Anisotropic carrier transport properties of highly aligned oligophenylenevinylenes in organic field-effect transistors

We have studied the electronic excitations of fluorinated copper phthalocyanine, CuPcF₁₆, thin films using electron energy-loss spectroscopy in transmission. This allowed for the derivation of the dielectric function in a wide energy range. Furthermore, we have analyzed the lowest excitation feature using a Lorentz model, which enabled the determination of the dielectric background in the energy range of the gap excitation, and an analysis of the momentum dependence of the spectral intensities at low energies.     

Kinks and d-waves from phonons: The intermediate coupling story

I present results from an approach that extends the Eliashberg theory by systematic expansion in the vertex function; an essential extension at large phonon frequencies, even for weak coupling. In order to deal with computationally expensive double sums over momenta, a dynamical cluster approximation (DCA) approach is used to incorporate momentum dependence into the Eliashberg equations. First, I consider the effects of introducing partial momentum dependence on the standard Eliashberg theory using a quasi-local approximation; which I use to demonstrate that it is essential to include corrections beyond the standard theory when investigating d-wave states. Using the extended theory with vertex corrections, I compute electron and phonon spectral functions. A kink in the electronic dispersion is found in the normal state along the major symmetry directions, similar to that found in photo-emission from cuprates. The phonon spectral function shows that for weak coupling Wλ<ω₀, the dispersion for phonons has weak momentum dependence, with consequences for the theory of optical phonon mediated d-wave superconductivity, which is shown to be 2nd order in λ. In particular, examination of the order parameter vs. filling shows that vertex corrections lead to d-wave superconductivity mediated via simple optical phonons. I map out the order parameters in detail, showing that there is significant induced anisotropy in the superconducting pairing in quasi-2D systems.     

Characterization of the electronic excitations in Alq3 using electron energy-loss spectroscopy

We have studied the electronic excitations of tris-(8-hydroxyquinoline)-aluminium(III) (Alq₃) thin films using electron energy-loss spectroscopy in transmission. This allowed us to derive an effective dielectric function of such films in a large energy range. Moreover, an analysis of the momentum dependence of the lowest lying excitation allowed us to gain information on its localization. We show that this excitation does not disperse, i.e., is localized in the condensed phase. In contrast to many other molecular organic semiconductors, its spectral weight does not follow the behavior of a simple dipole-allowed electronic transition.     

Preparation and electronic structure of phase pure K3C60

Phase pure K₃C₆₀ films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.     

Investigation of the hyperfine structure of Ta I lines (V)

We report on further investigations of the hyperfine structure of spectral lines of the neutral tantalum atom. Besides determination of the hyperfine constants A and B of 21 levels we report on the discovery of 9 up to now unknown fine structure levels for which we could determine their energy position, parity, angular momentum and the constants A and B. For a large number of up to now unclassified lines the combinations could be identified. Received 27 June 2000 and Received in final form 23 August 2000     

Stripes, electron fractionalization, and ARPES

Although structurally the high temperature superconductors are quasi-two-dimensional, there is both theoretical and experimental evidence of a substantial range of temperatures in which ‘stripe’ correlations make the electronic structure locally quasi-one-dimensional. We consider an array of Josephson coupled, spin gapped one dimensional electron gases as a model of the high temperature superconductors. For temperatures above T_c, this system exhibits electron fractionalization, yielding a single particle spectral response which is sharp as a function of momentum, but broad as a function of energy. For temperatures below the spin gap but above T_c, there are enhanced one-dimensional superconducting fluctuations and pseudogap phenomena. Pair tunneling induces a crossover to three-dimensional physics as T_c is approached. Below T_c, solitons are confined in multiplets with quantum numbers which are simply related to the electron, and a coherent piece of the single particle spectral function appears. The weight of this coherent piece vanishes in the neighborhood of T_c in proportion to a positive power of the interchain superfluid density. This behavior is highly reminiscent of recent ARPES measurements on the high temperature superconductors.     

Influence of electron diffraction on measured energy-resolved momentum densities in single-crystalline silicon

Electron momentum spectroscopy is used to determine the spectral function of silicon single crystals. In these experiments 50 keV electrons impinge on a self-supporting thin silicon film and scattered and ejected electrons emerging from this sample with energies near 25 keV are detected in coincidence. Diffraction effects are present that give rise to additional structures in the measured spectral momentum densities. Spectra for a specific momentum value can be obtained at different orientations of the crystal relative to the analysers. By comparing these spectra for which the measured momentum density is the same, but the diffraction conditions of the incoming and outgoing electron trajectories differ, one can distinguish between features due to diffraction of the incoming and/or outgoing electrons, and those due to the electronic structure of the target itself.     

Atomic coherent states studied by virtue of the EPR entangled state and their Wigner functions

Based on the newly constructed Einstein, Podolsky and Rosen (EPR) entangled state representation we introduce macroscopic classical functions associated with atomic coherent state τ with angular momentum value j. These functions are proportional to the ordinary one-variable Hermite polynomials of order 2j. The corresponding Wigner quasiprobability function for τ in phase space is also derived which turns out to be a two-variable Hermite polynomial H _{2j, 2j}. In so doing, a new classical-quantum correspondence scheme for angular momentum system is established. Received 7 August 2002 / Received in final form 14 December 2002 Published online 24 April 2003 RID="a" ID="a"Work supported by the National Natural Science Foundation of China under grant 10175057. RID="b" ID="b"e-mail: fhym@sjtu.edu.en     

Conservation of mass and momentum of the least-squares spectral collocation scheme for the Stokes problem

From the literature it is known that spectral least-squares schemes perform poorly with respect to mass conservation and compensate this lack by a superior conservation of momentum. This should be revised, since the here presented new least-squares spectral collocation scheme leads to an outstanding performance with respect to conservation of momentum and mass. The reasons can be found in using only a few elements, each with high polynomial degree, avoiding normal equations for solving the overdetermined linear systems of equations and by introducing the Clenshaw–Curtis quadrature rule for imposing the average pressure to be zero. Furthermore, we combined the transformation of Gordon and Hall (transfinite mapping) with our least-squares spectral collocation scheme to discretize the internal flow problems.     

Dispersion approach to quark-binding effects in weak decays of heavy mesons

The dispersion approach based on the constituent quark picture and its applications to weak decays of heavy mesons are reviewed. Meson interaction amplitudes are represented within this approach as relativistic spectral integrals over the mass variables in terms of the meson wave functions and spectral densities of the corresponding Feynman diagrams. Various applications of this approach are discussed:Relativistic spectral representations for meson elastic and transition form factors at spacelike momentum transfers are constructed. Form factors at q² > 0 are obtained by the analytical continuation. As a result of this procedure, form factors are given in the full q² range of the weak decay in terms of the wave functions of the participating mesons.The 1/m_Q expansion of the obtained spectral representations for the form factors for the particular limits of the heavy-to-heavy and heavy-to-light transitions are analysed. Their full consistency with the constraints provided by QCD for these limits is demonstrated.Predictions for form factors for B_(s) and D_(s) decays to light mesons are given.The B → γℓν decay and the weak annihilation in rare radiative decays are considered. Nonfactorizable corrections to the B⁰ mixing are calculated.Inclusive weak B decays are analysed and the differential distributions are obtained in terms of the B meson wave function.     

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS₂, HfS₂, MoS₂ and WS₂ using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS₂ and WS₂ cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS₂ is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.     

Isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8 due to quadratic electron-phonon coupling

In connection with the experiment on oxygen isotope effect of Bi₂Sr₂CaCu₂O₈ with the angle-resolved photoemission spectroscopy (ARPES), we theoretically study the isotope-induced band shift in ARPES by the Hartree-Fork and quantum Monte Carlo methods. We find that this band shift can be clarified based on a quadratically coupled electron-phonon (e-ph) model. The large ratio of band shift versus phonon energy change is connected with the softening effect of phonon, and the positive-negative sign change is due to the momentum dependence of the e-ph coupling.     

Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

2 . Dynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation threshold, the ablated mass, and the particle size were investigated. The ablation plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm² the average initial velocity of the ejected particles was deduced from the extension of the plume to range from 120–400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopically determined absorption coefficient of cornea α_cornea. Where absorption due to proteins is high (at λ=6.2 and 6.5 μm), ablated mass, velocity and recoil momentum behave according to α_cornea. For the first time, variations of the ablation plume from pulse to pulse were observed. Those, as well as the particle size, not only depend on the absorption coefficient, but also on the predominant absorber. Received: 4 November 1997/Revised version: 7 September 1998     

Temperature dependence of the spectral weight in p- and n-type cuprates: A study of normal state partial gaps and electronic kinetic energy

The optical conductivity of CuO₂ (copper-oxygen) planes in p- and n-type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part σ₁ (ω) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr_2−xCe_xCuO₄ (PCCO) but not of p-type Bi₂Sr₂CaCu₂O_8+δ (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy E_kin—deduced from the spectral weight—at the superconducting transition is found to cross over from a conventional BCS behavior (increase of E_kin below T_c) to an unconventional behavior (decrease of E_kin below T_c) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics.     

Spectral line broadening mechanism of Er3+ transitions in Er:Ti:LiNbO3 channel waveguides

In order to elucidate the interaction effects among the various defects present in a LiNbO₃-based integrated optical device, we investigated the change of the optical properties of Er³⁺ ions under the application of an external electric field and hydrostatic pressure. We obtained for stoichiometric bulk material a complete picture of the field-induced spectral shifts as a function of transition and site. As a first important application of these results we were able to clarify the mechanism of spectral broadening of the Er³⁺ transitions in Ti:Er:LiNbO₃ channel waveguides. By selecting different waveguide modes for excitation and using highly selective double-resonance excitation with two lasers, we found that the [Ti⁴⁺] concentration gradient caused by the indiffusion results in an internal E-field gradient. This translates, due to the averaging within the guided mode, into mode-dependent spectral line broadening. Received: 24 May 2001 / Published online: 23 October 2001     

Multiplicity dependence of particle spectra: A peripheral phase-space description with application to 19 GeV/c pp reactions

As an attempt to describe the overall features of spectra of particles produced in hadronic interactions at high energy, we propose a peripheral phase-space model which incorporates restricted transverse momenta to the outgoing particles as well as a leading particle effect. Simulating proton-proton interactions at 19 GeV/c beam momentum, we examine the effect of modifying Monte-Carlo generated events by a matrix element containing a factor (?bp_⊥²) for each outgoing particle and an exponential damping exp (at) in the momentum transfer to each of the final baryons. With a fixed set of cut-off constants, we give a survey of spectral shapes obtained for particles of different types, pions, kaons, nucleons and hyperons. In particular, the generated longitudinal spectra are shown to have a distinct dependence on multiplicity. A comparison is carried out with existing data on 19 GeV/c pp interactions obtained by the Scandinavian Bubble Chamber Collaboration.     

Scaling properties of fluctuation results from the PHENIX experiment at RHIC

The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has made measurements of event-by-event fluctuations in the charged-particle multiplicity as a function of collision energy, centrality, collision species, and transverse momentum in several heavy-ion collision systems. It is observed that the fluctuations in terms of σ ²/μ ² exhibit a universal power law scaling as a function of N_participants that is independent of the transverse momentum range of the measurement.     

Spectral properties of doped bilayer cuprates at finite temperatures

Recently, angle-resolved photoemission spectroscopy measurements on Bi₂Sr₂CaCu₂O_8+δ, which possesses two CuO₂ layers in the same unit cell, have yielded very interesting results. For the overdoped samples, these results show a splitting of electronic states near k=(π, 0) point of Brillioun zone. On the other hand, no splitting is observed in the underdoped samples. In view of this, the detailed studies including the doping and temperature dependence of the spectral properties become desirable. In this paper, we consider cuprates possessing two CuO₂ layers per unit cell. Each layer in the system is described by the t-t ¹-J model and the two layers are coupled via an intrabilayer hopping term (t _⊥) and an intrabilayer exchange coupling (J _⊥). A self-consistent perturbation approach is used to calculate the electronic spectral function for different values of hole density, hole momentum and temperature. We find that the imaginary part of the self energy is strongly momentum dependent which contradicts the suggestion that the Fermi surface of cuprates may be described by marginal Fermi liquid theory. We have calculated the spectral function for various values of intrabilayer parameters t _⊥ and J _⊥. For larger values of intrabilayer interactions we observe the splitting in the quasi-particle peak at k=(π, 0) which is in agreement with the recent observations. The splitting is also found to be sensitive to the hole concentration as well as the temperature of the system. We have also discussed the reasons why the splitting is absent in underdoped bilayer cuprates at low temperature.     

Electronic excitations in hole-doped La1−xSrxMnO3 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La_1−xSr_xMnO₃ (x=0, 0.2, and 0.4) at Mn K-absorption edge. Hole-doping effect on the electronic excitations in the strongly correlated electron systems is elucidated by comparing with undoped LaMnO₃. The scattering spectra of metallic La_0.6Sr_0.4MnO₃ show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the spectral weight shifts toward lower energies. Though the peak position of the excitations shows weak dispersion in momentum dependence, RIXS intensity changes as a function of the scattering angle (2θ), which is related to the anisotropy. Furthermore, anisotropic temperature dependence is observed in La_0.8Sr_0.2MnO₃ which shows a metal-insulator transition associated with a ferromagnetic transition. We consider that the anisotropy in the RIXS spectra is possibly attributed to the correlation of the orbital degrees of freedom. The anisotropy is large in LaMnO₃ with long-range orbital order, while it is small but finite in hole-doped La_1−xSr_xMnO₃ which indicates persistence of short-range orbital correlation.