Weak Coulomb Interaction Research Articles

Momentum-Dependent Variational Approach to Correlated Electron System

We present here the momentum dependent local-ansatz wavefunction approach (MLA) with the best choice of the self-consistent variational parameters to describe the ground-state properties of the correlated electron systems in solids. With use of the self-consistent variational scheme, we performed the numerical calculations for the half-filled band as well as non-half-filled band Hubbard model on the hypercubic lattice in infinite dimensions. The ground-state energy in the MLA is lower than those of the local-ansatz approach (LA) and the Gutzwiller approach (GA) in weak and intermediate Coulomb interaction regimes. The double occupation number is suppressed as compared with the LA. We observe the distinct momentum dependence of the momentum distribution functions which is qualitatively different from those of the LA and the GA.

Thermoelectric effects of a laterally coupled double-quantum-dot structure

We investigate the thermoelectric properties of a laterally coupled double-quantum-dot structure. For this structure, a one-dimensional quantum dot (QD) chain between two leads forms a main channel for electron transmission, and each QD in the chain laterally couples to an additional QD. It is found that at low temperature, similar insulating bands emerge around the antiresonant points in the electronic and thermal conductance spectra. And, the edges of the insulating bands become steep rapidly with the increase of QD numbers. What’s interesting is that striking thermoelectric effect exists in the energy region where the insulating bands appear. Furthermore, with the formation of the insulation bands, the magnitude of the Seebeck coefficient becomes stable, whereas the thermoelectric efficiency is increased. By plotting the Lorentz number spectrum, we observe that in such a structure, the Lorentz number strongly violates the Wiedemann-Franz law in the insulating-band region with its maximum at the point of antiresonance. When weak intradot Coulomb interaction is taken into account, the weakened thermoelectric effect can still be improved with the increase of QD numbers.

Wonderful life at weak Coulomb interaction: increasing of superconducting/superfluid transition temperature by disorder

We have shown that in systems where the Coulomb interaction is strongly suppressed, the superconducting transition temperature may be enhanced by disorder close to the Anderson localization transition. This phenomenon is based on the enhancement by disorder of the matrix element of attraction in the Cooper channel. For enhancement to take place one needs (i) strong disorder which makes the single-particle wave functions strongly inhomogeneous in space and (ii) strong correlation of the patterns of inhomogeneity for different wavefunctions. One case where such correlation is well known is the system close to the Anderson transition. We review the notion of multifractality of wavefunctions in this region and show how the enhancement of Tc arises out of the multifractal correlations.

Electric Dipolar Kondo Effect Emerging from a Vibrating Magnetic Ion

When a magnetic ion vibrates in a metal, it inevitably introduces a new channel of hybridization with conduction electrons, and in general, the vibrating ion induces an electric dipole moment. In such a situation, we find that magnetic and nonmagnetic Kondo effects alternatively occur due to the screening of the spin moment and electric dipole moment of the vibrating ion. In particular, the electric dipolar two-channel Kondo effect is found to occur for a weak Coulomb interaction. We also show that a magnetically robust heavy-electron state appears near the fixed point of the electric dipolar two-channel Kondo effect. We believe that the vibrating magnetic ion opens a new door in Kondo physics.

The Binding Energy of Diquark-Antidiquark System in Nanometer and Subnanometer Scales

In this paper, using Monte Carlo Fortran code, we have obtained the binding energies for three different systems of diquark–antidiquark in distances from 0.01 to 15 nm. In [0.1 - 15] nm interval, we made use of Coulomb potential because in this interval, strong interaction is negligible. We have compared the binding energies of the systems with one another. The results of these comparisons were close to our anticipations. We also obtained the binding energy of one of the systems in the interval below 1 fm, where diquark-antidiquark systems comprise a tetraquark and the potential is of strong interaction type. Because of weak Coulomb interaction, strong interaction has been used as the basis of the calculations. The binding energy resulted is consistent with the existing references.

Chiral symmetry breaking in graphene

The question of whether the Coulomb interaction is strong enough to break the sublattice symmetry of undoped graphene is discussed. We formulate a strong coupling expansion where the ground state of the Coulomb Hamiltonian is found exactly and the kinetic hopping Hamiltonian is treated as a perturbation. We argue that many of the properties of the resulting system would be shared by graphene with a Hubbard model interaction. In particular, the best candidate sublattice symmetry-breaking ground state is an antiferromagnetic Mott insulator. We discuss the results of some numerical simulations which indicate that the Coulomb interaction is indeed subcritical. We also point out the curious fact that if the electron did not have spin degeneracy, the tendency to break chiral symmetry would be much greater and even relatively weak Coulomb interactions would likely gap the spectrum.

Momentum Dependent Local-Ansatz Approach to Correlated Electron Systems: Non Half-Filled Case

Momentum dependent local-ansatz wavefunction approach (MLA) to the correlated electron systems in solids has been further developed to solve best a self-consistent equation for variational parameters at non half-filling. With use of the improved variational scheme we performed the numerical calculations for the non-half-filled band Hubbard model on the hypercubic lattice in infinite dimensions. We verified that the self-consistent scheme significantly improves the correlation energy and the momentum distribution as compared with the original scheme in the MLA. We also demonstrate that the theory improves the standard variational methods such as the Local-Ansatz approach (LA) and the Gutzwiller wavefunction approach (GA); the ground-state energy in the MLA is lower than those of the LA and the GA in the weak and intermediate Coulomb interaction regimes. The double occupation number is shown to be suppressed as compared with the LA. Calculated momentum distribution functions show a distinct momentum dependence, which is qualitatively different from those of the LA and the GA.

Generating distributed entanglement from electron currents

Several recent experiments have demonstrated the viability of a passive device that can generate spin-entangled currents in two separate leads. However, manipulation and measurement of individual flying qubits in a solid state system has yet to be achieved. This is particularly difficult when a macroscopic number of these indistinguishable qubits are present. In order to access such an entangled current resource, we therefore show how to use it to generate distributed, static entanglement. The spatial separation between the entangled static pair can be much higher than that achieved by only exploiting the tunnelling effects between quantum dots. Our device is completely passive, and requires only weak Coulomb interactions between static and flying spins. We show that the entanglement generated is robust to decoherence for large enough currents.

Metallic glass electronic structure peculiarities revealed by UHV STM/STS

We present the results of ultrahigh vacuum scanning tunneling microscopy/spectroscopy investigation of metallic glass surface. The topography and electronic structure of Ni63.5Nb36.5 have been studied. A great number of clusters with size about 5–10 nm have been found on constant current scanning tunneling microscopy images. The tunneling spectra of normalized tunneling conductivity revealed the energy pseudogap in the vicinity of Fermi energy. For energy values above 0.1 eV the normalized tunneling conductivity changes linearly with increasing of tunneling bias. The obtained results can be understood within suggested theoretical model based on the interplay of elastic electron scattering on random defects and weak intra-cluster Coulomb interaction. The effects of the finite edges of electron spectrum of each cluster have to be taken into account to explain the experimental data. The tunneling conductivity behavior and peculiarities in current images of individual clusters can also be qualitatively analyzed in the framework of suggested model.

Bipolaron recombination in conjugated polymers

By using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term and an external electric field, we investigate the scattering process between a negative and a positive bipolaron in a system composed of two coupled polymer chains. Our results show that the Coulomb interactions do not favor the bipolaron recombination. In the region of weak Coulomb interactions, the two bipolarons recombine into a localized excited state, while in the region of strong Coulomb interactions they can not recombine. Our calculations show that there are mainly four channels for the bipolaron recombination reaction: (1) forming a biexciton, (2) forming an excited negative polaron and a free hole, (3) forming an excited positive polaron and a free electron, (4) forming an exciton, a free electron, and a free hole. The yields for the four channels are also calculated.

Correlation-Hole Induced Paired Quantum Hall States in the Lowest Landau Level

A theory is developed for the paired even-denominator fractional quantum Hall states in the lowest Landau level. We show that electrons bind to quantized vortices to form composite fermions, interacting through an exact instantaneous interaction that favors chiral p-wave pairing. There are two canonically dual pairing gap functions related by the bosonic Laughlin wave function (Jastrow factor) due to the correlation holes. We find that the ground state is the Moore-Read Pfaffian in the long-wavelength limit for weak Coulomb interactions, a new Pfaffian with an oscillatory pairing function for intermediate interactions, and a Read-Rezayi composite Fermi liquid beyond a critical interaction strength. Our findings are consistent with recent experimental observations of the 1/2 and 1/4 fractional quantum Hall effects in asymmetric wide quantum wells.

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

We re-examine the three-dimensional spinless Falicov-Kimball model with dispersive $f$ electrons at half-filling, addressing the dispute about the formation of an excitonic condensate, which is closely related to the problem of electronic ferroelectricity. To this end, we work out a slave-boson functional integral representation of the suchlike extended Falicov-Kimball model that preserves the $SO(2)\otimes U(1)^{\otimes 2}$ invariance of the action. We find a spontaneous pairing of $c$ electrons with $f$ holes, building an excitonic insulator state at low temperatures, also for the case of initially non-degenerate orbitals. This is in contrast to recent predictions of scalar slave-boson mean-field theory but corroborates previous Hartree-Fock and RPA results. Our more precise treatment of correlation effects, however, leads to a substantial reduction of the critical temperature. The different behavior of the partial densities of states in the weak and strong inter-orbital Coulomb interaction regimes supports a BCS-BEC transition scenario.

Mesoscale hydrodynamic simulation of short polyelectrolytes in electric fields

The dynamical, conformational, and transport properties of short flexible polyelectrolytes are studied in salt-free solution under the influence of an external electric field taking hydrodynamic interactions into account. A coarse-grained polymer model is applied and the multiparticle collision dynamics approach is adopted to account for the solvent. We consider various Coulomb interaction strengths and polymer lengths. The scaling behavior of the diffusion coefficient with respect to polymer length strongly depends on the charge interaction strength due to changes in the polyelectrolyte conformations by counterion condensation. The diffusion coefficients at weak Coulomb interaction strengths can well be described by the expression of rodlike objects, whereas at stronger electrostatic interactions, we observe a Zimm-like behavior. The polyelectrolyte mobilities obey the Nernst-Einstein relation at weak charge interactions. For large interaction strengths and long polymers, the mobilities are independent of molecular weight because of the screening of the intramolecular Coulomb interactions by counterions. Our studies demonstrated that the dynamics of polyelectrolytes in dilute solution and in an external field is governed by an interplay of hydrodynamic interactions and counterion condensation.

Relativistic Mott criticality in graphene

We formulate the effective Gross-Neveu-Yukawa theory of the semimetal-insulator transitions on the honeycomb lattice and compute its quantum critical behavior near three (spatial) dimensions. We find that at the critical point Dirac fermions do not survive as coherent excitations and that the $\ensuremath{\sim}1/r$ tail of the weak Coulomb interaction is an irrelevant coupling. The emergent Lorentz invariance near criticality implies a universal ratio of the low-temperature specific heats of the metallic and the rotational-symmetry-broken insulating phase.

Kondo Effect in the Helical Edge Liquid of the Quantum Spin Hall State

Following the recent observation of the quantum spin Hall (QSH) effect in HgTe quantum wells, an important issue is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator as a function of temperature in the presence of a magnetic impurity. At high temperatures, Kondo and/or two-particle scattering give rise to a logarithmic temperature dependence. At low temperatures, for weak Coulomb interactions in the edge liquid, the conductance is restored to unitarity with unusual power laws characteristic of a "local helical liquid," while for strong interactions, transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event.

Evolution of structure of SiO2nanoparticles upon cooling from the melt

Evolution of structure of spherical SiO2 nanoparticles upon cooling from the melt has been investigated via molecular-dynamics (MD) simulations under non-periodic boundary conditions (NPBC). We use the pair interatomic potentials which have weak Coulomb interaction and Morse type short-range interaction. The change in structure of SiO2 nanoparticles upon cooling process has been studied through the partial radial distribution functions (PRDFs), coordination number and bond-angle distributions at different temperatures. The core and surface structures of nanoparticles have been studied in details. Our results show significant temperature dependence of structure of nanoparticles. Moreover, temperature dependence of concentration of structural defects in nanoparticles upon cooling from the melt toward glassy state has been found and discussed.

Structural properties of simulated amorphous GeO2 nanoparticles

AbstractStructural properties of amorphous GeO2 nanoparticles have been studied by using molecular dynamics (MD) simulations. Four models with different sizes of 2, 3, 4 and 5 nm have been obtained by cooling from the melt via the MD method under nonperiodic boundary conditions. We used the interatomic potentials that have a weak Coulomb interaction and Morse‐type short‐range interaction. Structural properties of amorphous nanoparticles obtained at 300 K have been analyzed in detail through the partial radial distribution functions (PRDFs), mean interatomic distances, coordination number and bond‐angle distributions compared with those observed for the bulk counterpart. Moreover, the radial density profile in nanoparticles was found and discussed. Calculations showed that size effects on structure of amorphous GeO2 nanoparticles are significant in that if the size is larger than 3 nm amorphous GeO2 nanoparticle has a distorted tetrahedral network structure with the mean coordination number ZGe–O ≈ 4.0 and ZO–Ge ≈ 2.0 like those observed in the bulk counterpart. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hartree-Fock study of electronic ferroelectricity in the Falicov-Kimball model withf−fhopping

The Hartree-Fock (HF) approximation with the charge-density-wave (CDW) instability is used to study the ground-state phase diagram of the spinless Falicov-Kimball model (FKM) extended by $f\text{\ensuremath{-}}f$ hopping in two and three dimensions. It is shown that the HF solutions with the CDW instability perfectly reproduce the two-dimensional intermediate-coupling phase diagram of the FKM model with $f\text{\ensuremath{-}}f$ hopping, which was recently calculated using the constrained path Monte Carlo (CPMC) method. Using this fact, we have extended our HF study on cases that have been not described by CPMC, namely, (i) the case of small values of $f$-electron hopping integrals, (ii) the case of weak Coulomb interactions, and (iii) the three-dimensional case. We have found that ferroelectricity remains robust with respect to the reducing strength of coupling ($f$-electron hopping) as well as with respect to the increasing dimension of the system.

Microstructural analysis of liquid and amorphous SiO 2 nanoparticles

Microstructural properties of liquid and amorphous SiO 2 nanoparticles have been investigated via molecular dynamics (MD) simulations with the interatomic potentials that have weak Coulomb interaction and Morse-type short-range interaction under non-periodic boundary conditions. Structural properties of spherical nanoparticles with different sizes of 2, 4 and 6 nm obtained at 3500 K have been studied through partial radial distribution functions (PRDFs), coordination number and bond-angle distributions, and compared with those observed in the bulk. The core and surface structures of liquid SiO 2 nanoparticles have been studied in detail. We found significant size effects on structure of nanoparticles. Calculations also show that if the size is larger than 4 nm, liquid SiO 2 nanoparticles at the temperature of 3500 K have a lightly distorted tetrahedral network structure with the mean coordination number Z Si–O≈4.0 and Z O–Si≈2.0 like those observed in the bulk. Moreover, temperature dependence of structural defects and SiO x stoichiometry in nanoparticles on cooling from the melt has been found and presented.

Phase diagram of the asymmetric Hubbard model

The ground-state phase diagram of the asymmetric Hubbard model is studied in one and two dimensions by a well-controlled numerical method. The method allows us to calculate directly the probabilities of particular phases in the approximate ground state and thus to specify the stability domains corresponding to phases with the highest probabilities. Depending on the electron filling $n$ and the magnitude of the asymmetry ${t}_{f}∕{t}_{d}$ between the hopping integrals of $f$ and $d$ electrons two different scenarios in formation of ground states are observed. At low electron fillings $(n\ensuremath{\leqslant}1∕3)$ the ground states are always phase segregated in the limit of strong asymmetry $({t}_{d}⪢{t}_{f})$. With decreasing asymmetry the system undergoes a transition to the phase-separated state and then to the homogeneous state. For electron fillings $ng1∕3$ and weak Coulomb interactions the ground state is homogeneous for all values of asymmetry, while for intermediate and strong interactions the system exhibits the same sequence of phase transitions as for $n$ small. Moreover, it is shown that the segregated phase is significantly stabilized with increasing electron filling, while the separated phases disappear gradually from the ground-state phase diagrams.