Strong Electron-electron Interactions Research Articles

Ionic interactions in platinum chloride linear-chain compounds.

Halogen-bridged transition-metal chain compounds have recently emerged as an important class of low-dimensional electronic materials with strong electron-lattice and electron-electron interactions. We introduce a many-body model for such materials which is quantitatively successful for highly valence-localized (strong charge-density-wave) members of the class, e.g., PtCl with various ligand structures. The model introduces nearest-neighbor Coulomb attraction and metal ion-ion electrostatic repulsion directly rather than through a linear Peierls-Hubbard Hamiltonian. These interactions have the effects of (i) modifying the on-site orbital energies, (ii) generating an effective anharmonic intrasite electron-lattice coupling (leading to the formation of a charge-density wave even if the intersite electron-lattice coupling is weak), and (iii) acting as an effective anharmonic elastic force between neighboring metal and halogen atoms. The stoichiometric ground state and various defect states (polaron, bipolaron, kink, exciton) are studied within this framework by computing their optical absorption, Raman, and infrared spectra: The results agree well with available experimental data. Buckling of Cl atoms out of the chain axis in certain PtCl compounds is predicted to be important in order to obtain the observed Raman frequencies for electron polarons and bipolarons. Finally, a strong ``template'' effect is discussed by comparing two PtCl materials with different ligands and counterions.

The new AlPdRe icosahedral phase: Towards universal electronic behaviour for quasicrystals?

Electronic properties have been measured for different new compositions close to Al 70.5Pd 22Re 7.5 of the icosahedral phase. They exhibit the same exceptional behaviours that were previously found for other stable quasicrystalline phases exept for the resistivity magnitude which is much higher that what is observed for the parent AlPdMn icosahedral phase. For high quality quasicrystals, the conductivity reaches the same order of magnitude as the one for doped semiconductors. The conductivity σ(T) increases rapidly with temperature. σ(T) curves appear to be parallel for all i-AlPdRe samples, although conductivity values strongly varies with the composition and the structural quality. High field magnetoconductivity indicates the presence of quantum interference effects including strong electron-electron interactions. All these features strengthen the concept of universal electronic behaviour for these complex phases, with particularly enhanced anomalous behaviour for i-AlPdRe.

Spin polarization at the Fe/V interface.

The local magnetic moments and magnetic order are investigated at the Fe/V interface of thin ${\mathrm{Fe}}_{3}$/${\mathrm{V}}_{\mathit{n}}$ films and thin V films adsorbed on Fe substrate. Different crystallographic directions (001), (101), (111), and a two-atom-width and monoatomic-height-per-step stepped surface [corresponding to the vicinal (103) surface] are considered. We calculate the spin-polarized electronic-charge distribution of these systems as a function of the exchange integral ${\mathit{J}}_{\mathrm{V}}$ of vanadium in the range 0\ensuremath{\le}${\mathit{J}}_{\mathrm{V}}$\ensuremath{\le}0.8 eV using a self-consistent tight-binding real-space model within the unrestricted Hartree-Fock approximation to the Hubbard Hamiltonian. For (001), (011), and (111) crystallographic faces, the spin polarization of the V atoms at the Fe/V interface is nonzero for any ${\mathit{J}}_{\mathrm{V}}$ value, with antiferromagnetic coupling between the Fe and V interface atoms. Due to the hybridization between V and Fe orbitals, the spin polarization at the interface tends to increase in V with respect to the corresponding pure-V system, whereas the opposite happens in Fe, resulting in a magnetic moment induced at the interface V atoms by the Fe substrate. This effect is less pronounced in the (101) orientation. Both results can be qualitatively explained by considering two facts: (a) the stronger electron-electron interaction of Fe with respect to V; (b) Fe atoms at the interface have fewer V atoms at nearest-neighbor positions for the (101) orientation than for the (001) orientation. In the case of V adsorbed on the (103) vicinal surface of Fe, the internal and external V atoms at the surface layer of each step are coupled antiferromagnetically. For this particular case, there is a strong competition between the tendencies of V to couple antiferromagnetically with the ferromagnetic Fe substrate and with its V nearest neighbors.

Electron-electron interactions and superconducting fluctuations in weakly localized K3C60.

Magnetotransport studies on superconducting ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ films are presented. These films are a three-dimensional (3D) weakly localized system with strong electron-electron interactions, reflected by the large positive magnetoresistance (MR). The effects of superconducting fluctuations on the MR are observed in this 3D weakly localized system. The localization is primarily caused by doping-induced disorder rather than granularity and the upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) follows the Werthamer-Helfand-Hohenberg model. An electron mean free path of 10--20 \AA{} is estimated in the normal state just above ${\mathit{T}}_{\mathit{c}}$.

Electronic properties of stable quasicrystals: towards a semiconducting quasicrystal?

Anomalous transport properties are presented of a series of pure AlCuFe icosahedral phases of very high structural quality including a millimetre size single grain. The most salient feature is that all these metallic samples present very high resistivity values close to the metal-insulator transition (MIT) rising up to 10 000 μΩ cm at 4 K in Al 62.5Cu 25Fe 12.5. The resistivity at 4 K depends strongly on the structural quality in a very peculiar way, since it increases when the structural defects are removed. It also varies strongly with the nominal composition of the sample increasing by a factor of almost 2.5 for only a 0.5% change in the composition. On the contrary, it is remarkable that the temperature dependence of the conductivity is almost independent of the composition and the structural quality. It is shown that, at low temperature, the temperature - and the magnetic field - dependence of the conductivity can be analyzed with classical weak localization theories including, however, strong electron-electron interaction similarly to that observed in systems close to a MIT. Similar peculiar behaviours are observed by preliminary measurements in the new AIPdMn stable icosahedral phase.

Transport and thermodynamic properties near a metal-insulator transition in AlCuFe icosahedral phases

Quasicrystalline samples of various compositions close to Al 63Cu 25Fe 12 have been produced by melt spinning or conventional casting. A slight change in the nominal composition induces a steep increase of the resistivity, ranging from 4600 μμ cm in Al 63Cu 25Fe 12 to 10000 μμ cm in Al 62.5Cu 25Fe 12.5. The temperature and magnetic field dependences of the conductivity present peculiar behaviours which can be attributed to strong electron-electron interaction effects as already observed in heavily doped semiconductors on the metallic side of the metal-insulator transition. The temperature dependence of the specific heat at very low temperature can also be analyzed within metal-insulator theories.

Electron-electron interactions and solitons in polyacetylene.

The electron-electron interactions and the creation energy of a soliton in highly conducting polymers were calculated by using a correlated-basis-function method with a general screening Coulomb potential within the Kronig-Penny-model framework. Our results showed that even when the electron-electron interactions are considered, solitons are still the main charge carriers. The strong long-range electron-electron interaction is seen to reduce the creation energy of a soliton. A comparison with results based on the Hubbard model has been made. The limitations of the Hubbard model in this application were discussed.

Electron correlations in narrow band alloys: Hubbard subbands and cohesive energies

The effect of strong electron-electron interactions (described by the single-band Hubbard Hamiltonian) on the electronic spectrum and cohesive energies of substitutionally disordered alloys has been studied. The authors use the Hubbard III solution to treat the correlations and coherent-potential approximation for configurational averaging. The use of the Lorentzian form of the host metal density of states enables them to obtain analytical expressions for various quantities of interest. Questions related to the problem of the calculations of cohesive energies in disordered systems with correlations are discussed and partially resolved.

Balance-Equation Approach to Hot-Carrier Transport in Semiconductors

The balance-equation approach to nonlinear hot-carrier transport theory, formulated by Lei and Ting (1984), is addressed in this comprehensive review. A central feature is the role of strong electron-electron interactions in promoting rapid thermalization about the drifted transport state and the concomitant substantial simplification of the transport theory. This physical feature is embodied in the initial density matrix chosen to represent the unperturbed carrier system. Force and energy balance equations are formulated for the dc steady state, ac dynamic and transient cases of charge conduction, including memory effects. The scattering mechanisms include impurity and phonon interactions along with dynamic nonlocal screening effects due to carrier-carrier interactions. Both linear and nonlinear resistivities are discussed in the degenerate and nondegenerate statistical regimes. Interesting phenomena such as electron cooling and thermal noise and diffusion are discussed as well. Semiconductor microstructure transport is described for both linear and nonlinear hot carrier conduction. In this connection, quasi-2D-systems, heterojunctions, and quantum well superlattices are treated with attention to steady state, transient and high frequency transport, including, for example, superlattice plasmon resonance structure. Type-II superlattice transport is reviewed as well as type-I, and electron-hole drag is treated in conjunction with negative minority electron mobility in a quantum well. Multivalley semiconductors are discussed in some detail. Furthermore, attention is also focused on the center-of-mass velocity fluctuation, Langevin-type equation and thermal noise and diffusion for microstructures and multivalley systems. A number of particularly important phenomena are examined from the balance-equation point of view, such as nonequilibrium phonons, higher order scattering effects and weak localization, hydrodynamic equations for weakly nonuniform systems, and the intracollisional field effect. Alternative formulations and interpretations of the balance-equation approach are reviewed. The distinction between this many-particle, isothermal, balance-equation theory and the noninteracting (single-particle) adiabatic transport theory is discussed to clarify issues subject to controversy in the literature. Finally, we give a brief review of recent developments in the balance-equation approach: investigation of the distribution function in balance-equation theory, improved calculations for GaAs/AlGaAs heterojunctions, extension of the balance equations to an abitrary energy band and recent work on superlattice miniband transport.

Strong electron-electron interaction effects in highly resistive Al-Cu-Fe icosahedral phases.

We have analyzed the temperature and magnetic-field (up to 35 T in pulsed fields) dependences of the conductivity of pure Al-Cu-Fe quasicrystalline samples. The results of this analysis are consistent with predictions of weak-localization and electron-electron interaction theories. A maximum is observed in the high-temperature magnetoconductivity, which could be due to competing effects between these two contributions. The more resistive sample shows a peculiar low-temperature behavior, which could be attributed to band-structure effects.

Effects of electron-electron scattering on impurity resistivity

All important theories of impurity resistivity each give one of two results. These two results coincide for zero temperature but differ for higher temperatures. The electron-electron scattering does not give a direct contribution to the resistivity. However, it has a indirect effect. It affects the occupation numbers of the electron states and thereby modifies the impurity scattering. The two results discussed are both obtained in the absence of explicit electron-electron scattering. The solution of the Boltzmann equation in absence of electron-electron scattering gives one of two results. In the limit of strong electron-electron scattering the second result is obtained; strong electron-electron interaction leads to full thermalisation in the centre-of-mass system of the carriers. Thus the two results are the extreme limits of the results from the Boltzmann equation for varying strength of the electron-electron scattering. The authors make a realistic estimate of the electron-electron scattering-rate and calculate the Boltzmann result for doped GaAs.

Restriction of microwave enhancement of superconductivity in impure superconductors due to electron-electron interaction

Transition from microwave enhancement of supercurrent to superconductivity suppression is investigated in impure superconductors. It is demonstrated that frequency range of enhancement effect narrows with the decrease of electron mean free path, ℓ, and at ℓ⩽1nm electron heating is observed in the whole frequency range. Dependences of frequency boundaries on ℓ are explained by taking into account strong electron-electron interaction in impure metals.

Restriction of microwave enhancement of superconductivity in impure superconductors due to electron-electron interaction

Transition from microwave enhancement of supercurrent to superconductivity suppression is investigated in impure superconductors. It is demonstrated that the frequency range of the enhancement effect narrows with the decrease of the electron mean free path, l, and at l ⩽ 1 nm electron heating is observed in the whole frequency range. Dependences of frequency boundaries on l are explained by taking into account strong electron-electron interaction in impure metals.

Kinetic properties of surface electrons over liquid helium under strong electron-electron interaction

The mobility μ and lifetime of ground-level electrons τ⊥ are studied experimentally in a two-dimensional electron system at the surface of liquid helium at temperatures of0.4 to1.4 K and charge concentrations of (2.8–12.0) × 108 cm−2. It is shown that for fairly low temperature and high concentration where the frequency of interelectronic collisions is much higher than that of electron-ripplon ones, the so-called complete control condition is realized in the electron system, i.e., when the average drift velocity of electrons and the effective electron temperature can be introduced. This model is found to describe well the kinetic properties of surface electrons in the range of charge concentrations up to 4×108 cm−2, where the one-particle approach is no longer applicable. For the value of parameter Γ=e2(πns)1/2/T=47 that corresponds to a strong electron-electron interaction, the lifetime τ⊥ is found to increase sharply and the short-range order typical of a liquid state appears in the two-dimensional electron layer.

The electrodynamics of correlated metals

The electrodynamic response of several heavy-fermion materials, such as CeAl 3, UPt 3 and CePd 3, is reviewed with particular emphasis on low frequency coherent state effects. A narrow zero-frequency resonance is observed in the conductivity at millimeter-wave frequencies which is a manifestation of strong electron-electron interactions. The renormalized relaxation time, the renormalized plasma frequency (or equivalently the spectral weight in the zero-frequency mode) and the enhanced effective mass are derived from the frequency dependent response. Finally, we discuss the relationship between the electrodynamics and thermodynamics in heavy fermion systems.

Negative density of states of 2D electrons in a strong magnetic field

By measuring the magnetocapacitance of high-mobility Si-MOSFETs, we have found that the thermodynamic density of states of 2D electrons in a strong magnetic field and at low temperatures can be negative when energy levels are partially filled. This effect seems to be due to strong electron-electron interaction.

The spectrum of two-particle excitations of the Hubbard Hamiltonian

The spectra of two-hole and two-electron excitations are calculated for a single-band Hubbard Hamiltonian. These spectra are directly connected with the intensity of CVV Auger spectra and with the appearance potential spectra of transition metals with strong electron-electron interactions. The theory is based on a local decoupling procedure applied to the retarded two-particle double-time Green functions in the site representation. It is exact in four important limiting cases: in the non-interacting limit (U to 0), in the atomic limit (band width W to 0), in the limit of low concentration of electrons (n to 0) and holes (n to 2), and it provides a physically reasonable interpolation between these limits. In contrast with previous similar theories, it has proper transformation properties under the electron-hole transformation. The theory is illustrated numerically in a simplified model.

Magnetic characteristics of normal and superconductive states in SmBa 2Cu 3O 7

The electric and magnetic measurements have been carried out on a polycrystalline single phase SmBa 2Cu 3O 7 sample. The normal state susceptibility of it consists of a temperature-dependent Curie-Weiss paramagnetic term with an effective Bohr magneton number p eff=1.1 and the Curie-Weiss temperature θ=38 K and temperature-independent terms: the Van Vleck and Pauli paramagnetism and the ionic core and Landau diagmagnetism. The Pauli susceptibility derived from the temperature-independent normal-state susceptibility of YBa 2Cu 3O 7 is so large that it is due to the strong electron-electron interactions among conduction electrons. We deduce the low critical field H c1(1.5 K), which is 580 Oe and the critical magnetization current density which is of the order of 10 4 A/cm 2 at 5 kOe and decreases with increasing applied field from the magnetization hysteresis loop of SmBa 2Cu 3O 7 for fields up to 60 kOe and temperature at 1.5 K. Some magnetic and superconductive parameters such as H c(0), k GL, χ, λ, etc. are also given by using the values of the low and upper critical fields and the energy gap within the Ginzburg-Landau theory.

Electron-electron interaction and magnon scattering in the d-electron dominated resistivity of Ni80?xFe x Si8B12 glasses

Our investigation of the electrical resistivity ρ(T) betweenT=0.1 K and 295 K revealed for all Fe-concentrations, 2.4≤x≤16, a minimum of ρ betweenTmin=7 K and 15 K. Above the minimum, scattering from structural disorder prevails in the paramagnetic alloy (x=2.4), while in the ferromagnetic glasses (x≥5.6) coherent magnon scattering appears to dominate. With one exception, ρ(T≤Tmin) increases as\(\sqrt T \) down to the lowest temperatures for all investigated Fe-contents (x≤10.4) in zero magnetic field as well as in applied 6 T. The value of the\(\sqrt T \) slopes, 6.3(6) (Ω cm)−1 K−1/2, agrees with the universal scaling behaviour reported recently by Cochrane and Strom-Olsen for systems with strong electron-electron interaction. A large, linear rise of ρ(T<Tmin,B=0) observed in the most diluted sample (x=2.4) is tentatively attributed to quantum localization in the non-reentrant spin glass phase. The magnitude of the temperature effects on ρ(T) above as well as belowTmin indicates significantd-band contributions to the conductivity.

Superconductivity from predominantly repulsive interactions in quasi one-dimensional systems.

We show that for a one-dimensional electron gas with strong repulsive electron-electron interactions and a weak retarded attractive interaction, superconducting fluctuations can occur at high temperatures. The dominant driving force for these fluctuations is the repulsive interactions! Although the results are for a one-dimensional model, a strong analogy exists between our results and the short-ranged resonating-valence-bond picture of superconductivity.