Electronic structure and work function of potassium-doped PTCDA thin films

The electronic properties of thin films of potassium-doped 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA) have been studied using photoemission spectroscopy. Potassium addition results in a charge transfer to PTCDA molecules as signaled by the evolution of the valence band photoemission data, and the potassium ions mainly interact with the carbonyl end groups of the PTCDA molecules. None of the potassium-doped PTCDA films prepared in our studies is metallic, which is attributed to the effect of electronic correlations. PACS 71.20.Rv; 79.60.-i     

The motion of ions in potassium-doped polyacetylene

Abstract

When the computer-simulated potassium-doped polyaceteylene lattice is relaxed free of restrictions, the chains of the host polymer deviate from strict planarity. On introducing a K⁺ vacancy in two lattices containing different amounts of dopant, defective lattice calculations show that the structure with the lower doping level is favoured, but that samples probably also contain regions with the higher doping level. The mobility of the K⁺ ions is expected to be very high when vacancies are present in the dopant sublattice.     

The optical dielectric function of ZnO

In this work, an analytical expression for the complex dielectric function is proposed, which includes both discrete and continuum exciton effects. We have introduced Lorentzian broadening in Elliott’s formula and compare our model to other models based on Elliott’s formula. The results show that our model is superior to other models for the dielectric function of ZnO in the literature. Excellent agreement with experimental data for the dielectric function of ZnO for both perpendicular and parallel polarizations has been obtained using our model. Received: 18 April 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002 RID="*" ID="*"Corresponding author. (Fax: +852-2559-8738, E-mail: dalek@eee.hku.hk)     

Thermal dielectric function

Thermal dielectric functions ε(k, ω) for homogeneous electron gas were determined and discussed. The ground state of the gas is described by the Fermi-Dirac momentum distribution. The low and high temperature limits of ε(k,ω) were related to the Lind-hard dielectric function and to ε(k,ω) derived for Boltzmann and for classical momentum distributions, respectively.     

The intensity-dependent dielectric function of an exciton system

The nonlinear dielectric function of a direct band gap semiconductor is calculated using non-equilibrium Green's function technique. The interaction between the semiconductor and the intense radiation field is assumed to occur via excitons. As an example we calculate the polariton dispersion relation and the intensity-dependent changes of refraction index for GaAs.The author would like to thank Prof. Nguyen Van Hieu for projecting the problem and helpful discussions.     

光吸收介质的吸收系数与介电函数虚部的关系

讨论了相对介电函数虚部和消光系数与光吸收的关系,指出吸收系数与消光系数成正比,而吸收系数与相对介电函数虚部成正比是有条件的.     

Mode mixing and dielectric function

We consider a model in which a bilinear Hamiltonian couples an optically inactive mode (magnon) with optically active modes (phonons). The dielectric function is derived both by a classical method and by the use of linear response theory. It is shown that the low frequency dielectric constant is increased above the value which it has in the absence of interaction.     

Two-impurity Kondo effect in potassium-doped single-layer p-sexiphenyl films

Herein, we show that a self-assembled phase of potassium(K)-doped single-layer para-sexiphenyl(PSP) film on a gold substrate is an excellent platform for studying the two-impurity Kondo model. On K-doped PSP molecules well separated from others, we observe a Kondo resonance peak close to EFwith a Kondo temperature of 30 K. The Kondo resonance peak splits when another K-doped PSP molecule is present in the vicinity, and the splitting gradually increases with the decrease in intermolecular distance without signs of phase transition. Our data demonstrate how a Kondo singlet state gradually evolves into an antiferromagnetic singlet state due to the competition between Kondo screening and antiferromagnetic Ruderman-Kittel-Kasuya-Yosida coupling,as described in the two-impurity Kondo model. Intriguingly, the antiferromagnetic singlet is quickly destroyed on increasing temperature and transforms back to a Kondo singlet below the Kondo temperature. Our data provide a comprehensive picture and quantitative constraints on related theories and calculations of the two-impurity Kondo model.     

The inversion of the dielectric function in the presence of an interface

For a system containing a metal-vacuum interface an exact formal solution is presented for the inverse RPA dielectric function. The solution is valid for an arbitrary shape of the electron density profile at the surface. It contains(a) the inverse dielectric Lindhard function representing the bulk screening effects and (b) surface terms which describe the modification of the screening propertles near the interface. From the pole contribution of the surface term an exact equation for the surface plasmon dispersion is derived. In order to check the applicability of the general result for ?^?1, the surface plasmon dispersion and the screening length of a homogenous field applied perpendicular to the surface are briefly discussed by adopting the infinite barrier model.     

The dielectric function of two- and three-dimensional rotators with permanent electric dipoles

Summary The dynamical theory of the dielectric function of classical dipolar liquids is extended by taking into account the dynamical behaviour of the Kirkwood correction factor and improving the treatment of the relaxation functions. Two- and three-dimensional systems are investigated. It is shown that the Kirkwood correction, related to shortrange interactions, has two dynamical effects: it leads to a viscous force and an orienting force, which decays exponentially with frequency. The formalism developed for liquids is extended for frozen rotators in solids.

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Riassunto La teoria dinamica della funzione dielettrica dei liquidi dipolari classici è estesa prendendo in considerazione il comportamento dinamico del fattore di correzione di Kirkwood e migliorando la trattazione delle funzioni di rilassamento. Si studiano sistemi bi- e tridimensionali. Si mostra che la correzione di Kirkwood, connessa con le interazioni a corto raggio, ha due effetti dinamici: porta ad una forza viscosa e ad una forza di orientamento, che decade esponenzialmente con la frequenza. Il formalismo sviluppato per i liquidi ⪻ esteso per rotori ghiacciati nei solidi.

     

Calculation of the dielectric function of Li metal

The inverse microscopic dielectric function of Li metal is evaluated within the time dependent local density approximation (TLDA) using one-electron energies and wave-functions obtained by SCF band structure calculations. Even at larger values of transferred momentum the influence of exchange and correlation is found to be quite small. Local fields are approximated by considering all elements of the dielectric matrix corresponding to the first and second shell of the reciprocal lattice vectors. Their influence is found to be orientation-dependent but in most cases quite small. However, in some cases they even cause a change of the shape of the inverse dielectric function. Our theoretical results agree quite well with measurements of the dynamic structure function by use of inelastic X-ray scattering with transferred energy up to 18 eV. Unrealistic oscillations at still higher energies as well as the narrow width of the peaks indicate the limits of a one-particle scheme.     

On the dielectric function in zero-gap semiconductors

The q and ω dependent dielectric function is examined in the random phase approximation for zinc-blende type, zero-gap semiconductors such as HgTe or HgSe. A detailed analysis of the properties of this function based on the analytical formulae obtained for the case of a highly degenerate free-electron gas is performed. In the limiting cases of the static and dynamic dielectric functions a comparison to previous theories is given.     

Broadband dielectric function of nonequilibrium warm dense gold

We report on the first single-state measurement of the broadband (450-800 nm) dielectric function of gold isochorically heated by a femtosecond laser pulse to energy densities of 10(6) - 10(7) J/kg. A Drude and an interband component are clearly seen in the imaginary part of the dielectric function. The Drude component increases with energy density while the interband component shows both enhancement and redshift. This is in strong disagreement with predictions of a recent calculation of dielectric function based on limited Brillouin zone sampling.     

Anisotropy of the dielectric function for wurtzite InN

A -plane InN film grown by molecular beam epitaxy on -plane sapphire substrate with an AlN nucleation layer and a GaN buffer was studied by spectroscopic ellipsometry. The data analysis yields both the ordinary and the extraordinary dielectric tensor components perpendicular and parallel to the optical axis, respectively. Strong optical anisotropy is demonstrated over the whole energy range from 0.72 up to 9.5 eV. The line shapes of the tensor components and the polarisation behaviour are in very good agreement with the results of recently published band structure and dielectric function calculations. Above the band gap, five van Hove singularities are evidenced from the ordinary component, while three are resolved from the extraordinary part. The polarisation dependence below 1 eV can be interpreted in terms of optical selection rules for three energetically split valence bands around the Γ-point of the Brillouin zone, similar to the well known behaviour of wurtzite GaN. This emphasises a band gap of hexagonal InN of about 0.7 eV.