorticity. A recent experiment by the JILA group vindicated the formation of a square vortex lattice in this system.

Luttinger liquid versus charge density wave behaviour in the one-dimensional spinless fermion Holstein model

We discuss the nature of the different ground states of the half-filled Holstein model of spinless fermions in 1D. In the metallic regime we determine the renormalised effective coupling constant and the velocity of the charge excitations by a density-matrix renormalisation group (DMRG) finite-size scaling approach. At low (high) phonon frequencies the Luttinger liquid is characterised by an attractive (repulsive) effective interaction. In the charge-density wave Peierls-distorted state the charge structure factor scales to a finite value indicating long-range order.

Exciton sizes of conducting polymers predicted by time-dependent density functional theory

The electronic structure and size scaling of spectroscopic observables in conjugated polymers are investigated using time-dependent density functional theory. We show that local density approximations and gradient-corrected functionals do not have an effective attractive Coulomb interaction between photoexcited electron-hole pairs to form bound states and therefore do not reproduce finite exciton sizes. Long-range nonlocal and nonadiabatic density functional corrections (such as hybrid mixing with an exact Hartree-Fock exchange) are necessary to capture correct delocalization of photoexcitations in one-dimensional polymeric chains.