Many-body Interaction Effects Research Articles

Equilibrium and dynamics of a trapped superfluid Fermi gas with unequal masses

Interacting Fermi gases with equal populations but unequal masses are investigated at zero temperature using local density approximation and the hydrodynamic theory of superfluids in the presence of harmonic trapping. We derive the conditions of energetic stability of the superfluid configuration with respect to phase separation and the frequencies of the collective oscillations in terms of the mass ratio and the trapping frequencies of the two components. We discuss the behavior of the gas after the trapping potential of a single component is switched off and show that, near a Feshbach resonance, the released component can still remain trapped due to many-body interaction effects. Explicit predictions are presented for a mixture of $^{6}\text{L}\text{i}$ and $^{40}\text{K}$ with resonant interaction.

Effect of Many Body Interactions on Cohesive Energy of Alkalihalides

Effect of Many Body Interactions on Cohesive Energy of Alkalihalides

Observation of many-body Coulomb interaction effects on the photoluminescence spectra of InAs/GaAs quantum dots

InAs quantum dots (QDs) on GaAs (0 0 1) substrates were grown by Molecular Beam Epitaxy (MBE) using two growth temperatures. Photoluminescence (PL) pump power dependence measurements at low temperature were carried out for sample grown at higher temperature (520 °C). With increasing excitation density, the ground-state transition energy is found to decrease by 8 meV, while the excited-state transition energies exhibit resonance behaviour. The redshift of the ground-state emission was related to the band-gap renomalization (BGR) effect whereas the blueshift of the excited-state emissions was assigned to the compensation between filling of fine structure states and BGR effects. Using a quasi-resonant PL measurement, we have shown that the renormalization of the band-gap had to occur in the QD barrier.

Many-body interaction in semiconductors probed with two-dimensional Fourier spectroscopy

A particular difficulty in studying many-body interactions in a solid is the absence of an experimental technique that can directly probe their key characteristics. We show that 2D Fourier spectroscopy provides an efficient tool for the measurement of critical parameters describing the effect of many-body interactions on the optical response of semiconductors. We develop the basic microscopic theory of 2D Fourier spectroscopy of semiconductors in the framework of the three-band model (heavy holes, light holes, and electrons). The theory includes many-body correlations nonperturbatively and can be generalized straightforwardly in order to describe 2D Fourier spectra obtained in atomic physics. We establish a relation between the 2D Fourier spectrum and the many-body correlations. It is shown, in particular, that 2D Fourier spectroscopy provides a principal possibility to establish experimentally the origin of the fast decay of the memory term describing the Coulomb interaction between heavy- and light-hole excitons. The theory is applied to an analysis of the available experimental data. Experiments providing more detailed information are suggested.

High precision investigations of the hyperfine structure of metastable levels in a chromium atom

A series of precise measurements of the hyperfine structure (hfs) of nine metastable levels in a chromium atom 53Cr has been performed. For eight levels the hfs has been investigated with the method of laser induced fluorescence (LIF) on an atomic beam, and for the lowest-lying metastable level 3d54s 5S2 a still more precise method of laser-rf double resonance on an atomic beam (ABMR-LIRF) has been applied. A semi-empirical analysis of the fine and the hyperfine structure of the chromium atom, including many-body interaction effects, has been carried out in order to determine the value of the nuclear electric quadrupole moment, which amounts to Q = −0.22(1) barn.

Many-body interaction effects in doped and undoped graphene: Fermi liquid versus non-Fermi liquid

We consider theoretically the electron-electron interaction induced many-body effects in undoped (``intrinsic'') and doped (``extrinsic'') two-dimensional (2D) graphene layers. We find that (1) intrinsic graphene is a marginal Fermi liquid with the imaginary part of the self-energy, $\mathrm{Im}\phantom{\rule{0.2em}{0ex}}\ensuremath{\Sigma}(\ensuremath{\omega})$, varying linearly in energy $\ensuremath{\omega}$ for small $\ensuremath{\omega}$, implying that the quasiparticle spectral weight vanishes at the Dirac point as ${(\mathrm{ln}\phantom{\rule{0.2em}{0ex}}\ensuremath{\omega})}^{\ensuremath{-}1}$; and (2) extrinsic graphene is a well-defined Fermi liquid with $\mathrm{Im}\phantom{\rule{0.2em}{0ex}}\ensuremath{\Sigma}(\ensuremath{\omega})\ensuremath{\sim}{\ensuremath{\omega}}^{2}\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}\ensuremath{\omega}$ near the Fermi surface similar to 2D carrier systems with parabolic energy dispersion. We provide analytical and numerical results for quasiparticle renormalization in graphene, concluding that all experimental graphene systems are ordinary 2D Fermi liquids since any doping automatically induces generic Fermi liquid behavior.

Coherent Zeeman resonance from electron spin coherence in a mixed-typeGaAs/AlAs quantum well

Coherent Zeeman resonance from electron spin coherence is demonstrated in a Lambda-type three-level system, coupling electron spin states via trions. The optical control of electron density that is characteristic of a mixed-type quantum-well facilitates the study of trion formation as well as the effects of many-body interactions on the manifestation of electron spin coherence in the nonlinear optical response.

Phase behavior of a suspension of colloidal hard rods and nonadsorbing polymer

We study the phase behavior of a mixture of colloidal hard rods with a length-to-diameter ratio of L/sigma(c)=5 and nonadsorbing ideal polymer. We map our binary mixture onto an effective one-component system by integrating out the degrees of freedom of the polymer coils. We derive a formal expression for the exact effective Hamiltonian of the colloidal rods, i.e., it includes all effective many-body interactions and it is related to the exact free volume available for the polymer. We determine numerically on a grid the free volume available for the ideal polymer coils "on the fly" for each colloidal rod configuration during our Monte Carlo simulations. This allows us to go beyond first-order perturbation theory, which employs the pure hard-rod system as reference state. We perform free energy calculations for the isotropic, nematic, smectic, and crystal phase using thermodynamic integration and common tangent constructions are used at fixed polymer fugacities to map out the phase diagram. The phase behavior is determined for size ratios q=sigma(p)/sigma(c)=0.15, 0.5, and 1, where sigma(p) is the diameter of the polymer coils. The phase diagrams based on the full effective Hamiltonian are compared with those obtained from first-order perturbation theory, from simulations using the effective pair potential approximation to the effective Hamiltonian, and with those based on an empiric effective depletion potential for the rods. We find that the many-body character of the effective interactions stabilizes the nematic and smectic phases for large q, while the effective pair potential description overestimates the attractive interactions and favors, hence, a broad isotropic-crystal coexistence.

A First-Principles Description of Liquid BeF2 and Its Mixtures with LiF: 1. Potential Development and Pure BeF2

The construction of an interaction potential for BeF2 and its mixtures with LiF on a purely first-principles basis is described. The quality of the representation of the forces on the ions obtained from ab initio electronic structure calculations by various potentials, which include many-body interaction effects to different extents, are considered. The predictions of the properties of pure BeF2 obtained in simulations with a polarizable potential are then compared with experimental values. In the subsequent paper, a more extensive comparison of the predicted properties of LiF-BeF2 mixtures with experiment is considered.

Effect of many-body interactions on the bulk and interfacial phase behavior of a model colloid-polymer mixture

We study a model suspension of sterically stabilized colloidal particles and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. By integrating out the degrees of freedom of the polymer coils, we derive a formal expression for the effective one-component Hamiltonian of the colloids. We employ an efficient Monte Carlo simulation scheme for this mixture based on the exact effective colloid Hamiltonian; i.e., it incorporates all many-body interactions. The many-body character of the polymer-mediated effective interactions between the colloids yields bulk phase behavior and adsorption phenomena that differ substantially from those found for pairwise simple fluids. We determine the phase behavior for size ratios q=sigma(p)/sigma(c)=1, 0.6, and 0.1, where sigma(c) and sigma(p) denote the diameters of the colloids and polymer coils, respectively. For q=1 and 0.6, we find both a fluid-solid and a stable colloidal gas-liquid transition with an anomalously large bulk liquid regime caused by the many-body interactions. We compare the phase diagrams obtained from simulations with the results of the free-volume approach and with direct simulations of the true binary mixture. Although we did not simulate the polymer coils explicitly, we are able to obtain the three partial structure factors and radial distribution functions. We compare our results with those obtained from density functional theory and the Percus-Yevick approximation. We find good agreement between all results for the structure. We also study the mixture in contact with a single hard wall for q=1. Upon approach of the gas-liquid binodal, we find far from the triple point, three layering transitions in the partial wetting regime.

Melting line of charged colloids from primitive model simulations

We develop an efficient simulation method to study suspensions of charged spherical colloids using the primitive model. In this model, the colloids and the co- and counterions are represented by charged hard spheres, whereas the solvent is treated as a dielectric continuum. In order to speed up the simulations, we restrict the positions of the particles to a cubic lattice, which allows precalculation of the Coulombic interactions at the beginning of the simulation. Moreover, we use multiparticle cluster moves that make the Monte Carlo sampling more efficient. The simulations are performed in the semigrand canonical ensemble, where the chemical potential of the salt is fixed. Employing our method, we study a system consisting of colloids carrying a charge of 80 elementary charges and monovalent co- and counterions. At the colloid densities of our interest, we show that lattice effects are negligible for sufficiently fine lattices. We determine the fluid-solid melting line in a packing fraction eta-inverse screening length kappa plane and compare it with the melting line of charged colloids predicted by the Yukawa potential of the Derjaguin-Landau-Verwey-Overbeek theory. We find qualitative agreement with the Yukawa results, and we do not find any effects of many-body interactions. We discuss the difficulties involved in the mapping between the primitive model and the Yukawa model at high colloid packing fractions (eta>0.2).

Effects of collective excitations on the quantum well intersubband absorption

The dependence of the intersubband absorption spectra on the Coulomb interaction and quantum well (QW) width is studied. Rather than following the Fermi–Dirac distribution, we have solved the intersubband equations of motion to determine the subband population self-consistently. We have gone beyond the linear absorption theory to show the effect of various many-body interactions on the absorption spectra. It is found that the redistribution of electrons in excited states reduces the absorption. Our results indicate that the line shape and peak position are determined by the interplay of different collective excitations, such as the Fermi edge singularity and the intersubband plasmon. The dependence of the absorption spectrum on the QW width and the subband effective masses is also discussed.

Three-body interactions improve the prediction of rate and mechanism in protein folding models.

Here we study the effects of many-body interactions on rate and mechanism in protein folding by using the results of molecular dynamics simulations on numerous coarse-grained Calpha-model single-domain proteins. After adding three-body interactions explicitly as a perturbation to a Gō-like Hamiltonian with native pairwise interactions only, we have found (i) a significantly increased correlation with experimental phi values and folding rates, (ii) a stronger correlation of folding rate with contact order, matching the experimental range in rates when the fraction of three-body energy in the native state is approximately 20%, and (iii) a considerably larger amount of three-body energy present in chymotripsin inhibitor than in the other proteins studied.

Nonlinear screening and effective electrostatic interactions in charge-stabilized colloidal suspensions

A nonlinear response theory is developed and applied to electrostatic interactions between spherical macroions, screened by surrounding microions, in charge-stabilized colloidal suspensions. The theory describes leading-order nonlinear response of the microions (counterions, salt ions) to the electrostatic potential of the macroions and predicts microion-induced effective many-body interactions between macroions. A linear response approximation [A.R. Denton, Phys. Rev. E 62, 3855 (2000)] yields an effective pair potential of screened-Coulomb (Yukawa) form, as well as a one-body volume energy, which contributes to the free energy. Nonlinear response generates effective many-body interactions and essential corrections to both the effective pair potential and the volume energy. By adopting a random-phase approximation (RPA) for the response functions, and thus neglecting microion correlations, practical expressions are derived for the effective pair and triplet potentials and for the volume energy. Nonlinear screening is found to weaken repulsive pair interactions, induce attractive triplet interactions, and modify the volume energy. Numerical results for monovalent microions are in good agreement with available ab initio simulation data and demonstrate that nonlinear effects grow with increasing macroion charge and concentration and with decreasing salt concentration. In the dilute limit of zero macroion concentration, leading-order nonlinear corrections vanish. Finally, it is shown that nonlinear response theory, when combined with the RPA, is formally equivalent to the mean-field Poisson-Boltzmann theory and that the linear response approximation corresponds, within integral-equation theory, to a linearized hypernetted-chain closure.

On the origin of time-resolved pump–probe differential signal from excitons in quantum wells

We investigate the origin and nature of femtosecond time-resolved (TR) pump–probe differential (PPD) signal from the exciton resonance in semiconductor quantum wells (QWs) in the coherent regime. The excitonic PPD signal is expressed as a sum of interferences of the excitonic nonlinear polarization with the incident probe field and the linear polarization induced by the probe. A nonzero excitonic TR-PPD signal from QWs can occur for time t larger than the pulse width ( t p) when the exciton dephasing time is longer than t p, and this signal can originate only from the second interference term for both the negative and positive pump–probe delays, the contribution from the first interference term being zero for t> t p. This is in contrast with the case of PPD signal in the spectral domain where the first interference term plays a major role. We discuss the effects of many body interactions on the TR-PPD signal. It is shown that excitation-induced dephasing can cause a significant increase in the rise time of the TR-PPD signal.

Destruction of interference by many-body interactions in cold atomic Bose gases

We study the effects of many-body interactions on the interference in a Mach-Zehnder interferometer for matter waves of ultracold Bose atoms. After switching off an axial trapping potential, the thermal initial wavepacket expands, and subsequently interference fringes may be observed in a circular 1D trap. These are computed for axial harmonic or $\delta$-function traps, and for interaction strengths from the Thomas-Fermi regime to the Tonks-Girardeau limit. It is shown that many-body correlations in a realistic setup destroy interference to a large degree. Analytical expressions allowing to infer the observability of phase coherence and interference are provided.

The effect of many-body interactions on the electrostatic force on a finite chain of spheres confined in a long charged tube

The effect of many-body interactions on the electrostatic force between spheres in a chain of charged spheres confined in a long charged pore has been quantified by solving the non-linear Poisson–Boltzmann equation (PBE), using the adaptive finite element method (AFEM) combined with error minimisation techniques. The equation is solved for conditions of constant surface potential and constant surface charge density. The computed force indicates that the force between any two spheres in a long chain of spheres in a long charged tube does not differ significantly from the force between only two spheres in a tube. This trend is also observed for an unconfined chain of spheres, where the force between two isolated spheres is again very similar to the force between any two spheres in a long unbounded chain. The results also quantify the effect of many-body interactions on the reduction of the repulsion force between the spheres. A significant reduction in the repulsion force between the spheres is observed when the radial distance between the pore wall and the chain of spheres is reduced. The effect of the dimensionless radius of the spheres on the electrostatic force between them has been determined and significant reduction of the force observed as the dimensionless radius is reduced.

Simple two-state protein folding kinetics requires near-levinthal thermodynamic cooperativity.

Simple two-state folding kinetics of many small single-domain proteins are characterized by chevron plots with linear folding and unfolding arms consistent with an apparent two-state description of equilibrium thermodynamics. This phenomenon is hereby recognized as a nontrivial heteropolymer property capable of providing fundamental insight into protein energetics. Many current protein chain models, including common lattice and continuum Gō models with explicit native biases, fail to reproduce this generic protein property. Here we show that simple two-state kinetics is obtainable from models with a cooperative interplay between core burial and local conformational propensities or an extra strongly favorable energy for the native structure. These predictions suggest that intramolecular recognition in real two-state proteins is more specific than that envisioned by common Gō-like constructs with pairwise additive energies. The many-body interactions in the present kinetically two-state models lead to high thermodynamic cooperativity as measured by their van't Hoff to calorimetric enthalpy ratios, implying that the native and denatured conformational populations are well separated in enthalpy by a high free-energy barrier. It has been observed experimentally that deviations from Arrhenius behavior are often more severe for folding than for unfolding. This asymmetry may be rationalized by one of the present modeling scenarios if the effective many-body cooperative interactions stabilizing the native structure against unfolding is less dependent on temperature than the interactions that drive the folding kinetics.

A model for many-body interaction effects in open quantum dot systems

We discuss the influence of the electron–electron interaction on transportproperties of open quantum dot systems. Based on the idea of the Andersonmodel, we present interaction-induced temperature-dependent corrections to theconductance beyond the single-particle picture.

Many-body interactions in colloidal suspensions

At low ionic strength the interaction between any two charged colloids in aconcentrated charge-stabilized suspension is strongly influenced by the chargedistributions of other colloids in the neighbourhood of the two interactingcolloids. When such many-body interactions become important, a colloidalsuspension can no longer be treated as a simple Yukawa liquid. We here (i)discuss experimental data showing that many-body interactions are indeedpresent in low-salt-content colloidal suspensions, (ii) report on three-bodycalculations in colloidal systems that help in understanding the conceptof ‘macroion shielding’, (iii) demonstrate the configuration dependenceof effective forces and (iv) illustrate, using simulations based on (a) atruncated Yukawa potential with a density-dependent cut-off and (b) afull Poisson–Boltzmann mean-field description, the effect of many-bodyinteractions on the solid–liquid phase behaviour of colloidal suspensions.