Strong Electron-electron Interactions Research Articles

Transmission zero in a quantum dot with strong electron-electron interaction: Perturbative conductance calculations

A pioneering experiment [E. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, and Hadas Shtrikman, Nature 385, 417 (1997)] reported the measurement of the transmission phase of an electron traversing a quantum dot and found the intriguing feature of a sudden phase drop in the conductance valleys. Based on the Friedel sum rule for a spinless effective one-dimensional system, it has been previously argued [H.-W. Lee, Phys. Rev. Lett. 82, 2358 (1999)] that the sudden phase drop should be accompanied by the vanishing of the transmission amplitude, or transmission zero. Here we address roles of strong electron-electron interactions on the electron transport through a two-level quantum dot where one level couples with the leads much more strongly than the other level does [P. G. Silvestrov and Y. Imry, Phys. Rev. Lett. 85, 2565 (2000)]. We perform a perturbative conductance calculation with an explicit account of large charging energy and verify that the resulting conductance exhibits transmission zero, in agreement with the analysis based on the Friedel sum rule.

ARPES on high-temperature superconductors: Simplicity vs. complexity (Review Article)

A notable role in understanding of the microscopic electronic properties of high-temperature superconductors (HTSC) belongs to angle-resolved photoemission spectroscopy (ARPES). This technique supplies a direct window into the reciprocal space of solids: the momentum-energy space where quasiparticles (electrons dressed in clouds of interactions) dwell. Any interaction in the electronic system, e.g., superconducting pairing, leads to modification of the quasiparticle spectrum—to redistribution of the spectral weight over the momentum-energy space probed by ARPES. Continued development of the technique had the effect that the picture seen through the ARPES window became clearer and sharper until the complexity of the electronic band structure of the cuprates had been resolved. Now, in the doping range optimal for superconductivity, the cuprates much resemble a normal metal with well-predicted electronic structure, though with rather strong electron-electron interaction. This principal disentanglement of the complex physics from complex structure reduced the mystery of HTSC to the tangible problem of the interaction responsible for quasiparticle formation. Here we present a short overview of recent ARPES results, which, we believe, suggest a way to resolve the HTSC puzzle.

Magnetic properties and disorder effects in diluted magnetic semiconductors

We present calculations of the exchange interactions and critical temperatures for several diluted magnetic semiconductor systems. It is shown that the exchange interactions are dominated by short-ranged interactions that have a strong directional dependence. Using a combination of first-principles calculations of the exchange interactions together with Monte Carlo simulations of the classical Heisenberg model, in which the positional disorder and spin fluctuations are properly included, the calculated critical temperatures are in good agreement with experimantal observations. It is shown that agreement between theory and experiment, as regards ordering temperatures, is obtained only when the magnetic atoms are randomly positioned in a simulation cell which proves that disorder effects play a very important role. The effect of strong electron-electron interaction has been studied by means of the $\mathrm{LSDA}+U$ scheme. We investigate in detail the nature of the anisotropic exchange interactions by means of a Fermi surface analysis.

Phononless thermally activated transport through a disordered array of quantum wires

Phononless plasmon assisted transport through a long disordered array of finite length quantum wires is investigated analytically. Two temperature regimes, the low- and the high-temperature ones, with qualitatively different temperature dependencies of thermally activated resistance are identified. The characteristics of plasmon assisted and phonon assisted transport mechanisms are compared. Generically strong electron-electron interaction in quantum wires results in a qualitative change of the temperature dependence of thermally activated resistance in comparison to phonon assisted transport. At high temperatures, the thermally activated resistance is determined by the Luttinger liquid interaction parameter of the wires.

Charge fluctuations in coupled systems: Ring coupled to a wire or ring

Coupled systems in the mesoscopic regime are of interest as charge fluctuation between the sub-systems will depend on electron-electron interactions and will play a dominant role in determining their thermodynamic properties. We study some simple systems like a stub or a bubble strongly coupled to a ring. We show that for a strong electron-electron interaction, there are some regimes where these charge fluctuations are quenched and charge is individually conserved in the two subsystems. This feature does not depend on choice of parameters or charge distribution.

Low‐lying states of two‐electron quasi‐one‐dimensional ring

One-dimensional wave equation for two electrons in nanoring whose height and width are much smaller than the diameter is derived. The energies of the two-electron low-lying states arising from the two-electron rotation around the axis and their dependencies on the nanoring diameter, height and width are analysed. The level-ordering and crossovers of the corresponding curves related to the strong electron-electron interaction are studied and discussed. In the limit case as the height and width of the ring tend to zero our results coincide with those obtained previously for a one-dimensional nanoring. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic transport in single polyaniline and polypyrrole microtubes

Completely doped submicrotubes (90--560 nm in diameter) of conducting polyaniline and polypyrrole have been synthesized by a template-free method. The measurements of resistivity, $I\text{\ensuremath{-}}V$ curve and magnetoresistance of single polyaniline tube by a standard four-probe technique are presented. Due to the elimination of large intertubular contact resistance, the single polyaniline tube shows a considerably high conductivity and a small positive magnetoresistance. In particular, a crossover from Mott to Efros-Shklovskii variable-range hopping conduction at about 66 K is observed in the single polyaniline tube owing to a strong electron-electron interaction with a Coulomb gap of 11.2 meV. This strong Coulomb interaction is also proved by the $I\text{\ensuremath{-}}V$ curves, which show an obvious zero-bias anomaly at low temperatures. In addition, the temperature dependences of electrical conductivity of single polypyrrole submicrotubes with different diameter have also been studied. The room-temperature conductivity of single polypyrrole tube increases from 0.13 to $73\phantom{\rule{0.3em}{0ex}}\mathrm{S}∕\mathrm{cm}$ when the outer diameter decreases.

Hopping conduction in single ZnO nanowires

ZnO nanowires were synthesized by chemical vapour deposition (CVD). The dc electricalconductivity of a single ZnO nanowire was investigated over a wide temperature range from300 to 6 K. It is found that the temperature dependence of conductivity follows the relation. The conductivity data suggest that the dominant conduction mechanismis Efros–Shklovskii variable-range hopping conduction. The strongelectron–electron interaction in the nanowire is also proved by theI–V and d I/d V curves, on which there emerges a Coulomb gap-like structure at low temperatures.

On electron transport in ZrB12, ZrB2, and MgB2 in normal state

We report on measurements of the temperature dependence of resistivity, $\rho(T)$, for single crystal samples of ZrB$_{12}$, ZrB$_{2}$ and polycrystalline samples of MgB$_{2}$. It is shown that cluster compound ZrB$_{12}$ behaves like a simple metal in the normal state, with a typical Bloch -- Gr\"uneisen $\rho(T)$ dependence. However, the resistive Debye temperature, $T_{R}=300 K$, is three times smaller than $T_{D}$ obtained from specific heat data. We observe the $T^{2}$ term in $\rho(T)$ of these borides, which could be interpreted as an indication of strong electron-electron interaction. Although the $\rho (T)$ dependence of ZrB$_{12}$ reveals a sharp superconductive transition at $T_{c}=6.0 K$, no superconductivity was observed for single crystal samples of ZrB$_{2}$ down to $1.3 K$.

Electron-hole symmetry in a semiconducting carbon nanotube quantum dot.

Optical and electronic phenomena in solids arise from the behaviour of electrons and holes (unoccupied states in a filled electron sea). Electron-hole symmetry can often be invoked as a simplifying description, which states that electrons with energy above the Fermi sea behave the same as holes below the Fermi energy. In semiconductors, however, electron-hole symmetry is generally absent, because the energy-band structure of the conduction band differs from the valence band. Here we report on measurements of the discrete, quantized-energy spectrum of electrons and holes in a semiconducting carbon nanotube. By applying a voltage to a gate electrode, an individual nanotube is filled controllably with a precise number of either electrons or holes, starting from one. The discrete excitation spectrum for a nanotube with N holes is strikingly similar to the corresponding spectrum for N electrons. This observation of near-perfect electron-hole symmetry demonstrates that a semiconducting nanotube can be free of charged impurities, even in the limit of few electrons or holes. We furthermore find an anomalously small Zeeman spin splitting and an excitation spectrum indicating strong electron-electron interactions.

Electronic reconstruction at an interface between a Mott insulator and a band insulator.

Surface science is an important and well-established branch of materials science involving the study of changes in material properties near a surface or interface. A fundamental issue has been atomic reconstruction: how the surface lattice symmetry differs from the bulk. 'Correlated-electron compounds' are materials in which strong electron-electron and electron-lattice interactions produce new electronic phases, including interaction-induced (Mott) insulators, many forms of spin, charge and orbital ordering, and (presumably) high-transition-temperature superconductivity. Here we propose that the fundamental issue for the new field of correlated-electron surface/interface science is 'electronic reconstruction': how does the surface/interface electronic phase differ from that in the bulk? As a step towards a general understanding of such phenomena, we present a theoretical study of an interface between a strongly correlated Mott insulator and a band insulator. We find dramatic interface-induced electronic reconstructions: in wide parameter ranges, the near-interface region is metallic and ferromagnetic, whereas the bulk phase on either side is insulating and antiferromagnetic. Extending the analysis to a wider range of interfaces and surfaces is a fundamental scientific challenge and may lead to new applications for correlated electron materials.

Magnetic circular dichroism in Fe2presonant photoemission of magnetite

Measurements of magnetic circular dichroism in Fe $2p$ resonant photoemission and calculations based on a full-multiplet cluster model reveal the parameters of the electronic structure of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ We obtained on-site Coulomb energy ${U}_{\mathrm{dd}}$ of $3d$ electrons, charge-transfer energy $\ensuremath{\Delta}$ which is defined as the energy required to transfer an electron from O $2p$ to Fe $3d,$ and the hybridization strength between Fe $3d$ and O $2p$ at different Fe ion sites of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ by analyzing the data of magnetic circular dichroism. The charge-transfer energy of ${\mathrm{Fe}}^{3+}$ ions was found to be significantly smaller than that of ${\mathrm{Fe}}^{2+}$ ions. We determined the energy positions of the low-lying multiplets of each Fe ion site. These energy positions for ${\mathrm{Fe}}^{3+}$ ion sites are 1.5\char21{}2.5 eV higher than those obtained from band-structure calculations. Our calculations also show that the photoemission spectral weight in binding energy bigger than 10 eV is substantial, in contrast to the results of band-structure calculations. Our findings conclude that ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is a system with strong electron-electron interactions.

Giant positive magnetoresistance in metallicVOxthin films

We report on giant positive magnetoresistance effect observed in VOx thin films, epitaxially grown on SrTiO3 substrate. The MR effect depends strongly on temperature and oxygen content and is anisotropic. At low temperatures its magnitude reaches 70% in a magnetic field of 5 T. Strong electron-electron interactions in the presence of strong disorder may qualitatively explain the results. An alternative explanation, related to a possible magnetic instability, is also discussed.

Study of magnetization and transport properties in La 0.7Pb 0.3MnO 3 with the addition of nonmagnetic Ag

Magnetization and transport properties of Ag containing La 0.7Pb 0.3MnO 3+ x wt% Ag ( x=0–20) system have been reported. The samples with high Ag content ( x>10), Ag phase is observed along with the perovskite structure. Presence of non-magnetic Ag in La 0.7Pb 0.3MnO 3 decreases the d.c magnetization along with increase of ferromagnetic Curie temperature. All the samples show metal-insulator transition (MIT) with peaks at the MIT temperatures (T p ∼240–340 K depending on Ag content). Resistivity ( ρ) decreases and T p increase with increase of Ag content. For higher concentration of Ag (>10 wt%) decrease of ρ is small and T p remains almost constant. Magnetoresistance ratio [MRR=−( ρ 1.5T− ρ 0 T )/ ρ 0 T )] also decreases with increasing Ag concentration. In the semiconducting regime (above T p), resistivity data can be well fitted with small polaron hopping (SPH) model. Activation energy calculated from the SPH model decreases with Ag content. In the low temperature phase ( T< T p), T 2 dependency of resistivity is observed prominent than T 2.5 dependency for the samples with Ag ≥5 wt%, indicating the appearance of strong electron-electron interaction in La 0.7Pb 0.3MnO 3+ x Ag system. Present study indicates the importance of both electron-electron and electron–magnon interactions for explaining transport properties in low temperature phase ( T< T p) of the present La-Pb-Mn-O+Ag system.

Spin entropy as the likely source of enhanced thermopower in Na(x)Co2O4.

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.

Magnetic ordering, orbital ordering, and resonant x-ray scattering in perovskite titanates

The effective Hamiltonian for perovskite titanates is derived by taking into account the three-fold degeneracy of $t_{2g}$ orbitals and the strong electron-electron interactions. The magnetic and orbital ordered phases are studied in the mean-field approximation applied to the effective Hamiltonian. A large degeneracy of the orbital states in the ferromagnetic phase is found in contrast to the case of the doubly degenerate $e_g$ orbitals. Lifting of this orbital degeneracy due to lattice distortions and spin-orbit coupling is examined. A general form for the scattering cross section of the resonant x-ray scattering is derived and is applied to the recent experimental results in YTiO$_3$. The spin wave dispersion relation in the orbital ordered YTiO$_3$ is also studied.

Conjugated polymers in a strong longitudinal electric field

The effect of a strong longitudinal electric field on the dimerization amplitude and the formation energy of solitons is considered by exact diagonalization studies within the self-consistent adiabatic approximation for open chains described by the extended Peierls-Hubbard-model. The interplay of intermediately strong electron-electron (el-el) and electron-lattice (el-ph) interactions as well as the external electric fields are considered. Possible implications for devices based on conjugated polymers which support polarons and bipolarons are suggested.

Triangular lattices of charge-and spin-density waves in chemisorbed metal monolayers on the surface of diamond structure semiconductors

The electronic spectrum of a planar triangular lattice is analyzed, and the possible occurrence of charge (CDWs) and spin density waves (SDWs) in the lattice is discussed. Commensurate CDW and SDW structures of two types are considered in the weak and strong electron-electron interaction approximations. The CDW and SDW models are applied to the specific case of chemisorbed metal monolayers on the (111) surface of diamond structure semiconductors with a coverage close to 1/3.

COEXISTENCE OF CHARGES TRAPPED IN LOCAL LATTICE DISTORTIONS AND FREE CARRIERS IN CUPRATES

The interplay of strong local electron-lattice and electron-electron interaction is analyzed in the two-dimensional Hubbard-Holstein model using an effective single band Hamiltonian. The effect of the strain field acting on the CuO2 plane and the local cooperative pseudo Jahn Teller vibronic electron-lattice coupling appropriate to cuprate superconductors at optimum doping (i.e.; at doping larger than 1/8) are included via the effective next near neighbor electronic hopping integral t′ (determined by the strain field) close to the local vibronic frequency ω0 (involved in the pseudo Jahn Teller electron lattice coupling). The system shows a characteristic feature of the physics of complex systems, the competition and fluctuation between two states: 1) a first phase of charges self trapped in domains with local lattice distortions (a generalization to a many polaron system of the idea of single intermediate polaron) and 2) a second phase of free carriers.

Reflection Spectroscopic Study of Organic Conductors

Abstract The author reviews the low-energy optical transitions in quasi one- and two-dimensional organic conductors studied by polarized reflection spectroscopy. The characteristic optical properties of organic conductors are the strong anisotropy, electron-electron interaction, and coupling with intra-molecular vibration. The method to extract the transfer integrals in anisotropic tight-binding bands is described for several organic conductors. The correlation effect has a big influence on the line shape of the optical conductivity spectrum in the intra-band transition region. The electron-molecular vibration coupling provides us with rich information on the structural fluctuations and structural changes during the phase transition. The observation of plasmons resulting from the mixing with the transverse excitation is caused by the strong anisotropy in electronic structure.