Strong Electron-electron Correlations Research Articles

On the influence of electron-electron correlation on the current-voltage characteristics of a short molecular wire

It is demonstrated that detailed information can be gained on the inelastic mechanism of correlated electron transfer through a molecular wire if one analyses the current behavior in dependence on the strength of the external electric field. The approach is complementary to the derivation of the current-voltage characteristics, and the given analysis of the current-length effect observed at a fixed electric field-strength allows to specify the influence of the blocking transmission factor on incoherent electron transfer. It is found that at small external electric field-strength the current through a regular molecular wire exhibits an Ohmic behaviour while at strong fields electron-electron correlations lead to an exponential decrease of the current.

Experimental constraints on the physics of cuprates

Abstract Masny anomalous physical properties have been observed in the high-Tc cuprate superconductors for the last decade. These includet (1) the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the spin-glass freezing temperature and on the antiferromagnetic ordering temperature; (2) the exotic pairing symmetry shown by many bulk-sensitive experiments; (3) the magnetic resonance peak in the superconducting state revealed by inelastic neutron scattering; (4) the peak, dip and hump features seen in angle-resolved photoemission spectroscopy and in tunnelling spectra; (5) the strong-coupling features in optical and tunnelling data; (6) the pseudogap in the normal state of underdoped cuprates and (7) the unusually large supercarrier mass anisotropy and its novel doping dependence. We review these important experimental results, which can place crucial constraints on the physics of cuprates. The conclusion is that high-Tc superconductivity in cuprates is a cooperative phenomenon between the strong electron-electron correlation and strong electron-phonon coupling.

Experimental constraints on the physics of cuprates

Abstract Masny anomalous physical properties have been observed in the high-Tc cuprate superconductors for the last decade. These includet (1) the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the spin-glass freezing temperature and on the antiferromagnetic ordering temperature; (2) the exotic pairing symmetry shown by many bulk-sensitive experiments; (3) the magnetic resonance peak in the superconducting state revealed by inelastic neutron scattering; (4) the peak, dip and hump features seen in angle-resolved photoemission spectroscopy and in tunnelling spectra; (5) the strong-coupling features in optical and tunnelling data; (6) the pseudogap in the normal state of underdoped cuprates and (7) the unusually large supercarrier mass anisotropy and its novel doping dependence. We review these important experimental results, which can place crucial constraints on the physics of cuprates. The conclusion is that high-Tc superconductivity in cuprates is a cooperative phenomenon between the strong electron-electron correlation and strong electron-phonon coupling.

Triplet energies of pi-conjugated polymers.

Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different pi-conjugated polymers. In all cases we find that the 1 (3)B(u) is of order 0.6 to 1 eV below the 1 (1)B(u), indicative of localized triplet states with strong electron-electron correlation. We also observe that the 1 (1)A(g)-1 (3)B(u) gap decreases linearly as the 1 (1)A(g)-1 (1)B(u) gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap <1.3 eV will have a triplet ground state.

Charge excitations in heavy electron metals

We show that the optical response of metals with strong electron-electron correlation consists of two excitations, a renormalized Drude response at zero energy and a mid-infrared peak occurring at frequencies around 2000 cm-1. The latter originates from a dynamical, correlation-induced gap, as evinced from a many body theoretical approach based on the periodic Anderson model. At very low temperatures, it can be viewed as optical gap between two renormalized quasi-particle bands. The gap size is proportional to the geometric mean of the characteristic lattice Kondo temperature of the material and its bandwidth.

Cobalt Oxides and Kondo Semiconductors: A Pseudogap System as a Thermoelectric Material

Layered cobalt oxides and heavy-fermion compounds are quantitatively compared from the viewpoint of thermoelectric materials. These two systems exhibit anomalously large thermopowers with good electric conductivity, which is attributed to a strong electron-electron correlation. In certain materials the pseudogap increases at low temperature, which further enhances the thermopower.

Jahn-Teller effect and many-body correlation effect inLaMnO3

Effects of the Jahn-Teller (JT) distortion and the strong electron-electron correlation on the electronic structure of perovskite oxides LaMnO3 are studied by using the linear muffin-tin orbital method. Results show that the Jahn-Teller distortion has evident influence on the electronic structure and its influence varies with different magnetic configurations. On the other hand, the effect of strong electron-electron correlation depends on crystal structures. For distorted LaMnO3 structure, the strong correlation affects the density of states and the band structure lightly, while for undistorted structure, its effect is much stronger. Our calculations indicate that the effect of Jahn-Teller distortion is more pronounced than that of strong electron-electron correlation in LaMnO3 but the latter is important for calculation of total energy. To get the correct ground state as experiments reported for LaMnO3, it is necessary to take the Jahn-Teller distortion, the electron-electron correlation, and antiferromagnetic ordering into consideration simultaneously.

133Cs and 13C-NMR in CsC 60 polymers under pressure

Alkali metal doped fullerides reveal a polymerized crystal structure with parallel chains of covalently bounded C 60 molecules. The electronic structure shows a narrow π-electron band. The existence of strong electron-electron correlations in this fulleride is supported by the establishment of an antiferromagnetic ground state at T N =30K. We report the measurements of 133Cs and 13C-NMR relaxation rates on CsC 60 polymers which have been performed versus pressure and temperature. Our results show that the hydrostatic pressure suppresses the antiferromagnetic order and stabilizes a spin-singlet long range order at 5kbar. A study of the quadrupolar spin-echo of 133Cs performed at ambient pressure reveals a second order structural transition at T S =13.8K towards a spin-singlet ground state which coexists with the preexisting antiferromagnetic order.

Electronic structure of PrBa2Cu3O7 and YBa2Cu3O7: A local spin density approximation with the on-site Coulomb interaction study

Using the self-consistent linear muffin-tin orbitals and the atomic sphere approximation method, the band structure calculations for PrBa2Cu3O7 and YBa2Cu3O7 are performed in the regime of the local spin density approximation with the on-site Coulomb interaction (LSDA+U). The strong electron-electron correlation effects are considered for both the Pr 4f and Cu 3d states. Comparing with the LSDA calculations, the LSDA+U calculations show that, while the essential feature of energy band of YBa2Cu3O7 is preserved, the electronic structure of PrBa2Cu3O7 is quite different. For the bands arisen from the CuO2 planes, a gap with a value of 0.5 eV is opened, indicating the insulating or semiconducting character of the CuO2 planes. The pdσ bands associated with the CuO chains are across the Fermi level, revealing the metallic character of the CuO chains.

Temperature dependence of resistivity of quasi-one-dimensional conductors with nested electronic spectra

We computed the resistivity of $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6}$ under pressure for $Tl60 \mathrm{K}$ using a simple model of anisotropic metal with open Fermi sheets and weakly interacting electrons. The results agree well with experiment. Nesting features in the electron spectrum cause notable deviations from the ${\mathrm{T}}^{2}$ behavior even in the framework of standard kinetic approach. There are no indications of strong electron-electron correlation effects.

Excitons in quasi-one-dimensional organics: Strong correlation approximation

An exciton theory for quasi-one-dimensional organic materials is developed in the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short-range extended Hubbard interactions. Within a strong electron-electron correlation approximation, the exciton properties are extensively studied. Using scattering theory, we analytically obtain the exciton energy and wave function and derive a criterion for the existence of a ${B}_{u}$ exciton. We also systematically investigate the effect of impurities on the coherent motion of an exciton. The coherence is measured by a suitably defined electron-hole correlation function. It is shown that, for impurities with an on-site potential, a crossover behavior will occur if the impurity strength is comparable to the bandwidth of the exciton, corresponding to exciton localization. For a charged impurity with a spatially extended potential, in addition to localization the exciton will dissociate into an uncorrelated electron-hole pair when the impurity is sufficiently strong to overcome the Coulomb interaction which binds the electron-hole pair. Interchain coupling effects are also discussed by considering two polymer chains coupled through nearest-neighbor interchain hopping ${t}_{\ensuremath{\perp}}$ and interchain Coulomb interaction ${V}_{\ensuremath{\perp}}.$ Within the $t$ matrix scattering formalism, for every center-of-mass momentum, we find two poles determined only by ${V}_{\ensuremath{\perp}},$ which correspond to the interchain excitons, and four poles only involving intrachain Coulomb $V$, which are intrachain excitons. The interchain exciton wave function is analyzed in terms of inter- and intra-chain character. Finally, the exciton state is used to study the charge transfer from a polymer chain to an adjacent dopant molecule. From a variational wave function for the total system, we explore the dependence of the probability of charge transfer on the acceptor level, the hopping, and the wave function of the exciton.

On a model of F centres in alkali halide crystals

On the basis of analysis of available experimental data it is shown that a model of the F centre in alkali halide crystals (AHC) as a self-trapped electron (STEL) can more naturally explain some of these data than the commonly accepted de Boer model (an electron bound at an anion vacancy). It is supposed that the high binding energy of STEL in AHC may be caused by a strong electron-electron correlation.

Nonmagnetic molecular Jahn-Teller Mott insulators

Narrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a non-magnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of non-magnetic insulator in a very simple $E\otimes e$ Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K_{4}C_{60} and Rb_{4}C_{60}.

Se-Substitution and Cation Effects on the High-Pressure Molecular Superconducior, β-Me4N[Pd(dmit)2]2 -A Unique Two-Band System

Crystal and electronic structures and physical properties of the molecular conductors Me4Z [Pd(L)2]2 (Z = N, P, As, and Sb; L = dmit and dmise) are described. Every salt has very similar crystal structure with “solid-crossing” columns. The behavior of resistivity under pressure depends on the Se-substitution and the choice of the cation. Tight-binding band calculations indicate that these salts form a unique two-band system. The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band. Magnetic properties suggest that the insulating state under ambient pressure comes from the strong electron-electron correlation. The one-dimensional LUMO (bonding) band is located below the HOMO band. The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state. The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with the LUMO band. Se-substitution and cation effects are observed in the band structure and the shape of Fermi surface. Our results suggest an interrelation between the correlation effect and the dimensionality of the electronic structure.

Electron‐Ion Recombination of Fe v

Ionization balance in the low-ionization stages of iron is important in a variety of astrophysical plasmas. Accurate recombination rates are needed but are difficult to compute in a detailed quantum mechanical treatment. The main difficulty lies in the complexity of strong electron-electron correlation arising due to the open 3d shell. We present the total electron-ion recombination rate coefficients for the process, e+Fe VI → Fe V, obtained in an ab initio manner. Large-scale computations are carried out in the close coupling (CC) approximation using the R-matrix method employing a unified treatment that considers the infinite number of states of the recombined ion and incorporates both the radiative recombiantion (RR) and the dielectronic recombination (DR) processes in a self-consistent manner. It involves calculations of detailed photoionization cross sections, σPI, with autoionizing resonances of a large number of bound states with n≤nmax, such as 1054 bound states for the present case of Fe V. The number also equals the total number of state-specific recombination rates obtained for the ion. The high-n states are treated through the DR theory by Bell & Seaton. The same wavefunction expansion is employed in all photoionization/recombination calculations, thereby ensuring self-consistency. Rates are presented at a wide range of temperature for all practical applications. Present rates for total recombination differ considerably from currently used values obtained using simpler approximations. Application of the new recombination rate coefficients and photoionization cross sections for the ionization structure of iron in planetary nebulae show that under typical conditions the relative fractions of Fe V and Fe VI change by nearly a factor of 2.

Remote and spatially separatedD−centers in quasi-two-dimensional semiconductor structures

The electronic structure, binding energy, and correlation functions of three different spatial configurations of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ centers in GaAs-based semiconductor structures are studied using a numerical technique that solves the two-electron Schr\"odinger equation for such systems. We compare our results for the quantum well ${\mathrm{D}}_{\mathrm{w}}^{\mathrm{\ensuremath{-}}}$ with experimental data and variational and diffusion Monte Carlo calculations. We demonstrate the existence of the spatially separated ${\mathrm{D}}_{\mathrm{s}}^{\mathrm{\ensuremath{-}}}$ center as a bound state, even in the regime of very low magnetic fields, and find interesting anticrossings among the energy levels in its electronic structure, which underlie a spectrum composition of a ${\mathrm{D}}^{0}$ donor center and a free electron in a magnetic field. For the remote ${\mathrm{D}}_{\mathrm{r}}^{\mathrm{\ensuremath{-}}}$ center we find that strong electron-electron correlations can give rise to magnetic-field-induced angular-momentum transitions, and as the magnetic field increases, these correlations weaken, while eventually the system magnetically evaporates approaching the classical unbound regime.

Molecular conductors based on Pd(dmit) 2 and related metal dithiolene complexes —A unique two-band system—

Physical properties and electronic structures of molecular conductors (cation) [Pd(dmit) 2] 2 (cation = Me 4Z, Et 2Me 2Z; Z = N, P, As, and Sb) are described. The Pd(dmit) 2 salts form a unique two-band system. The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band. The insulating state under ambient pressure is considered to come from the strong electron-electron correlation. The one-dimensional LUMO (bonding) band is located immediately below the HOMO band. The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state. The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with the LUMO band. The pressure effect strongly depends on the cation size.

When are thin films of metals metallic? Part III

A large amount of experimental information has indicated that very thin films of metallic elements can exhibit nonmetallic behavior, even on metal substrates. These films undergo a gradual nonmetal to metal transition with increasing film density or thickness. The nonmetallic behavior can be related to electron localization due to strong electron-electron correlation in low dimensional systems, as indicated by the strong enhancement of electron effective mass. The evolution in the electronic structure associated with the nonmetal to metal transition bears a striking resemblance to the behavior observed for free metal clusters. Part I [1], outlined the general concepts of a nonmetal to metal transition in a thin film, while part II [2] critically examined three examples of nonmetal to metal transitions for thin film overlayers. This paper will discuss some surprising correlations between the electronic structure of divalent thin films on metal substrates and divalent metal clusters that have been reported in the literature.

Frozen electron solid in the presence of small concentrations of defects.

We investigate the freezing of a low-density electron liquid for a two-dimensional electron layer in the presence of low levels of defects typical of a high-quality semiconductor interface. We use a memory function approach with mode-coupling approximation and include the effect of strong electron-electron correlations, which we find are crucial for the transition. For a range of low impurity concentrations we find a stable frozen solid with a liquidlike short-range order. At higher impurity concentrations the electrons localize separately and there is no short-range order. Our electron-density vs peak-mobility phase diagram at zero temperature is in agreement with recent metal-insulator transition experiments in silicon heterostructures. \textcopyright{} 1996 The American Physical Society.

Phonon-induced d-wave pairing in the two-dimensional Hubbard model.

We discuss phonon-induced anisotropic superconductivity of strongly correlated quasiparticles within the two-dimensional Hubbard model. The generalized Fr\olich-like canonical transformation leads to pairing interactions that become repulsive for vanishing concentration of holes but are attractive at low dopings. Our results indicate that the electron-phonon coupling in the presence of strong electron-electron correlations favors the formation of d-wave superconductivity.