Nearest-neighbor Coulomb Interaction Research Articles

Influence of nearest-neighbor Coulomb interactions on the phase diagram of the ferromagnetic Kondo model

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized $S=1/2$ spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at $V=0,$ the Coulomb interaction breaks the large domains of the two competing phases into small ``islands'' of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density $n=0.5,$ the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of ``charge-ordered ferromagnetism'' may be related with experimental observations of a similar phase by Chen and Cheong [Phys. Rev. Lett. 76, 4042 (1996)]. Our results reinforce the recently introduced notion [see, e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)] that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation--charge-ordering tendencies.

Charge-density waves and surface Mott insulators for adlayer structures on semiconductors: Extended Hubbard modeling

Motivated by the recent experimental evidence of commensurate surface charge density waves (CDW) in Pb/Ge(111) and Sn/Ge(111) sqrt{3}-adlayer structures, as well as by the insulating states found on K/Si(111):B and SiC(0001), we have investigated the role of electron-electron interactions, and also of electron-phonon coupling, on the narrow surface state band originating from the outer dangling bond orbitals of the surface. We model the sqrt{3} dangling bond lattice by an extended two-dimensional Hubbard model at half-filling on a triangular lattice. We include an on-site Hubbard repulsion U and a nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The electron-phonon interaction is treated in the deformation potential approximation. We have explored the phase diagram of this model including the possibility of commensurate 3x3 phases, using mainly the Hartree-Fock approximation. For U larger than the bandwidth we find a non-collinear antiferromagnetic SDW insulator, possibly corresponding to the situation on the SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram arises, with several phases involving combinations of charge and spin-density-waves (SDW), with or without a net magnetization. We find that insulating, or partly metallic 3x3 CDW phases can be stabilized by two different physical mechanisms. One is the inter-site repulsion V, that together with electron-phonon coupling can lower the energy of a charge modulation. The other is a novel magnetically-induced Fermi surface nesting, stabilizing a net cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW. Comparison with available experimental evidence, and also with first-principle calculations is made.

Exact-diagonalization study on the effect of the long-range Coulomb interaction to the superconducting ground state in the two-chain Hubbard model

We present exact-diagonalization results clearly indicating that the long-range part of Coulomb interaction does not sweep away the superconducting region of the two-chain Hubbard model driven by the short-range part. When the model contains the nearest-neighbor Coulomb interaction both for the interchain and the intrachain directions, the region of developed superconducting correlations in the ground state is reduced. The CDW correlation is found to increase, intervening with the competition between superconductivity and CDW. When we include the long-range Coulomb interaction of the type of 1/ r, the instability to the CDW state weakens with the superconducting region recovered to some extent compared with the above case.

Manifestation of Spin-Charge Separation in the Dynamic Dielectric Response of One-DimensionalSr2CuO3

We have determined the dynamical dielectric response of a one-dimensional, correlated insulator by carrying out electron energy-loss spectroscopy on Sr2CuO3 single crystals. The observed momentum and energy dependence of the low-energy features, which correspond to collective transitions across the gap, are well described by an extended one-band Hubbard model with moderate nearest neighbor Coulomb interaction strength. An exciton-like peak appears with increasing momentum transfer. These observations provide experimental evidence for spin-charge separation in the relevant excitations of this compound, as theoretically expected for the one-dimensional Hubbard model.

Effects of the nearest-neighbor Coulomb interactions on the ground state of the periodic Anderson model

The magnetic and non-magnetic ground states of the periodic Anderson model with Coulomb interaction between $f$-electrons on the nearest-neighbour(NN) sites are investigated using a variational method, which gives exact calculation of the expectation values in the limit of infinite dimensions. It is shown that for a critical value of NN Coulomb interactions the magnetic ground state of the periodic Anderson model in the Kondo regime is unstable. Factors in terms of the physical processes responsible for instability of the magnetic ground state are also discussed. Our study indicates the importance of the NN Coulomb interactions for correlated two band models.

Gutzwiller and slave-boson methods for intersite Coulomb interactions

We investigate the application of the slave-boson method to the treatment of finite intersite interactions. In the saddle-point approximation of the formalism the results obtained with an extended Gutzwiller-type local ansatz are recovered. We discuss the application of the method to a spinless fermion model with nearest-neighbor and long-range Coulomb interactions, respectively.

Low density electron pairs in the extended Hubbard model

We study the extended Hubbard model in the parameter region where it exhibits superconducting instabilities. The model is defined by onsite and nearest-neighbor Coulomb interactions U and V, and hopping integral t. We calculate pair binding energies for various finite clusters by performing exact diagonalization of the Hamiltonian. We show results for one-dimensional chains and discuss results for 4 × 4, 6 × 6, and 8 × 8 two-dimensional square lattices. Results of solving the BCS equation for the pair binding energy as functions of carrier density n, and Coulomb interactions U and V, are compared with the exact diagonalization results. In the dilute concentration limit ( n → 0), the BCS equation is expected to be exact and, in the low electron density region, the results for the pair binding energy are found to be in fairly good agreement with the exact solutions.

Few-electron ground states of charge-tunable self-assembled quantum dots

The few-electron ground states of self-assembled InAs quantum dots are investigated using high-resolution capacitance spectroscopy in magnetic fields up to 23 T. The level structure reveals distinct shells which are labeled as $s$-, $p$-, and $d$-like according to their symmetry. Our measurements enable us to resolve the single-electron charging not only of the lowest $(s)$ state with two electrons but also of the second lowest $(p)$ state with four electrons as pronounced maxima in the capacitance spectra. Furthermore, two peaks at higher energy can be attributed to charging of the $d$ shell with the first two electrons. We discuss the energy spectrum in terms of spatial quantization energy, Coulomb blockade, and many-particle effects. At around $B=15$ T we observe a magnetic-field-induced intermixing of the $p$ and $d$ shell. Additional fine structure in the capacitance spectra is observed and discussed both in terms of nearest-neighbor Coulomb interactions and monolayer fluctuations of the dot size.

Correlation effects on the electronic state and transport through a small interacting region

We investigate the effect of the on-site and the nearest-neighbor Coulomb interaction on the electronic state and transport phenomena of the quasi-one-dimensional quantum wire including a small interacting region. The stability and the conductance for several electronic states are calculated as a function of the chemical potential μ using the recursive Green's function method and the Kubo formula. It is found that electronic states, in which spins in the small region are aligned antiferromagnetically in the y-direction and ferromagnetically in the x-direction, are stabilized over a wide range of the chemical potential.

Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors

The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.

Theory of biexcitons in conjugated polymers

A tight binding, two-band Hamiltonian is used to investigate the formation and nonlinear optical properties of biexcitons in ID conjugated polymers. Calculations are performed using the full single and double electron-hole pair basis sets. The biexciton phase region in (α,β) space is numerically calculated for long chains in the polymer limit α and β are the nearest neighbor Coulomb interaction energy and charge transfer energies respectively, made dimensionless through division by the onsite Coulomb energy. Both truncated and extended potentials are studied. The former supports a single biexciton state while the latter supports multiple biexciton states. Calculated two-photon absorption (TPA) spectra contain two types of peaks: one at lower energy due to a charge-transfer, single-exciton and a higher energy peak arising from a biexciton state. Good agreement for the TPA spectrum of poly (di- n-hexylsilane) is obtained.

Simulations of the In situ cyclic voltammetry dependent EPR spectra and DC conductivity

In situ cyclic voltammetry (CV) dependent EPR and DC conductivity of polyaniline have been reported by many groups. However, the variation of the EPR intensities at two half-wave potentials of the CV scan and the asymmetric CV dependent DC conductivity have remained to be fully accounted for. Here we present in detail a novel quasi-random oxidation model and the related simulation results to interpret the reported in situ experimental results. This model quantitatively describes many phenomena and physical properties found in polyaniline including the origin of the defect states, the variations of the in situ EPR signal during CV potential scan, the effects of the hydrolysis, and the pH-dependent DC conductivity data. The statistical nature of this model suggests its general applicability to the oxidation processes of other conducting polymers. The important roles of nearest neighbor Coulomb interaction and formation of a metallic polaron lattice in the computer modeling are evaluated and discussed.

Exact diagonalization study of the hole distribution inCuO3chains within the four-band dp model

We consider the hole distribution in ${\mathrm{CuO}}_{3}$ chains as a function of the total hole number 0\ensuremath{\leqslant} n \ensuremath{\leqslant}2 per chain unit within the standard dp model containing one orbital per site. The nonequivalency of the apical and the chain oxygen sites is taken into account explicitly. Using slightly modified standard ${\mathrm{CuO}}_{2}$ plane parameter sets, we compare the results of exact diagonalization studies of periodic and open ${\mathrm{CuO}}_{3}$ chains with experimental data for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) and Ca(Sr${)}_{2}$${\mathrm{CuO}}_{3}$, and with theoretical results obtained within the following often used approximations: the Hartree-Fock and the local ansatz approximations, as well as with the exclusion of double occupancies at Cu sites and also that at oxygen ones (spinless fermion picture). We have found out that the ratio of the hole densities on the apical and the chain oxygen sites is sensitive to the magnitude of the nearest-neighbor Coulomb interaction ${\mathrm{V}}_{\mathrm{pd}}$ and the difference of their site energies ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{pp}}$. Adopting values ${\mathrm{V}}_{\mathrm{pd}}$\ensuremath{\sim} 1 eV and ${\mathrm{V}}_{\mathrm{pp}}$\ensuremath{\sim}0.${5\mathrm{V}}_{\mathrm{pd}}$, we estimate ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{pp}}$=1.5 to 2.5 eV for YBCO from the comparison with O 1s x-ray absorption spectroscopy and $^{17}\mathrm{O}$ nuclear magnetic resonance data. Using experimental values of the corresponding binding energies and the components of the electric field gradient tensor, the site energy of the apical oxygen ${2\mathrm{p}}_{\mathrm{z}}$ states relative to the planar copper Cu(2) ${3\mathrm{d}}_{{\mathrm{x}}^{2}\mathrm{\ensuremath{-}}{\mathrm{y}}^{2}}$ states is estimated as \ensuremath{\sim}5 to 6 eV. With increasing hole doping and/or the strength of ${\mathrm{V}}_{\mathrm{pd}}$, the holes are increasingly localized at the apical oxygen sites. Various chain aspects of proposed scenarios for the ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ problem are briefly discussed.

Persistent current in disordered Aharonov-Bohm rings with interacting electrons.

The role of repulsive on-site and nearest-neighbor Coulomb interactions in disordered half-filled Aharanov-Bohm rings is studied by world-line quantum Monte Carlo simulations. The diverse dependence of the equilibrium persistent current on the couplings is found to relate systematically to the magnetic phase of the model: the maximum charge stiffness (or the persistent current) coexists with the phase-transition line between the dominant charge-density-wave state and the dominant spin-density-wave state. The stiffness vanishes with an increasing departure from the transition line. Thus in the disordered rings the Coulomb interactions can enhance the charge stiffness over the noninteracting limit in such a way as to drive the system toward the phase-transition regime. \textcopyright{} 1996 The American Physical Society.

Effects of quantum fluctuations of electrons and lattice in doped polyacetylene.

The effects of quantum fluctuations of electrons and the lattice on the electronic and lattice structures of doped polyacetylene are studied using the quantum Monte Carlo method. We adopt the model where the on-site (U) and the nearest-neighbor (V) Coulomb interaction terms are added to the Su-Schrieffer-Heeger (SSH) model. In the SSH model, a charged soliton lattice survives quantum fluctuations of the lattice even in the heavily doped regime. However, the magnitude of bond-length alternation in the interface regions between charged solitons is reduced by them. In the model where U>0 and V=0, the ground state becomes a charged soliton lattice in the low doping regime by introducing quantum fluctuations of electrons. The magnitude of the bond-length alternation in the interface regions decreases with increasing doping concentration and becomes almost zero in the heavily doped regime. In the model where U>0 and V>0, a charged soliton lattice survives quantum fluctuations of electrons and the lattice even in the heavily doped regime. However, both the magnitude of the bond-length alternation in the interface regions and the magnitude of the charge-density alternation around the soliton centers are reduced by them. The interaction between charged solitons is significantly weakened by quantum fluctuations of electrons, which results in a significant reduction of the charged soliton formation energy in the heavily doped regime. \textcopyright{} 1996 The American Physical Society.

Ferromagnetism in correlated electron systems: Generalization of Nagaoka's theorem.

Nagaoka's theorem on ferromagnetism in the Hubbard model with one electron less than half filling is generalized to the case where all possible nearest-neighbor Coulomb interactions (the density-density interaction $V$, bond-charge interaction $X$, exchange interaction $F$, and hopping of double occupancies $F'$) are included. It is shown that for ferromagnetic exchange coupling ($F>0$) ground states with maximum spin are stable already at finite Hubbard interaction $U>U_c$. For non-bipartite lattices this requires a hopping amplitude $t\leq0$. For vanishing $F$ one obtains $U_c\to\infty$ as in Nagaoka's theorem. This shows that the exchange interaction $F$ is important for stabilizing ferromagnetism at finite $U$. Only in the special case $X=t$ the ferromagnetic state is stable even for $F=0$, provided the lattice allows the hole to move around loops.

New phases in an extended Hubbard model explicitly including atomic polarizabilities.

We consider the influence of a nearest-neighbor Coulomb interaction in an extended Hubbard model and introduce a new interaction term which simulates atomic polarizabilities. This has the effect of screening the on-site Coulomb interaction for charged excitations, unlike a neighbor Coulomb interaction which reduces energies of locally neutral excitations. We find that the spin density to charge density wave phase transition, however, is determined by the unscreened on-site Coulomb repulsion. The order of this phase transition is affected by polarization. We show that new phases appear, one of which is ferroelectric, when atomic polarizabilities are explicitly included.

Atomic screening and intersite Coulomb repulsion in strongly correlated systems.

We consider the influence of a nearest-neighbor Coulomb interaction in an extended Hubbard model and introduce an interaction term which simulates atomic polarizabilities. The inclusion of atomic polarizabilities in the model has the effect of screening the on-site Coulomb interaction for charged excitations, unlike a neighbor Coulomb interaction which reduces energies of neutral local excitations, leaving the conductivity gap unchanged. This is supported by an exact diagonalization study on small, mainly one-dimensional, extended Mott-Hubbard systems. For large V, the system undergoes a phase transition from a spin-density wave (SDW) to a charge-density wave (CDW). It is shown that the SDW/CDW phase transition cannot be described correctly when using effective parameters, but that screening effects have to be included explicitly. Also other phases, one of which is ferroelectric, appear when atomic polarizabilities are included in the model Hamiltonian. We study ${\mathrm{C}}_{60}$ in terms of the degenerate extended Hubbard model with screening effects.

Metallic polyacetylene is a soliton lattice

We present a series of calculations which suggest that a soliton lattice persists into the metallic phase of polycetylene and can account for the onset of the Pauli susceptibility at a doping level of 5%. The starting point for analysis is the continuum version of the SSH Hamiltonian. We solve exactly for the single-particle states that arise when n-doped electrons are added to a single polymer chain. The role of on-site ( U), nearest-neighbor ( V) and bond repulsion ( W) Coulomb interactions are then obtained from a first-order perturbative calculation with the exact single-particle states. We minimize the total energy and hence are able to assess the relative stability of soliton and polaron configurations. We show that as the doping level approaches metal concentrations, the critical values of U and V at which a soliton lattice converts to a polaron lattice increase significantly beyond experimentally accepted estimates. W is also shown to favor solitons. In fact, we estimate that a doping level corresponding polaron lattice. We then show that the bound-state soliton levels merge to fill the gap sufficiently that the Pauli susceptibility becomes non-zero and comparable to the corresponding experimental values.

Phase diagram for strongly correlated doped trans-polyacetylene chains.

We study here the stability of soliton and polaron excitations in a single strand of trans-polyacetylene as a function of the strength of the electron-electron correlations as well as the doping level. We proceed by first solving exactly the continuum version of the Su, Schrieffer, and Heeger Hamiltonian for the single-particle states that arise when electrons are added to a single polymer chain. The role of on-site (U), nearest-neighbor (V), and bond repulsion (W) Coulomb interactions are then obtained from a first-order perturbative calculation with the exact single-particle states. By minimizing the total energy, we are able to determine the relative stability of polaron and soliton configurations. We show that, at a fixed doping level, a polaron lattice is favored over a soliton configuration provided that U and V exceed critical values. However, as the doping level is increased, we show that the critical values of U and V at which a soliton lattice converts to a polaron lattice increase significantly beyond experimentally accepted estimates. W is also shown to favor solitons. We estimate that at a doping level corresponding to the insulator-metal transition (5%), U=4 eV and V=0.4 eV, a soliton lattice is 0.6 eV lower in energy than the corresponding polaron lattice. If we argue that the insulator-metal transition in polyacetylene results from the onset of a polaron lattice at 5%, then our work places restrictions on the magnitude of such effects as interchain coupling which have been proposed in the literature as a principal player in the metal transition in polyacetylene.