Strong Electron-electron Interactions Research Articles

Kink structure in the electronic dispersion of high-Tcsuperconductors from the electron-phonon interaction

We investigate the electronic dispersion of high-Tc superconductor on the basis of the two-dimensional three-band Hubbard model with the electron-phonon interaction together with the strong electron-electron interaction. In our model, it is shown across the hole-doped region of high-Tc superconductor that the electron-phonon interaction makes a dispersion kink, observed along the nodal direction, and that the small isotope effect appears on the electronic dispersion.

Graphene via largeN: A renormalization group study

We analyze the competing effects of moderate to strong Coulomb electron-electron interactions and weak quenched disorder in graphene. By using a one-loop renormalization group calculation controlled within the large-$N$ approximation, we demonstrate that at successively lower energy (temperature or chemical potential) scales, a type of non-Abelian vector potential disorder always asserts itself as the dominant elastic scattering mechanism for generic short-ranged microscopic defect distributions. Vector potential disorder is tied to both elastic lattice deformations (``ripples'') and topological lattice defects. We identify several well-defined scaling regimes, for which we provide scaling predictions for the electrical conductivity and thermopower, valid when the inelastic lifetime due to interactions exceeds the elastic lifetime due to disorder. Coulomb interaction effects should strongly figure into the physics of suspended graphene films, where ${r}_{s}g1$; we expect the vector potential disorder to play an important role in the description of transport in such films.

Electronic Origin of the Inhomogeneous Pairing Interaction in the High- T c Superconductor Bi 2 Sr 2 CaCu 2 O 8+δ

Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+delta using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.

Coupled force-balance and particle-occupation rate equations for high-field electron transport

It is pointed out that in the framework of balance-equation approach, the coupled force-balance and particle-occupation rate equations can be used as a complete set of equations to determine the high-field transport of semiconductors in both strong and weak electron-electron interaction limits. We call to attention that the occupation rate equation conserves the total particle number and maintains the energy balance of the relative electron system, and there is no need to introduce any other term in it. The addition of an energy-drift term in the particle-occupation rate equation [Phys. Rev. B 71, 195205 (2005)] is physically inadequate for the violation of the total particle-number conservation and the energy balance. It may lead to a substantial unphysical increase of the total particle number by the application of a dc electric field.

Relaxation Process of Transient Current Through Nanoscale Systems; Density Matrix Calculations

Transient current behavior through a nanoscale system responding to a sudden voltage application was investigated, based on the Liouville equation of density matrix. It was found that the transient current behavior is characterized with the oscillation and relaxation processes. The oscillation originates from the time-dependent electron transitions between the electrode and nanoscale system, and the period depends on the Fermi energy and the bandwidth of electrodes. On the other hand, the relaxation occurs due to the energy dissipation into a number of electronic states in electrodes, thus, the relaxation time depends on the density of states in electrode and the electron transfer energy between the electrode and nanoscale system. Furthermore, we clarified that the strong electron-electron interaction decreases the relaxation time. [DOI: 10.1380/ejssnt.2008.213]

Enhanced thermoelectric properties in a layered rhodium oxide with a trigonal symmetry

We have synthesized an original layered rhodium oxide in the Bi-Sr-Rh-O system with alternately stacked conductive $\mathrm{Rh}{\mathrm{O}}_{x}$ layer and insulating $({\mathrm{Bi}}_{0.75}{\mathrm{Sr}}_{0.25}){\mathrm{O}}_{1.5}$ fluorite-type layers. Thermopower, resistivity, Hall coefficient, magnetoresistance, and magnetization measurements evidence high thermoelectric properties with Pauli-like paramagnetism and large carrier concentration $(\ensuremath{\sim}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$. The room-temperature resistivity, thermopower, and power factor are $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{m}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}$, $63\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{V}∕\mathrm{K}$, and $2.48\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{W}∕\mathrm{cm}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$, respectively, which are comparable with those of the best existing $p$-type thermoelectric oxides, the misfit-layered cobalt oxides with the hexagonal $\mathrm{Cd}{\mathrm{I}}_{2}$-type $\mathrm{Co}{\mathrm{O}}_{2}$ layer. In these crystals, the temperature dependence of the resistivity and the magnetic susceptibility suggest the presence of strong electron-electron interaction. This two-dimensional material emphasizes the potential of the transition-metal oxides for thermoelectric applications.

Gradual disappearance of the Fermi surface near the metal-insulator transition inLa1−xSrxMnO3thin films

We report the observation of changes in the electronic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) across the filling-control metal-insulator (MI) transition by means of in situ angle-resolved photoemission spectroscopy (ARPES) of epitaxial thin films. The Fermi surface gradually disappears near the MI transition by transferring spectral weight from the coherent band near the Fermi level $({E}_{F})$ to the lower Hubbard band, whereas a pseudogap behavior also exists in the ARPES spectra in the close vicinity of ${E}_{F}$ for metallic LSMO. These results indicate that the spectral weight transfer derived from strong electron-electron interaction dominates the gap formation in LSMO associated with the filling-control MI transition.

Electrical and galvanomagnetic effects in new sodium-doped fullerene composites

The Hall effect, magnetoresistance, the temperature dependence of the electrical resistance, and the current-voltage characteristics of new C60-based carbon-carbon composites were studied in magnetic fields of up to 28 kOe at temperatures T = 1.8–336 K. Sodium-doped samples were used. It is shown that there is weak localization of charge carriers and strong electron-electron interactions. Superconductivity occurs in the range T ≤ 15 K. A mixed current may exist at higher temperatures. Cooper pairs are assumed to form due to the interaction of the conduction electrons with the π-electron system of a C60 molecule.

Fermi-liquid effects in the Hubbard model

Properties of an electronic system with a very strong electron-electron interaction are studied. The equation of state is obtained, and the temperature and concentration dependences of the hydrodynamic sound and magnetic susceptibility are calculated. A low-temperature domain of instability corresponding to phase segregation is found. This domain is located inside the wide region of Cooper instability. The equation for zero-sound oscillations is obtained with regard to the substantial anisotropy of the energy spectrum.

Tomonaga-Luttinger Liquid Features in Ballistic Single-Walled Carbon Nanotubes: Conductance and Shot Noise

We study the electrical transport properties of well-contacted ballistic single-walled carbon nanotubes in a three-terminal configuration at low temperatures. We observe signatures of strong electron-electron interactions: the conductance exhibits bias-voltage-dependent amplitudes of quantum interference oscillation, and both the current noise and Fano factor manifest bias-voltage-dependent power-law scalings. We analyze our data within the Tomonaga-Luttinger liquid model using the nonequilibrium Keldysh formalism and find qualitative and quantitative agreement between experiment and theory.

Ab initioelectron propagators in molecules with strong electron-phonon interaction: II. Electron Green’s function

Ab initio electron propagator methods are developed to study electronic properties of molecular systems with strong electron-electron and electron-phonon interactions. For the calculation of electron Green's functions we apply a canonical small polaron transformation that intrinsically contains strong electron-phonon effects. In the transformed Hamiltonian, the energy levels for the noninteracting particles are shifted down by the relaxation (solvation) energies. The Coulomb integrals are also renormalized by the electron-phonon interaction. For certain values of the electron-phonon coupling constants, the renormalized Coulomb integrals can be negative which implies the attraction between two electrons. Within the small polaron transformation we develop a diagrammatic technique for the calculation of electron Green's function in which the electron-phonon interaction is already included into the multiple phonon correlation functions. Since the decoupling of the phonon correlation functions is impossible, and therefore, a Wick's theorem for such correlation functions is invalid, there is no Dyson equation for the electron Green's function. To find the electron Green's function, we use different approximations. One of them is a link-cluster approximation that includes diagonal transitions for the renormalized zeroth Green's function. In the linked-cluster approach the Dyson equation is derived in the most general case, where the self-energy operator is an arbitrary functional (not only in the Hartree-Fock approximation). It is shown that even a Hartree-Fock electron (hole) is not a particle any longer. It is a quasiparticle with a finite lifetime that depends on energy of particle and hole states in different ways. As a consequence of this, a standard description of a Hartree-Fock approximation in terms of wave functions becomes inappropriate in this problem. To challenge the linked-cluster approximation we develop a different approach: a sequential propagation approximation where scattering events occur only for sequential transitions. A self-consistent Hartree-Fock equation for a four-index Green's function matrix is derived. In conclusion, the proposed schemes can be considered for future method developments for quantum chemical calculations for large molecules with strong nonadiabatic effects, e-e correlated electron transfer reactions, and electron transport in molecular transport junctions.

Ab initioelectron propagators in molecules with strong electron-phonon interaction. I. Phonon averages

Ab initio electron propagators in molecular systems with strong electron-electron and electron-phonon interactions are considered to study molecular electronic properties. This research is important in electron transfer reactions where the electron transition is not considered any longer as a single electron transfer process or in temperature dependences of current-voltage characteristics in molecular wires or aggregates. To calculate electron Green's functions, the authors apply a small polaron canonical transformation that intrinsically contains strong electron-phonon effects. According to this transformation, the excitation energies of the noninteracting Hamiltonian are shifted down by the relaxation (solvation) energy for each state. The electron-electron interaction is also renormalized by the electron-phonon coupling. For some values of the electron-phonon coupling constants, the renormalized Coulomb integrals can be negative resulting in the attraction between two electrons. Within this transformation, they develop a diagrammatic expansion for electron Green's function in which the electron-phonon interaction is included into the multiple phonon correlation functions. The multiple phonon correlation functions are exactly found. It is pointed out that Wick's theorem for such correlation functions is invalid. Consequently, there is no Dyson equation for electron Green's functions. The proposed approach can be considered for future method developments for quantum chemical calculations that include strong nonadiabatic (non-Born-Oppenheimer) effects.

Ordering in diluted magnetic semiconductors: A magnetic percolation phenomenon (invited)

We report on first principles calculations that describe the electronic structure, magnetic exchange interactions, and ordering temperatures of diluted magnetic semiconductors. The calculated interatomic exchange couplings are used in a Heisenberg Hamiltonian, and ordering temperatures are calculated with the use of Monte Carlo simulations. The accuracy of the method is analyzed by comparing observed and calculated ordering temperatures of several Mn and Cr doped III-V and II-VI semiconductors. The effect of magnetic percolation is discussed along with clustering phenomena and the effect of strong electron-electron interaction.

Low-energy electronic structure of thehigh-Tc cuprates La2−xSrxCuO4 studied by angle-resolved photoemission spectroscopy

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of thehigh-Tc cuprates La2−xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. Wehave revealed a systematic doping evolution of the band dispersions and (underlying)Fermi surfaces, pseudogap and quasi-particle features under the influence of strongelectron–electron interaction and electron–phonon interaction. The unusual transport andthermodynamic properties are explained by taking into account the pseudogap opening andthe Fermi arc formation, due to which the carrier number decreases as the doped holeconcentration decreases.

Electrical transport properties of individual disordered multiwalled carbon nanotubes

The electrical transport properties of individual disordered multiwalled carbon nanotubes (MWCNTs), synthesized with the floating catalyst method, have been investigated. The resistivity of such disordered MWCNTs shows T1∕2 dependence over a temperature range of 4.2–263K. The experimental finding can be interpreted in terms of Al’tshuler-Aronov model [Solid State Commun. 30, 115 (1979); Sov. Phys. JETP 50, 968 (1979); Pis’ma. Zh. Eksp. Teor. Fiz. 27, 700 (1978)], in which strong electron-electron interaction leads to a Coulomb gap in the density of electronic states at the Fermi level. Such a wide fitting range of temperature for T1∕2 dependence has never been reported for other materials, implying an extremely short carrier scattering time of the order of femtoseconds.

Valley Susceptibility of an Interacting Two-Dimensional Electron System

We report direct measurements of the valley susceptibility, the change of valley population in response to an applied symmetry-breaking strain, in an AlAs two-dimensional electron system. As the two-dimensional density is reduced, the valley susceptibility dramatically increases relative to its band value, reflecting the system's strong electron-electron interaction. The increase has a remarkable resemblance to the enhancement of the spin susceptibility and establishes the analogy between the spin and valley degrees of freedom.

Transport properties of a Luttinger liquid in the presence of several time-dependent impurities

We study the transport properties of a Luttinger liquid in the presence of several time-dependent weak point-like impurities. Our starting point is the bosonized form of the Luttinger liquid Hamiltonian with a potential introduced by the impurities. We find the correction to the total current due to the backscattering of the electrons by the impurities in first order of perturbation theory for a general time-dependent impurity potential. We then apply this result to a single time-dependent impurity, and show that it reproduces the result obtained by Feldman and Gefen in Ref. [7], in particular that the current enhancement occurs only for strong repulsive electron-electron interactions. For the case of two impurities oscillating with the same frequency, we find a novel effect, namely, that the dc-component of the backscattering current is positive {\it even for weak repulsive interactions}, due to the presence of the interference term induced by spatial correlations in the Luttinger liquid. We show that the impurity frequencies at which the enhancement takes place form a band-like structure. In the case of several impurities, the bands become broader, because of interference effects.

Quantized conductance in an AlAs two-dimensional electron system quantum point contact

We report experimental results on a quantum point contact (QPC) device formed in a wide AlAs quantum well where the two-dimensional electrons occupy two in-plane valleys with elliptical Fermi contours. To probe the closely-spaced, one-dimensional electric subbands, we fabricated a point contact device defined by shallow etching and a top gate that covers the entire device. The conductance versus top gate bias trace shows a series of weak plateaus at integer multiples of $2{e}^{2}∕h$, indicating a broken valley degeneracy in the QPC and implying the potential use of QPC as a simple ``valley filter'' device. A model is presented to describe the quantized energy levels and the role of the in-plane valleys in the transport. We also observe a well-developed conductance plateau near $0.7\ifmmode\times\else\texttimes\fi{}2{e}^{2}∕h$ which may reflect the strong electron-electron interaction in the system.

Spectral variety in a one-dimensional strongly correlated electron system: Role of electron-lattice interaction

Optical absorption spectra in a one-dimensional strongly correlated electron system are discussed with halogen-bridged Ni complexes as target materials. In particular, we have interest in the interplay between strong electron-electron interaction and at least intermediate electron-lattice interaction, because very small Raman Stokes shifts were observed and interpreted as suppression of the latter interaction. We thus treat the former interaction by a dynamical density-matrix renormalization group method and incorporate the latter interaction semiclassically. As a result, we again find the suppression of the electron-lattice interaction, which now manifests as a very sharp spectral width. The magnitude of broadening obtained almost reproduces that observed in a Br-bridged Ni system. Furthermore, we also show other one-dimensional cases that do not manifest such suppression, including a Holstein-type coupling case and a charge-density-wave system, and clarify the origin of such variety.

Density waves in atomic necklaces

O (1D) systems have always captured the imagination of physicists. In the ultimate 1D limit, a crystal is nothing but a string of atoms, akin to a necklace of tiny pearls. Most physics graduates undoubtedly associate 1D systems with the simple models used in introductory level quantum mechanics or statistical mechanics courses. Indeed, 1D model systems represent pedagogical and often most useful illustrations of the basic physics principles that govern more realistic condensed matter systems. These principles tell us, for instance, that an ideal atomic chain with a single metallic band should be unstable with respect to a Peierls distortion. A Peierls distortion is a condensed state of the 1D electron gas whose formation is triggered by the strong coupling between electrons and quantized lattice vibrations (phonons). The upshot is that such a wire would not conduct electricity at low temperature. Alternatively, atom wires could display exotic many-body physics. It has been predicted that, due to strong electron-electron interactions in 1D, the concept of single electrons should even break down and that it is only meaningful to discuss their collective behavior (i.e., excitations). Such a state of matter is referred to as the Luttinger-Tomonaga liquid. But even though the theory of ideal 1D systems has advanced much, experimental realization of truly 1D condensed matter systems remains a daunting task; it is impossible to suspend a long string of atoms in free space. Nonetheless, due to the progress in nanoscience, it is now possible to synthesize and tailor novel nanophase materials that mimic the ultimate 1D limit quite closely. As a result, crucial questions regarding the physics of atom wires can be experimentally addressed. For instance: would a string of atoms conduct if it is laid down on a substrate? How can the often detrimental effects of the inevitable structural imperfections or those of thermodynamic fluctuations be alleviated? Or perhaps the most fundamental and practical issue: would atom wires actually be thermodynamically stable? Experimental approaches toward creating 1D systems are often classified as ‘top down’ or ‘bottom up’ fabrication. The top-down procedure refers to artificial (lithographic) creation of 1D nanostructures. A natural way to produce atom wires is a bottom-up method that employs the most common 1D line-defects on surfaces, namely atomic step-edges, as a template for self-assembly. Moreover, artificially created nanostructures are generally unstable, whereas self-assembled (nano-) structures instead can be thermodynamically stable, just because the driving force in their formation is not a kinetically enforced low-dimensional ordering, but purely a drive toward the lowest energy state [1,2]. We use the symmetry breaking properties of surfaces and the reconstructions of their topmost atom layers to realize self-assembled macroscopic arrays of atom wires. To this end, we have used vicinal (stepped) Si surfaces. Besides the thermodynamic stability inherent to self-assembly based fabrication, an extra advantage of this approach is that the spatial access to these atom wire arrays allows for a detailed investigation of electronic instabilities in these wires, even at a local scale. The atom wires can reach macroscopic lengths on vicinal surfaces; their length is only limited by occasional step edges crossing the wires. On the other hand, producing atom wires via this bottom-up method inevitably introduces (intrinsic) defects. This should not necessarily be viewed as a drawback, since it provides a much-needed opportunity to study the dramatic influence of imperfections in atom wires on their (in-)stabilities. Because they are located at surfaces, the wires can be studied conveniently using well-developed techniques such as Scanning Tunneling Microscopy and Spectroscopy (STM and STS), and Angle Resolved Photoemission Spectroscopy (ARPES).